Cementitious Materials

CHAPTER 6 Cementitious materials

A. Fernández-Jiménez* and A. Palomo**

ABSTRACT

This chapter focuses on the application of single pulse MAS NMR to the 27Al and 29Si nuclei of the main components of the most prevalent construction binders, such as Portland cement and other alternative cements: Belite, Sulfobelite, Calcium Sulfoaluminate, Calcium Alumi- nate, Alkali Activated and Hybrid Alkaline Cements. A particular attention has also been paid with respect to the information obtained by the technique to the analysis of the different ce-

mentitious phases (C3S, C2S, C3A, C4A3s, etc.) and the reaction products formed in their hydration (AH3, CAH10, C2AH8, C3AH6, C6As3H32, C4AsH12, C4AcH11, etc.). Also a particular attention has been paid regarding the composition and structure of cementitious gels, main responsible for the good mechanical behaviour of these cements: C–S–H, C–A–S–H and N–A–S–H (synthetic cementitious gels, gels from real samples and compatibility studies) because given their amorphous nature, these gels are very difficult to characterise using XRD.

1. INTRODUCTION

Solid-state nuclear magnetic resonance spectroscopy (mainly the 27Al and 29Si nucleus) represents an important research tool for the characterisation and structural analysis of Portland cement and other aluminosilicate materials used as addition to OPC or even to produce alternative binders. When applying nuclear magnetic resonance (NMR) spectroscopy to solids, a series of dipolar interactions (in nucleus with I = 1/2), and quadrupolar interactions (in nucleus with I > 1/2) takes place; also some variations in the displacement of the chemical shift anisotropy are obseved. Theses main three interactions (dipolar, chemical shift anisotropy, quadrupolar) often lead to very broad and featureless lines. To improve the spectra resolution the Mag-

* Instituto Ciencias de la Construcción Eduardo Torroja (CSIC), [email protected]. ** Instituto Ciencias de la Construcción Eduardo Torroja (CSIC), [email protected].

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ic-Angle Spinning (MAS) tecnique is used. This method is based on the spinning of the sample on an inclined axis (54.736° exactly) with respect to the direction of the external magnetic field (Bo). Before citing examples of applications of MAS, a brief reminder about the main fundamentals on the chemistry of cement (including the thrust of today’s cement research) should help readers to a better understanding of this chapter. Portland cement (OPC), essentially a blend of clinker and gypsum, is indisputably the most widely used construction binder. The chief mineral phases found in

clinker include calcium aluminates (C3A and C4AF) and calcium silicates (C3S, C2S). Since this chapter, standard cement phase notation is used throughout, i. e.: C = CaO;

S = SiO2; A = Al2O3; F = Fe2O3; H = H2O; c = CO2; s = SO3 (Taylor 1997). OPC clinker contains fairly low (from 5 %-15 %) concentrations of calcium

aluminates such as C3A (3CaOAl2O3) and the ferrite C4AF (4CaO · Al2O3 · Fe2O3). In an initial exothermal reaction, C3A hydration with water yields metastable hexagonal calcium aluminate hydrates (see Equations 6.1 and 6.2) that later evolve into cubic hydrates. Attendant upon that early exothermal reaction is a physical event known as “flash cement setting”, which induces very rapid hardening and renders the

material unsuitable for most routine construction use. Gypsum (CaSO4 · 2H2O) is consequently added to control clinker setting (Taylor 1997, Lea’s 2003). C3A reacts with gypsum to form ettringite (Equation 6.3) and where there is an excess of C3A, ettringite evolves into calcium monosulfoaluminate (Equation 6.4).

2C3A + 27 H →→ C4AH19 + C2AH8 → 2C3AH6 + 15 H2O (6.1)

(r = very fast) (hexagonal hydrates) (cubic hydrate)

2C3A + 21 H →→ C4AH13 + C2AH8 → 2C3AH6 + 18/2 H2O (6.2)

C3A + 3CsH2 (gypsum) + 26H → C6As3H32 (ettringite) (r = fast, 1 or 2 min) (6.3)

C6As3H32 (ettringite) + 2C3A + 4H → 3C4AsH12 (monosulfoaluminate) (6.4)

Ferritic phase (C4AF) hydration follows a similar pattern. It reacts very quickly with water, inducing flash setting that, as in C3A, can be controlled by adding gypsum. Its hydration generates phases analogous to ettringite and monosulfoaluminate (Taylor 1997, Lea’s 2003). Nonetheless, since ferritic phase contain some iron, which hampers the exploration with NMR, it will not be ddressed in this chapter.

Monocalcium aluminate, CA (CaOAl2O3), is another compound of interest, although it may appear in very low proportions or not at all in OPC. It nonetheless plays a predominant role (75 %-80 %) in other products, such as calcium aluminate cement, CAC. This phase has even recently been used to formulate ternary OPC-CAC- gypsum cements with promising results (Torrens-Martin et al., 2013). CA also forms

metastable hexagonal hydrates that evolve towards C3A6 + AH3 when it reacts with

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water (Equation 6.5) (Skibsted et al., 1993). This same behaviour is observed in oth-

er calcium aluminates such as CA2 and C12A7 (phases that may appear in minor proportions in Portland cement clinkers, belite cements or CAC (Taylor 1997, Lea’s 2003): i. e., they react with water as per Equation 6.5.

CA H CAH10 CA2 ⎯⎯⎯⎯→ + AHn ⎯⎯⎯⎯→ C3AH6 + AH3 (6.5) C2AH8 conversion C12A7 }

A somewhat different aluminate, yelemite (C4A3s or Ca4Al6O12SO4), is also relevant to the study of cementitious materials. While absent in PC, it is the primary phase in sulfobelite (SAB) and calcium sulfoaluminate (CSAC) cements, with contents of around 50 % and 80 %, respectively. Calcium sulfoaluminate cements, known as low energy (LECs) (Glasser and Zhang 2001, Quillin 2001, Song and Young 2002,

Sánchez-Herrero et al., 2013) or low CO2 cements, differ from ordinary portland cement in their lower clinkering temperatures. Interest around their use has consequently been growing of late (Pelletier et al., 2010, Cuesta et al., 2014.) To date, research on calcium sulfoaluminate cement hydration has addressed its setting and hardening characteristics, mechanical properties and microstructural development (Glasser and Zhang 2001, Pelletier et al., 2010, Quillin 2001). This phase reacts quickly with water in the presence of calcium sulfate (gypsum, anhydrite...) to form ettringite and amorphous aluminium hydroxide (Equation 6.6). No consensus has been reached on the products precipitating when it is hydrated with water in the absence of sulfates. While some authors (Bizzozero et al., 2014; Song and Young, 2002) have detected calcium monosulfoaluminate and aluminium hydroxide (Equation 6.7), others (Winnefeld and Lothenbach, 2010; Sánchez-Herrero et al., 2013; Torréns-Martín et al., 2013) have found that ettringite, calcium monosul- 5 foaluminate, cubic hydrate and gibbsite (Equation 6.8) formed during C4A3 hydration.

C4A3s + 2CsHx + (38–2x)H → C6As3H32 + 2AH3 (6.6)

C4A3s + 18H = C4AsH12 + 2AH3 (6.7)

4C4A3s + 80H = C6As3H32 + C4AsH12 + 2C3AH6 + 8AH3 (6.8)

All the preceding reactions (Equations 6.1-6.8) entail changes in aluminium

coordination, from tetrahedral [AlO4 unit, AlT or Al (IV)] to octahedral [AlO6, Al 27 (VI) or AlO] units, changes that can be studied using Al MAS NMR, as discussed in section 2 below.

The calcium silicates C3S (alite) and C2S (belite), in turn, are the primary constituents in OPC and belite cement (CB) clinker mineralogy (Taylor 1997, Lea’s 2003).

As a general rule, portland cement is estimated to have from 60 wt %-80 wt % C3S and 10 wt %-20 wt % C2S, whereas belite cement contains approximately 50 %-60 % belite and 30 wt %-40 wt % alite. Belite cements emit significantly less CO2 than

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OPC. The drawback to the former is that as C2S hydrates much more slowly than C3S (Equations 6.9-6.10), its early mechanical strength is low (sufficiently low to occasion technological problems for which no ready solution has been found). Both the anhydrous and hydrated form of these phases can be studied with 29Si MAS NMR, as discussed in section 3.

The hydration of both silicates (C3S and C2S) yields portlandite, Ca(OH)2 and a non-crystalline calcium silicate hydrate generically represented as C–S–H. Cement owes its mechanical strength and durability to this C–S–H gel. Whilst alite and belite hydration generate the same products (albeit in different proportions), alite (Equa- tion 6.9) hydrates around 20 times faster than belite (Equation 6.10).

2C3S + 7H2O → → C3S2H4 (gel C–S–H) + 3CH AH = –111,4 kJ/mol (6.9) (60 %-80 %) (r = 6 to 10 hours) (61 %) (31 %)

2C2S + 5H2O → → C3S2H4 (gel C–S–H) + CH AH = –43,0 kJ/mol (6.10) (10 % to 20 %) (r = slow) (82 %) (18 %)

As C-S-H gel is an amorphous compound with a Ca/Si molar ratio that ranges

widely (from 1.0 to 2.0), it is assigned the generic formula (CaO)x SiO2(H2O)y. Struc- turally, it consists in silica tetrahedra connected as dimers (Q 1 units) or silica chains with 2, 5, 8 ... (3n – 1) links (Q 1 and Q 2 units) (Figure 6.1a). Section 4.1 contains a more exhaustive study of C–S–H gel and the heightened understanding of its nanostructural details afforded by NMR. The mass production of Portland cement raises certain energy and environ- mental issues, for it calls for temperatures of up to 1400 °C-1500 °C, entails the ex- traction of raw materials with the concomitant destruction of natural quarries and

emits GHGs such as CO2 and NOx. The 0.8 tonnes of CO2 emitted per tonne of cement manufactured contribute substantially to global air pollution (the cement

industry accounts for 7 % to 8 % of worldwide CO2 emissions). One of the solutions to these problems, widely accepted today by the scientific community, is to replace clinker with supplementary cementitious materials (SCMs), consisting in minerals such as natural pozzolans or industrial by-products such as blast furnace slag or fly ash. The resulting products are known as blended cements. Most of building codes specify both the type and maximum amount of SCMs that cements can include. The SCMs added to clinker are in general Si- and Al-higher and Ca-lower than clinker itself. The C–S–H gels forming in the presence of SCMs have a smaller Ca content than found in pure OPC and may also contain aluminium in their structure. An understanding of the amount of Al, its position in the C–S–H gel and the parameters that favour its uptake is important, for the presence of the element may affect the cohesion and durability of the material. Aluminium uptake

in C-S-H gel gives rise to what are called C–(A)–S–H or C–A–S–H (CaO–Al2O3–H2O) gels, depending on the aluminium content. The presence of aluminium in the C–S–H gel structure may favour the formation of inter-silicate chains and the conversion of a linear (1D) to a two-dimensional (2D) or crosslinked structure (Figure 6.1b). A more detailed study is given in section 4.2.

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Figure 6.1. Simplified depiction of gel structure: (a) linear (1D) C-S-H gel; (b) cross-linked (two-dimensional, 2D) C–A–S–H gel; (c) three-dimensional (3D) N–A–S–H gel.

Another possible, more innovative, approach to the overall problem facing the cement industry consists in developing more economical and eco-friendly binders

(alternative to Portland cement) generating low CO2 emissions, valuing industrial by-products and exhibiting good mechanical properties and good durability characteristics. A series of binders generically known as “alkaline cements” (Figure 6.2) con- stitute one such option. Alkaline cements are cementitious materials formed as the result of the dissolution of amorphous or vitreous natural materials or industrial waste in an alkaline medium. When mixed with alkaline activators, these materials set and harden, yielding a material with good binding properties. The two major alkali activation models in place can be distinguished by their starting conditions.

Procedure 1: involves the activation of calcium- and silicon-high materials; an example of this first model is the activation of blast furnace slag (Puertas, 1995; Fernández-Jiménez, 2000; Fernández Jiménez et al., 2003; Shi et al., 2006; Palomo et al., 2014). In this case the main reaction product is a C–A–S–H gel, similar to the C–S–H (calcium silicate hydrate) gel obtained during portland cement hydration but with a small percentage of Al in its structure.

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Figure 6.2. Position of hybrid alkaline cements relative to pozzolanic blended cements on the pure portland cement (PC) - pure alkaline cement spectrum; *ASM: aluminosilicate materials (adapted from García-Lodeiro et al., 2016, copyright Materials).

Procedure 2: in this model the materials activated comprise primarily aluminium and silicon with low CaO contents, such as metakaolin or type F fly ash. The main product here is a three-dimensional alkaline inorganic polymer, an alkaline

silicoaluminate hydrate, N–A–S–H gel (xNa2O · yAl2O3 · zSiO2 · nH2O) (Figure 6.1c) (Palomo et al., 1999, Fernández-Jiménez et al., 2005; Provis et al., 2009; Shi et al., 2011; Palomo et al., 2014)]. So-called hybrid or blended alkaline cements have also been studied of late. These materials are characterised by low clinker and high SCM contents, along with a small proportion of an alkaline activator to favour initial strength development (García-Loderio et al., 2013, García-Loderio et al., 2016, Alahrache et al., 2016). These cements are the outcome of combining the existing knowledge of OPC and alkaline cements. The inference drawn from the foregoing is that an understanding of the compatibility between the gels described is of utmost importance. Section 4.4 describes a number of examples of NMR-based compatibility studies conducted on synthetic cementitious gels (C–S–H, C–A–S–H and N–A–S–H) and gels from real samples. NMR has been an essential tool in cement research over the last 30 years, in particular due to the excellent ability to help to understand the composition and structure of the aforementioned cementitious gels (C–S–H, C–A–S–H and N–A–S–H). Given their amorphous nature, these substances are very difficult to characterise using XRD. One of the earliest studies in the literature on the application of NMR to cement, a paper on tobermorite structure, dates from 1974 (Sixth ICCC, held at Moscow). Since then, the number of studies using this technique to characterise binders has risen exponentially. Between 1980 and 1990, research focused primarily

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on the anhydrous and hydrated calcium silicate and aluminate constituents of portland cement (Lippmaa et al., 1980, Komarneni et al., 1985, Muller et al., 1986, Engel- hardt and Michel 1987). In the nineteen nineties, scientists concentrated mostly on C–S–H gel structure, changes with hydration time, Ca/Si ratio and aluminium uptake (Justnes et al., 1990, Richardson et al., 1992, 1993, Schilling et al., 1994, Co- lombet and Grimmer 1994, Kirkpatrick 1996, Skibsted et al., 1992, 1995, 2003; Foucon et al., 1998, 1999). Research has been so prolific since 2000 that it is difficult to highlight any studies in particular. Nonetheless, significant work has been conducted recently on the changes in C–S–H gel structure and composition due to the uptake of aluminium, nanosilica, alkaline ions and so on (Andersen et al., 2006, Brunet et al., 2004, García-Lodiero et al., 2010a, 2012; Pardal et al., 2009, 2012; Poulsen et al., 2009, Richardson 2004, Saez del Bosque et al., 2014), as well as on the application of NMR to the study of the structure and composition of both the precursors and the reaction products forming in the new binders. Hence the studies on C–A–S–H gels (Schilling et al., 1994, Fernández-Jiménez et al., 2000, 2003; Pardal et al., 2009, Puertas et al., 2011, Myers et al., 2013; Richardson 2014), N–A–S–H gels (Palomo et al., 2004, 2014, Fernández-Jiménez et al., 2006, Criado et al., 2007, 2008) and gel compatibility (Palomo 2007, García-Lodeiro et al., 2010b, 2011, 2013, 2016). MAS NMR has contributed to a substantial part of the progress in understanding the effect of huge different elements on cement structure and composition. How- ever a number of unknowns have yet to be unravelled. Actually, MAS NMR may play a very significant future role in the acquisition of a detailed understanding of the composition and structure of both precursors and the hydration products formed (gels in particular) in the development of more eco-friendly cements, such as men- tioned above: blended cements, sulfobelite cements, alkaline cements and blended or hybrid alkaline cements.

2. 27AL SPECTRA

Three regions have been identified on the 27Al MAS NMR spectra for cementitious

materials. Tetrahedrally coordinated aluminium (AlT or Al(IV), + 100/ + 50 ppm) is associated with a network-forming Al in anhydrous calcium aluminate compounds or with Al that replaces Si atoms in aluminosilicate chains (as discussed in

section 4). Al resonance for pentahedral aluminium (AlP or Al(V), + 35/ + 20 ppm), although as a rule associated with metastable aluminium compounds, may be generated in hydrated portland cements and C–S–H phases by Al3+ ions replacing Ca2+

ions in C–S–H interlayers (see Section 4). Lastly, octahedral aluminium (AlO or Al(VI), + 15/–10 ppm) is present in hydrated cement compounds such as Al(OH)3, AFm and AFt, calcium aluminate hydrates and carboaluminates (Skibsted et al., 1992, 1993, Andersen et al., 2006, Cong and Kirkpatrick 1993a Fernández-Jiménez et al., 2011, Sánchez-Herrerro et al., 2012, 2013). 27Al (I = 5/2) is a quadrupole nucleus and for structurally disorted 27Al sites this results in a strong quadrupolar broadening of resonances that is only partly reduced

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by MAS. The transition with the highest intensity (for I = 3/2, 5/2) is the so-called central transition (m = 1/2 ↔ m = –1/2), while transitions between the other energy levels normally are denoted like inner (m = ±1/2 ↔ m = ±3/2) and outer (m = ±3/2 ↔ m = ±5/2) satellite transitions. The quadrupole interaction requires consideration of both the first and second order. The second order quadrupolar interaction is in- versely proportional to the magnetic field strength; thus, improved resolution can be achieved at high magnetic fields and in combination with high-speed MAS. As spectra recording conditions depend largely on the material studied and the instruments used, they are omitted from this discussion (see Skibsted 1992 for further details). The aim in this section is to describe the use of 27Al MAS NMR to

study three specific mineralogical phases: anhydrous and hydrated CA, C3A and C4A3s.

2.1. Calcium Monoaluminate (CA) and Calcium Aluminate Cement (CAC)

Given the 100 % abundance of the 27Al isotope and hence its very high sensitivity to NMR exploration, even materials with a low aluminium content (1 %) can be studied with this technique. That notwithstanding, the aforementioned nuclear spin of I = 5/2 induces peak distortion in 27Al NMR spectroscopy, broadening and displacing

the signals to less positive chemical shifts δiso. That is attributed to the second order quadrupole effects of averaging chemical shift and first order quadrupole interactions. Such effects can be minimised but not eliminated entirely by applying the highest possible magnetic field and quickest MAS. A study in depth of these bands guarantees valuable information on quadrupolar interactions. Full spectra for anhydrous and water-hydrated CA and CAC are reproduced in Figures 6.3(a) and 6.3(b).

The quadrupolar coupling constants (CQ) and asymmetry parameters (ηQ ) can be obtained by combining 27Al MAS NMR spectra and numerical simulation/optimisa- tion procedures. Further information on how to study quadrupolar interactions can be found in (Skibsted 1992, Pena et al., 2008). The central area of the spectrum for synthetic CA (Figure 6.3(a), + 100/ + 40 ppm) exhibits a peak at + 79 ppm and a shoulder at + 76 ppm. This signal is the sum of the signals from six overlapping crystallographic positions other than Al(VI) present in CA that cannot be deconvoluted (Skibsted 2016, chapter 6 Book). When CA is hy-

drated (28 day CA + H2O spectrum), the intensity of the AlT signal declines, making it easier to distinguish between the main signal and the shoulder. The spectrum for the hydrated paste contains a new signal at around + 9/ + 10 ppm, associated with

the presence of AlO in hexagonal hydrates such as CAH10 and C2AH8. As shown in Equation 6.5, CA hydration generates hexagonal hydrates that overlap on 27Al MAS NMR spectra and are consequently difficult to distinguish. With time these hexago-

nal hydrates should convert to C3AH6, giving rise to a signal at around + 12 ppm. Figure 6.3(b) shows the full 27Al MAS NMR spectra for anhydrous and 28 day-water-hydrated calcium aluminate cement and a detail of the central signals. Here, as for CA, the focus is on the central signal (Skibsted 1992, Pena et al., 2008). The central area of the CAC spectrum exhibits a wide, intense band centred on + 78

ppm, associated with the presence of tetrahedral aluminium (AlT). A weak signal is

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also observed at around + 9 ppm, probably because the CAC analysed was slightly weathered. When CAC is water-hydrated and cured at ambient temperature for 28

days, the signal at + 78 ppm (AlT) practically disappears. The intense signal appearing at around + 9 ppm is associated with the presence of octahedral aluminium (AlO), in turn attributed to the formation of CAH10 and AH3 (Fernández-Jiménez et al., 2011). When CAC is hydrated in highly alkaline media (8M NaOH solution), the

signal at + 78 ppm associated with the AlT in the starting CAC disappears altogeth- er (Figure 6.3(b), spectrum B-28d.N). At the same time, the AlO signal in these materials shifts slightly to + 12 ppm. That displacement confirms the present authors’ proposed interpretation of XRD and FTIR data (Fernández-Jiménez et al., 2011) to the effect that when hydrated in the presence of alkalis, CAC gives rise to the cubic

hydrate C3AH6 directly, rather than via hexagonal hydrates.

27 Figure 6.3. Full Al MAS NMR spectra (Bruker AVANCE 400, 9.4T resonance frequency, νL = 104.3Mhz

and Spinning frecuency νR = vr 10kHz) and central band for: (a) anhydrous and 28 day water-hydrated CA; (b) anhydrous and 28 day water-hydrated (A) and 8 M NaOH-hydrated (B) CAC; * = sideband (Adapted from Fernández-Jiménez et al., 2011 and from Ph. D. thesis by M. J. Sánchez Herrero -2017).

2.2. Tricalcium Aluminate (C3A) and Ordinary Portland Cement (OPC)

27 The central area of a Al MAS NMR spectrum for C3A is reproduced in Fi gure 6.4. The many signals on this spectrum denote non-equivalent positions of the 2– 2– AlO4 groups present in the unit cell. This cell is based on rings containing six AlO4 tetrahedra, with the inter-ring voids occupied by calcium. Moreover, due to asymmetry and a high quadrupolar constant, each aluminium generates two signals rather than one, giving rise on the spectrum to a wide, low resolution peak (Sanchez-Her- rero et al., 2012, Foucon et al., 1998). In contrast, the spectrum for the 28 day hydrated phase exhibits a narrow signal at around + 12 ppm associated with the presence

of the octahedral Al in the cubic hydrate C3AH6. When deconvoluted, this spectrum

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also exhibits a low intensity signal at around + 9.5 ppm associated with C4AcH11 (Figure 6.4). The presence of that calcium carboaluminate is associated with paste carbonation during handling prior to recording the nuclear magnetic resonance spectrum. Figure 6.4(b) shows the 27Al MAS NMR spectrum for anhydrous and 7 and 28 day water-hydrated portland cement. The spectrum for the anhydrous material exhibits a single signal at around + 80 ppm, associated primarily with the tetrahedral

aluminium (AlT) surrounding C3A, together with other clinker phases such as C4AF (Colombet and Grimmer, 1994). This spectrum also contains a weak signal at around + 8 ppm associated with hydrated cement phases, indicating that this cement may have been slightly weathered when the spectra were recorded. The substantial

decline in AlT signal intensity between the 7 and 28 day materials confirms the reactions taking place in the calcium aluminate in the clinker. In some cases a low intensity signal is also detected at lower values, around + 66/ + 68 ppm, indicative of the

presence of a small amount of AlT in the C–S–H gel structure (Richardson et al., 1993, Sun et al., 2006). The greater the signal shift to less positive values, the larger is the number of Si atoms surrounding the aluminium (see Section 4).

27 Figure 6.4. Al MAS NMR spectrum for: (a) anhydrous and 28 day water-hydrated C3A; (b) anhydrous,

7 day and 28 day water-hydrated OPC. (9.4T, νL = 104.3MHz and νR = 10kHz) (adapted from Ph. D. thesis by M. J. Sánchez Herrero, 2017, and from Fernández-Jiménez et al., 2010).

The 7 day spectrum clearly exhibits an intense signal at + 13 ppm which, according to many authors (Skibsted et al., 1992, 1993; Andersen et al., 2006; Cong and Kirkpatrick 1993a; Fernández-Jiménez et al., 2011; Sanchez-Herrerro et al., 2012,

2017), can be associated with the AlO present in ettringite-like phases. The wide and poorly defined area at around + 8/ + 6 ppm is associated with the Al in gibbsite-

(Al(OH)3)-like phases. The poorly defined signal at lower values (around + 5/ + 4 ppm)

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is initially interpreted to be due to the presence of Al in amorphous hydrogarnet- or gibbsite-like phases. Anderson et al. (2006), however, proved that the wide and poorly defined signal appearing on some 27Al spectra for cement at values of + 5/ + 10 ppm may be associated with an amorphous or disordered aluminate hydrate, formed ei- ther as a separate phase or as a precipitate on the surface of the C-S-H gel. They called it the third aluminate hydrate (TAH), because it forms together with ettringite (AFt) and calcium monosulfoaluminate hydrates (AFm). The combination of the amorphous structure of TAH and the very small amounts (0.1 wt %-0.5 wt % of

equivalent Al2O3) that form in hydrated portland cements may explain why it is not detected by other analytical procedures such as X-ray diffraction. Inasmuch as solid-state NMR spectra primarily reflect the first and second coordination spheres of the 27Al spin nucleus, however, the technique is unable to deliver the molecular formula for TAH, establishing only the local environment surrounding the Al units and their thermal stability. This signal splits on the 28 day spectrum [Figure 6.4(b)]. The peak at + 13 ppm

associated with the AlO in ettringite is still visible, while a new signal at around + 9 ppm is associated here with the formation of calcium carboaluminates. The XRD identification of both ettringite and carboaluminates confirms that interpretation.

2.3. Calcium sulfoaluminate (C4A3s)

Figure 6.5 shows the 27Al NMR MAS spectrum for anhydrous and 28 day water-hy-

drated (at ambient temperature) synthetic C4A3s. The signal at + 67.7 ppm on the spectrum for the anhydrous phase is associated with tetrahedral aluminium. The spectrum for the 28 day hydrated pastes contains a narrow signal at around + 10/0 ppm

27 Figure 6.5. Al MAS NMR spectrum for anhydrous C4A3s and its 28 day hydrated paste (9.4T,

νL = 104.3MHz and νR = vr 10kHz) (adapted from PhD Thesis by Sánchez-Herrero, 2017).

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associated primarily with octahedral Al in the form of gibbsite, overlapping with monosulfoaluminate. Cross polarisation (CP) can be used to better interpret signals and generate spectra with more precisely defined, higher resolution environments, for a 28 day hydrated sample, for instance. When Sánchez-Herrero (2017) conducted 1H–27Al cross polarisation of this sample, they observed that the aforementioned signal was the result of the overlap of several. In addition to the highest intensity peak (+ 10.6 ppm, associated with gibbsite and monosulfoaluminate), they detected two other weaker signals, one at + 13.5 ppm, associated with ettringite, and the oth-

er at + 12 ppm, with the cubic hydrate. The data signify that in this case C4A3s hydration generated primarily calcium sulfoaluminate and gibbsite (see Equation 6.7), although minor amounts of ettringite and cubic hydrate (see Equation 6.8) were also found.

3. 29SI SPECTRA

29Si has a nuclear spin (I = 1/2) with dipolar interactions but not quadrupolar ones. However in solid materials 29Si NMR gives rise to relatively broad resonance signals mainly due to the shielding anisotropy of the 29Si nucleus. In addition, heteronuclear dipolar interactions between 29Si and 1H or 27Al (and likely other NMR active nuclei) may contribute to the linewidth if such atoms are present in close proximity to the 29Si nuclei. That line broadening due to both shielding anisotropy and dipolar interaction can be removed by the magic angle spinning. Normally to achieve good results in most 29Si MAS NMR experiments of solid silicates and aluminosilicates the rec-

ommended conditions are: utilization of moderate spinning speeds (νR = 5–10 kHz) and magnetic field strengths (e. g. 7.1-14.1 T). However single-pulse experiments often involve low sensitivity and consequently high repetition levels due to long spin-lattice relaxation times. It is important to keep in mind that the natural abundance of 29Si isotope, which is the sole NMR-active form of the element, accounts for only 4.7 % of the total silicon present in the sample (Engelhardth and Michel 1987, Colombet and Grimmer 1994). As a result the signal: noise ratio may be low in samples with low silica contents, for which reason 29Si-enriched samples are sometimes used to improve the quality of the spectra (Brough et al., 1994, Pustovgar et al., 2016). Other choice is the simultaneous application of high-power dipolar 1H decoupling (DD). It helps to remove residual dipolar interactions of 29Si spins with nearby 1H nuclei, and it im- proves the signal-to-noise ratio of the spectra. One additional option is to substantially enhance the intensities of the 29Si signals by means of the use of the 1H–29Si cross-polarization (CP) technique (see Section, 6.4.1, Fig. 6.9). Cross-polarization exploits the fact that in many solids the dilute and abundant nuclei are in close proximity and are thus coupled via the magnetic dipolar interaction. CP transfers the magnetization from an abundant spin to the dilute spin via the heteronuclear dipolar interaction. Historically, the mnemonics are I → S CP NMR experiment, where I = insensitive (high abundance, high sensitivity, often 1H), and S = sensitive (low abundance, low sensitivity, often 29Si). 1H–29Si CP/MAS NMR

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is interesting for the study of hydrated samples where CP experiment detects only the hydrated phases. Complementary to one-dimensional 29CP/MAS spectra other methodology can by applied as two-dimensional hetero (1H–29Si) o homo (29Si–29Si) nuclear shift correlation experiments at high spinning frecuencies (see section 6.4.1 Fig 6.10) In the interpretation of these spectra that follows, silicon tetrahedra are repre- n 4– sented as Q = SiO4 , where the value of n, which ranges from 0 to 4, denotes the degree of polymerisation: monomer, dimer, intermediate chain, trimer or tetramer. In 29Si MAS NMR spectra, the more negative the resonance values, the greater the degree of polymerisation. Five regions can therefore be distinguished on 29Si MAS NMR spectra (Engelhardth and Michel 1987, Colombet and Grimmer 1994), one for each standard unit: monomers or Q 0 (δ = 66 ppm to 74 ppm); dimers or ends of chain, Q 1 (δ = –75 ppm to –80 ppm); intra-chain groups, Q 2 (δ = –80 ppm to –86 ppm); flat structures, Q 3 (δ = –90 ppm to –100 ppm); and three-dimensional structures, Q 4 (δ = 103 ppm to 115 ppm). When an aluminium atom replaces a silicon atom in some aluminosilicates structures, the signal shifts by 3 ppm to 5 ppm to a more positive value. Therefore, signals denoting environments with different numbers of Si and Al atoms can be detected on the 29Si MAS NMR spectra for some aluminosilicates. Since in the ex- pression Q n(mAl) n may vary from 0 to 4 and m from 0 to n, theoretically there are 15 possible combinations. Nonetheless, detailed studies of the spectra for materials in which the Al and Si composition differs, show that the possibility of the replacement of Si by Al is subject to Loewenstein’s rule (Engelhardth and Michel, 1987, Colombet and Grimmer, 1994), according to which two adjacent tetrahedra may not both contain aluminium. Such studies also reveal that the Al not only fails to occupy adjacent tetrahedra but tends to be even more disperse to reduce the elec- trostatic repulsion generated in the silicates where aluminium is taken up in place of silicon.

3.1. Calcium silicates: C3S, C2S and portland and belite cements

29 The Si MAS NMR spectra for calcium silicates C3S and C2S are reproduced in Figure 6.6. Synthetic C3S has three polymorphisms (mono- and triclinic and rhom- bohedral) and nine crystallographic sites for silicon. Its 29Si MAS NMR spectrum, however, exhibits only seven of those sites [see Figure 6.6(a)], for two overlap (Co- lombet and Grimmer 1994, Cong and Kirkpatrck 1995). All these resonances lie 0 between –60 ppm and –74 ppm and are attributable to Q units. As synthetic C2S has five polymorphisms but a single crystallographic site for silicon, its 29Si MAS NMR spectrum shows only one sharp signal at around –71.3 ppm [Figure 6.6(a)] (Cong and Kirkpatrick 1993b, Justnes et al., 1990, Poulsen et al., 2009; Sánchez-Herrero 2017).

On the spectrum for 28 day water-hydrated C3S, the intensity of the anhydrous (Q 0) phase signals declines and the two new signals appearing at around –79 ppm and –85 ppm, respectively generated by Q 1 and Q 2 units, are associated with the C–S–H gel formed. The former denotes the presence of silica tetrahedra in the form of dimers or ends of chains, while the Q 2 units are attributed to intra-chain tetrahe-

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29 Figure 6.6. Si MAS NMR spectra for: (a) anhydrous synthetic C3S and C2S and 28 day hydrated; (b) anhydrous and 2 and 28 day hydrated OPC; (c) anhydrous and 2 and 28 day hydrated belite cement.

(9.4 T, νL = 79.5 MHz and νR = 10 kHz) (Adapted from Ph. D. thesis by M. I. García Lodeiro —2008, PhD thesis by M. J. Sánchez Herrero —2017— and from Fernández-Jiménez et al. —2010—).

dra. The spectra for 28 day hydrated C2S pastes show no signals that can be attributed to Q 1 or Q 2 units, confirming the slow reaction rate of this calcium silicate.

In portland cement the C3S and C2S appear not as pure phases, but in the form of solid solutions, alite and belite, respectively. The presence of polymorphic substitutions may distort their crystalline structures, lending them a disordered appearance. The 29Si MAS NMR spectrum for anhydrous OPC consequently ex-

hibits a wide signal around –65/–75 ppm, which envelopes all the C3S (alite or 29 C3Sss) as well as the C2S (belite or C2Sss) signal. The Si MAS NMR spectrum reproduced in Figure 6.6(b) for CEM I 42.5 cement contains a wide signal peaking at around –73 ppm, associated with the presence of alite, and a sharper signal at –71.5 ppm, attributed to belite. This spectrum for anhydrous cement can be deconvoluted to determine the alite and belite contents. Some authors use up to nine components to simulate this spectrum (Skibsted and Jakobsen 1995, Poulsen et al., 2009). Others (Sáez del Bosque et al., 2014) use just three, two for alite (–70.3 ppm and –75.8 ppm) and one for belite (–73.7 ppm). The latter simply quantify the alite and belite present in the cement (Sáez del Bosque et al., 2014) on the grounds

of the percentages of the areas assigned to these phases and the SiO2 content in the cement. The values obtained by quantifying those phases with NMR techniques are closely correlated to the values calculated using the modified Bogue method for the same cement. Most of the literature on the degree of reaction fails to distinguish between alite and belite hydration. In 1990 Justnes et al., proposed a set of equations for calculating the degree of reaction in alite and belite over time based on the relative areas found by deconvoluting the signals on 29Si MAS NMR spectra for anhydrous and hydrated pastes. These equations were ratified by Sáez del Bosque et al. (2014) in a study

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on the effect of temperature on the hydration of the two phases present in a white cement paste. These authors found that temperature affects the belite hydration rate intensely, especially at the early ages. At day 1 belite hydration grows six-fold when the curing temperature is raised, while alite rises by only a factor of 1.5. The spectrum for belite cement [Figure 6.6(c)] contains a sharp signal centred

over –71.5 ppm, similar to the signal on the C2S spectrum. As shown in Equations 6.9 and 10, C–S–H gel and Ca(OH)2 form in both portland and belite cement hydration as the main reaction products, inducing significant changes in the 29Si MAS NMR spectrum relative to the anhydrous materials. The intensity of these resonances, associated with anhydrous (Q 0) phases, declines, while new signals appear at more negative values (Figure 6.6). Such new signals are associated with the presence of Q 1 (dimer or end-of-chain) (–78/–79 ppm) and intra-chain or Q 2 (–85 ppm) units. The intensity of these new signals rises with hydration time. In OPC hydration, which yields primarily alite, the process takes place swiftly, with Q 1 and Q 2 signals clearly present after 2 days and much more intensely after 28. The reaction is much slower in belite cement, however, where the intensity of the 28 day Q 1 and Q 2 signals is lower than in 2 day OPC [Figure 6.6(b) and(c)]. The area associated with these signals, calculated by spectrum deconvolution and based on a “dreierketten” type structural model for C–S–H, has been related in the literature to a number of parameters, including degree of reaction, mean chain length (MCL) and gel Ca/Si ratio. In 1995 Cong and Kirkpatrik published one of the first studies to use this technique to systematically explore the effect of varying the Ca/Si ratio on C–S–H gel structure. These authors concluded that calcium silicate hydrates with a low (0.65 to 1.0) Ca/Si ratio consist primarily of long chains of Q 2 silicon tetrahedra forming silica-high, tobermorite-like gels. Calcium silicate hydrates with high (1.1 to 1.3) Ca/Si ratios, in turn, comprise primarily Q 1 dimers and short chains with Q 1 units at the ends and Q 2 units in intra-chain positions. These calcium-high gels are similar to jennite. All these C–S–H gel-related matters are discussed in greater detail in section 4. Since no resonances associated with anhydrous phases appear in CP/MAS NMR (technique that enhances the signal for Si atoms bonded to protons or OH– groups) (Colombet and Grimmer 1994, Cong and Kirkpatric 1995), studies of these materials, the Q 1 and Q 2 signals can be confidently attributed to hydrated cement phases. Those findings indicate that C–S–H gel is the result the formation of dimer, end-of-chain and intra-chain units from silica tetrahedra. Spectrum deconvolution may also be used to determine the Q 1/Q 2 ratio.

3.2. Octahedral silice coordination: Thaumasite

Thaumasite, Ca6[Si(OH)6]2 (CO3)2 (SO4)2 24H2O), is an expansive salt sometimes formed in Portland cement concrete as a result of the reaction between sulfates and carbonates with calcium silicate hydrate. Thaumasite is one of the few minerals containing octahedrally coordinated silicon, for which reason 29Si MAS NMR plays a significant role in its detection and characterisation.

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29Si MAS NMR spectrum for thaumasite exhibits signals between –170 ppm to –220 ppm because the Si is octahedrally coordinated with six OH– groups. Due to the low silicon concentrations in samples, however, if scans are taken under single pulse conditions as for tetrahedral Si, with 400 to 800 acquisitions and 5 second relaxation times, only a minimum signal is detected. Since at least 1000-2000 scans and 15-30 minute relaxation times are recommended to detect thaumasite, a single sample entails 15-20 hours of scan time. 29Si[1H] cross-polarisation CP/MAS NMR, in which magnetisation is transferred from 1H to 29Si, can be deployed to enhance signal sensitivity (Figure 6.7). The use of 29Si[lH] CP/MAS rather than 29Si MAS NMR to identify thaumasite reduces instrument working time by up to a factor of 1000 thanks 1 29 H Si to the huge gap between H and Si spin-lattice relaxation times (T 1 and T 1) and the CP-induced gain in 29Si intensity (Aguilera et al., 2001, Blanco-Varela et al., 2006). This technique raises sensitivity to a detection threshold of approximately 0.5 wt %. Nonetheless, to quantify the thaumasite present in a sample, a comparison must be run with an external sample of the mineral (with a threshold at 10 %-15 %) (Skibsted et al., 1995).

Figure 6.7. 29Si[1H] CP/MAS NMR of (a) 15 month synthetic thaumasite; (b) 1 year mortar (Adapt form references Aguilera et al., 2001, Blanco-Varela et al., 2006).

4. C–S–H, C–A–S–H AND N–A–S–H (CEMENTITIOUS GEL) STRUCTURE

4.1. C–S–H gel structure

C–S–H gel is the major hydration product precipitating in cement. It accounts for 50 %-70 % of the fully hydrated cement paste and is the product to which cement-based materials owe their strength and durability. C–S–H gel has a complex structure that is not yet wholly understood. Nonetheless, NMR along with other techniques has driven progress in the understanding of C–S–H gel structure and how it changes with variations in parameters such as reaction time, curing temperature, the presence of additions or admixtures and interaction with the surrounding environment. Two widely accepted models for

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C–S–H gel structure have been published. Taylor’s (1986) model describes a structure consisting in 1.4 nm tobermorite-like layers intermixed with jennite-like layers (T/J model). The Richardson and Groves (1992, 2004) model likens the structure to

tobermorite-type layers interstratified with layers of Ca(OH)2 (T/CH), in addition to the T/J arrangement. The latter can in fact be regarded as an enlargement on the former. Both deem C–S–H gel to have a disordered structure with tobermorite- or jennite-like layers in which each layer consists in “dreierketten”-type silica chains. This is also known as the “defect-tobermorite model” (Cong, R. J. Kirkpatrick 1996). Perfect tobermorite consists of two linear chains of silica tetrahedra connected across an interlayer of CaO (Figures 6.1(a) and 6.8). The linear chain comprises repetitive series of three tetrahedra, two of which (paired tetrahedra) are secured to the centre sheet of CaO with oxygen bridges while the third, known as the bridging tetrahedron, is not. Tobermorite therefore is composed of infinite linear chains of silica tetrahedra (Q 2 units), whereas in C–S–H gel many of the bridging tetrahedral are vacant, giving rise to finite two-, three- or five link chains (Figure 6.8).

Figure 6.8. Simplified model of C–S–H gel chains, as per the defect-tobermorite model: (a) dimers; (b) five-link chains; (c) eight-link chains.

Further to this model the infinite, tetrahedrally coordinated silicate chains (dreierketten) do not condense with another layer of dreierketten, but rather the interlayer space is occupied by water molecules and Ca2+ ions. This is the same model as proposed for jennite-like structures. In such structures, however, only half of the oxygen atoms in the central Ca–O layer are shared with the silicate chains: the rest

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form bonds with OH– groups. In the Taylor model, a vacancy in a tetrahedral site does not call for varying the amount of Ca2+ in the interlayer space (Taylor, 1986). Two characteristics of the model proposed by Richardson and Groves merit mention. Firstly, it also deems C–S–H gel to consist of finite tetrahedral silica chains (mix of T/J with 3n-1 links) but forming a “solid solution” with portlandite (the Ca

content may vary): tobermorite-like structures interstratified with Ca(OH)2 (T/CH) layers (Richardson and Groves 1992, 1993). Secondly, this model accommodates the uptake of Al in the C–S–H gel. Aluminium uptake in the C–S–H gel structure is addressed in detail in section 6.4.2. As noted earlier, most of the papers published on the “dreierketten”-type structure of C–S–H gel identify essentially two types of 29Si MAS NMR signals: one at around –79 ppm associated with the presence of Q 1 (dimer or end-of-chain) units and the other at about –85 ppm attributed to Q 2 (intra-chain) units [Figures 6.6(b) and 6.6(c)]. It is difficult to distinguish between intra-chain silicon tetrahedra (that would share their oxygens with the CaO interlayer) and bridging tetrahedra (known as Q 2B or Q 2(L) [Figure 6.8(b)]. According to studies by Faucon (1998, 1999), given their different environment, bridging tetrahedra, Q 2B, should generate a signal at about –83 ppm on 29Si spectra. Those results have been confirmed by other authors (Brunet et al., 2004, Bach et al., 2012, Pardal et al., 2012). The 29Si NMR spectra for synthetic C–S–H gels in Fig- ure 6.9 clearly exhibit the presence of a signal at –83 ppm in gels with a low (0.75) Ca/Si ratio (spectrum A). That signal is not detected when the Ca/Si ratio in the gel rises, however. In tobermorite-like (type I) C–S–H gel (spectrum B) with a Ca/Si ratio of 1.02, the space at the base of the spectrum between the signals at –79 ppm and –84.7 ppm is very wide, denoting the overlap of component Q 2B between the two. [1H]29Si CP/MAS NMR, a technique that transfers magnetisation from the high magnetic moment 1H nucleus to the more dilute 29Si nuclei in the vicinity, can be used to verify the presence of this critical component (Q 2B). The respective 1H/29Si MAS NMR spectrum contains two clear signals at –79 ppm and –85 ppm, along with a shoulder at –83 ppm. This shoulder can be associated with Q 2B units, confirming the identification of the signal in the deconvoluted spectrum in Figure 6.8. In contrast, as in the spectrum gel with a Ca/Si ratio of 1.54 (spectrum C), the signals at –78.2 ppm and –83.8 ppm, attributed to Q 1 and Q 2 units, respectively are clearly defined, with a low likelihood of the presence of a resonance associated with Q 2B. Garcia-Lodeiro et al. (2012) detected the presence of Q 2B units in highly polymerised, scantly crystalline C–S–H gels with low (< 1.2) Ca/Si ratios and mean chain lengths (MCLs) of over 12. A number of equations can be found in the literature with which to determine mean chain length. Those equations, the most significant of which are listed in Table 6.1, have obviously changed with progress in the understanding of these materials. The MCL declines, the percentage of Q 1 units rises and Q 2B units tend to disappear with rising Ca/Si ratios. Inasmuch as under such circumstances it is practically impossible to distinguish between Q 2 and Q 2B units, the signal is assumed to be generated by the former. For those reasons, the assumption that scantly polymerised gels with Ca/Si ratios of greater than 1 contain only Q 2 and Q 1 units may be regarded as valid in most studies.

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Figure 6.9. 29Si MAS NMR spectra for synthetic C–S–H gels (left); [1H]29Si CP/MAS NMR spectrum for sample B (right) (signals >[85] ppm in spectra B and C attributed to Q 3 and Q 4 units present in residual, 1 29 post-C–S–H synthesis silica). The conditions were 9 T, νL = 79.5 MHz and νR = 10 kHz, H/ Si CP MAS-NMR was recorded at short contact time (3 ms). (Adapted from Ph. D. thesis by M. I. García Lodeiro, 2008, and Reprinted with permission from authors García-Lodeiro et al., 2012, copyright J. Am. Ceram. Soc.)

Another factor that makes the presence of Q 2B units difficult to detect on NMR spectra is their intensity. Further to the defect-tobermorite model, the Q 2B/Q 2 ratio = 0.5; i. e., Q 2B signals should be only half as intense as the Q 2 signals. Moreover, as noted earlier, the only NMR-active silicon isotope, 29Si, accounts for just 4.7 % of the total abundance (Bach et al., 2012). Some authors have synthesised C–S–H gels with different Ca/Si ratios using 29Si-enriched constituents (Brunet et al., 2004), while others (Pustovgar et al., 2016) 29 have synthesised and subsequently hydrated Si-enriched triclinic Ca3SiO5 to improve spectrum resolution and attempt to differentiate among the various Q 2 positions. In those studies, authors used one-dimensional 29Si MAS NMR in conjunction with complementary NMR techniques, like for example two-dimensional 1H–29Si HETeteronuclear shift CORrelation experiments (HETCOR) at high spinning frequencies as well as double quantum homonuclear 29Si–29Si correlation experiments using the Back to Back (BABA) recoupling scheme preceded by a cross-polarization 1H–29Si excitation (see Figure 6.10). The combination of techniques allowed to distinguish not only between Q 2paired and Q 2B, but among different Q 2B environments in

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Table 6.1. Summary of ratios and equations used in the structural description of C–S–H, C–A–S–H and N–A–S–H gels

Gel Ratios and equations Schilling’s equations for C–S–H or C–(A)–S–H (1994) No. of vacant bridging tetrahedra → a = Q 1/2 No. of bridging tetrahedra occupied by Al → b = Q 2(1Al)/3 No. of bridging tetrahedra occupied by Si → c = Q 2(0Al)/2 Richardson’s equations for C–S–H gel (1994) Degree of reaction, α(%) = (1 – Q 0) 100 or α (%) = 100 – Σ Q 0 Al/Si = 1/2 Q 2(1Al) / [Q 1 + Q 2(0Al) + Q 2(1Al)] *MCL = (Q 1 + Q 2) / ½ Q 1 *MCL = [Q 1 + Q 2(0Al) + 3/2Q 2(1Al)] / ½ Q 1 C–S–H or C–(A)–S–H *Mean chain length Garcia Lodeiro’s equations for C–S–H Gel (2012) *MCL = (Q 1 + Q 2(0Al) + Q 2B) / ½Q 1 Richardson’s equations for C–A–S–H gel (2014) ν = Fraction of vacant tetrahedral sites present in double- or single-chain tobermorite or C–A–S–H ν = ½ Q 1 / [3/2 Q 1 + Q 2(0Al) + 3/2Q 2(1Al) + Q 3(0Al) + Q 3(1Al)] MCL = 2 [Q 1 + Q 2(0Al) + 3/2Q 2(1Al) + Q 3(0Al) + Q 3(1Al)] / Q 1 MCL = (1-ν) / ν or ν = 1 / (MCL + 1)

C–A–S–H f = Fraction of tetrahedral sites occupied by Al f = ½ Q 2(1Al) /[3/2 Q 1 + Q 2(0Al) + 3/2Q 2(1Al) + Q 3(0Al) + Q 3(1Al) Al/Si = f/(1 – f – ν) = ½ Q 2(1Al) / [Q 1 + Q 2(0Al) + Q 2(1Al) + Q 3(0Al) + Q 3(1Al)

Myers’s equations 2013

1 2 2 3 3 1 MCL[C] = 4 [Q + Q + Q (1Al) + Q + 2Q (1Al)] / Q 3 1 2 2 3 3 (Al/Si)[C] = Q (1Al) / [Q + Q + Q (1Al) + Q + Q (1Al)] C–(N)–A–S–H Engelhard’s equations for aluminosilicate materials 1987

n n = 1, 2, 3, 4 ∑m = 0 I Si (mAl) (Si/Al) = I Si(mAl) is the intensity of the Si-associa- RMN n m ted component surrounded by m Al and ∑m = 0 n¯ I Si (mAl) (4 – m)Si units.

In N–A–S–H gel n = 4 and 0 ≤ m ≤ 4 which involves (Q 4(mAl) units Gel N–A–S–H 4[Q 4(4Al) + Q 4(3Al) + Q 4(2Al) + Q 4(1AL) + Q 4(0Al)] Si/Al = [4(Q 4(4Al) + 3Q 4(3Al) + 2Q 4(2Al) + 1Q 4(1Al) + 0Q 4(0Al)]

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29Si-enriched C–S–H samples. The two unshared oxygens in bridging tetrahedra generate excess negative charge that must be neutralised by two protons located in the interlayer space in tobermorite-like structures or by a proton and an interlayer calcium ion in jennite-like structures. In these analyses the signal at –85 ppm continues to be attributed to intra-chain Q 2 (also known as Q 2paired) units, whilst for Q 2B units a distinction is drawn between a signal at around –82 ppm associated with bridging tetrahedra bonded to two protons (denominated Q 2BOH (Brunet et al., 2004, Bach et al., 2012) and another at around –83 ppm generated by bridging tetrahedra bonded to a proton and a calcium ion (denominated Q 2i or Q 2BCa); signal assignated to Q 2 site next to the Q 3 around –87/–88 ppm is colled Q 2v (Figure 6.7(c) and Figure 6.10).

Figure 6.10. 29Si MAS NMR for two enriched C-S-H samples (Bruker Avance 500 spectrometer) (a) 29Si 29 29 MAS NMR spectra recorded by single pulse (νR = 15 kHz); (b) 2D Si– Si CP/MAS NMR (νR = 10 kHz); 1 ––29 (c) 2D H Si- heteronuclear chemical shift correlation (HETCOR) NMR experiments (νR = 15 kHz). (Adapted from Brunet et al., 2004 copyright J. Phys. Chem. B).

4.2. The role of Al in the C–S–H gel structure: C–A–S–H gel

27Al and 29Si high resolution magic angle spinning (MAS) spectroscopy has been used by many authors to determine how aluminium is taken up into the C–S–H gel structure (Richardson et al., 1993, 1992, 2014; Schilling et al., 1994; Fernández-Jiménez et al., 2003; Pardal et al., 2009, 2012) All agree that tetrahedrally coordinated aluminium can replace silicon in bridging positions in the C–S–H gel structure, as noted earlier. When an Si is replaced by an Al, the signal generated is 3 ppm–5 ppm more positive. Consequently the replacement of a silicon in a Q 2B position by an aluminium tetrahedron shifts the 29Si signal to –82 ppm: 2Q 2 + Q 2B → (replacement) → 2Q 2(1Al) (in this case the intensity of Q 2 (1Al) could be greater than Q 2). Fig- ure 6.11(a) shows the structure of a C–S–H gel in which some of the Si bridging tetrahedra are replaced by Al (Fernández-Jiménez, 2000).

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Figure 6.11. Simplified structural model for (a) a dreierketten-type C–S–H gel in which some of the Si bridging tetrahedra have been replaced by Al tetrahedra; (b) a C–A–S–H gel in which linear chains are joined locally to form a cross-linked structure with Q 3(mAl) units.

Richardson et al. (1992, 1993, 1999, 2000 and 2014) conducted an exhaustive NMR and TEM study to determine how C–S–H gel structure changes in the presence of additions such as slag, fly ash, metakaolin and silica fume. Those studies showed that the Si/Ca and Al/Ca ratios in C–S–H gel rise with the inclusion of such additions because the Si and Al they contain is taken up into the gel structure. The presence of Al induces a new resonance (at around –82 ppm) on 29Si spectra, associated with the presence of Q 2(1Al) units. When studying synthetic gels to which aluminium had been added as sodium aluminate, Anderson et al. (2004) observed no aluminium replacements in paired or end-of-chain silicon tetrahedra (such replacements should give rise to Q 1(1Al) units). These authors reported that Al substitution for Si is not favoured in non-bridging sites. The charge imbalance generated when an Si4+ is replaced by an Al3+ is offset by the uptake in the interlayer of a monovalent alkaline cation or Ca2+. The presence of Al in tetrahedra bridging sites generates a resonance at around + 70/ + 65 ppm in 27Al MAS NMR spectra, characteristic of an aluminium surrounded by two or three silicons. In type I cements with a 95 %-100 % clinker content, only very minor amounts of Al can be taken up into the gel structure. The aluminium in the clinker comprises calcium aluminates that react with water to form calcium aluminate hydrates or AFt and AFm phases, as explained in section 2. Hence only a small amount of Al, present as an alite and belite impurity, is taken up in the C–S–H gel, leading to scant or no Q 2(1Al) units. The presence of Al in C–S–H gel structure is related consequently to additioned or blended cements. The longer chains, greater polymerisation and com- pactness and lower permeability resulting from aluminium uptake in C–S–H gel normally translates into greater cement durability. The presence of Al also modifies gel morphology. TEM exploration shows the presence of blast furnace slag to favour Al uptake in cements, which in turn induces a change from a fibrous to a leaf-shaped gel morphology (Richardson, 1999). Figure 6.12(a) contains examples of 29Si MAS NMR spectra for anhydrous and 28 day hydrated CEM I 42.5 OPC (spectra A and B, respectively) and Figure 6.12(b)

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for anhydrous and 28 day hydrated cement with 40 % blast furnace slag (spectra C and D, respectively). The intensity of the anhydrous phase signals at –71.5 ppm and a shoulder at –73 ppm (Q 0 units in alite and belite) is lower in the 28 day spectra. The signal about –74/–75 ppm in spectrum C for OPC + slag is associated with the presence of the vitreous phase in the slag (Schilling et al., 1994, Fernández-Jiménez 2000, 2003, Puertas et al., 2011). New signals appear at values of –79 ppm (Q 1), –82/–82.5 ppm Q 2(1Al) and –85 ppm. In OPC the Q 1 unit signal is more intense than the Q 2(1Al) and Q 2 signals, whereas in the slag-containing cement the percentage of Q 2 units is clearly greater. The addition of blast furnace slag induces a higher Al content in the C–S–H structure (more Q 2(1Al) units) and with it the formation of more polymerised, longer chains. In a word, the presence of slag prompts the formation of C–(A)–S–H gels.

Figure 6.12. 29Si MAS NMR spectra for anhydrous and 28 day hydrated (a) CEM I 42.5 cement; and (b) the same cement blended with 40 % blast furnace slag. The conditions were 9 T,

νL = 79.5 MHz and νR = 10 kHz.

The 29Si MAS NMR signal can be deconvoluted to calculate the area under each peak and determine the percentage of the various types of C–S–H gel units. In 1994 Shilling et al., established a series of expressions to determine: A, the number of vacant bridging sites in each linear chain of silicon tetrahedra (A = Q 1/2); B, the number of bridging tetrahedra occupied by Al (B = Q 2(1Al)/3); and C, the number of bridging tetrahedra occupied by Si (C = Q 2(0Al)/2) (Table 6.1). In that same year Richardson et al. (1994) published a series of equations to determine the degree of cement reaction and C–S–H or C–(A)–S–H gel mean chain length and Si/Al ratio based on the area assigned to each signal. Those equations are given in Table 6.1. The presence of Al in bridging tetrahedral sites in C–S–H gel favours local, inter-chain bonds. The resulting cross-linked chains form flat (2D) laminar silicate structures [Figure 6.11(b)], giving rise to Q 3(1Al) or Q 3(0Al) units. The presence of Q 3(mAl) units was detected by Fernandez-Jiménez et al., in 2000 in gels resulting from

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the alkaline activation of blast furnace slag (Fernández-Jiménez 2000, 2003). The presence of a certain amount of aluminium in C–S–H gel favours such bonds. These

laminar gels, known as C–A–S–H (xCaO · yAl2O3 · zSiO2 · nH2O) gels, are present- ly under study by a substantial number of researchers. They form in blended cements (Richardson and Groves 1992, Faucon et al., 1999, Pardal et al., 2012), in blast furnace slag-based alkaline cements (Schilling et al., 1994, Fernández-Jiménez 2000, 2003, Puertas et al., 2011) and even in some hybrid alkaline cements (García-Lodiero et al., 2010a, 2013, 2016). In 2014 Richardson, studying the existence of Q 3(mAl) units, published a revision of his equations for determining mean chain length and the Si/Al ratio in C–A–S–H gels (Table 6.1). By way of example, Figure 6.13 reproduces 29Si MAS NMR spectra for slag activated with 4 M NaOH and a sodium silicate solution (Fernández-Jiménez et al., 2000). Deconvolution of these spectra gives the position and area of each signal, to which the equations in Table 6.1 can be applied to calculate degree of reaction, MCL and Al/Si ratio. The deconvolution data and results of applying the equations are given in Table 6.2.

Figure 6.13. 29Si MAS NMR spectra for 7 day blast furnace slag activated with (a) 4 M NaOH; and

(b) a waterglass solution. The conditions were 9 T, νL = 79.5 MHz and νR = 4 kHz. (Adapted from P.h Thesis Fernández-Jiménez et al., 2000).

Figure 6.13. shows that when NaOH is involved in the reaction, the C–S–H gel

formed contains AlT, which occupies some of the bridging tetrahedra in the linear silicate chain. As a result, those gels have a mean chain length of ~8 links, which is longer than the three- or five-link chains in the C–S–H gels generated in OPC hydration. The Al/Si ratio in the gel is around 0.26, whereas when the slag is hydrated with a sodium silicate solution (which contains “waterglass”, a soluble silica), the result is a C–A–S–H gel with a lower Al/Si ratio than when caustic soda is the ac tivator. Nonetheless, the Q 3(0Al) and Q 3(1Al) units detected in the sodium silicate-activated material denotes the presence of cross-linked linear chains of silicates [Fig- ure 6.11(b)]. In other words, when sodium silicate is the activator, the product formed is what is known as C–A–S–H gel, a mix of cross-linked and non-cross-linked tobermorite-like structures. According to Richardson’s equations, this gel has a mean chain length of 12 links (longer than in the NaOH-activated material).

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Table 6.2. Data for deconvoluted 29Si MAS NMR spectra for slag pastes

Signal on spectrum Ratios (see Table 1)

Q 0 Q 0/Q 1 Q 1 Q 2(1Al) Q 2 Q 3(1Al) Q 3 α MCL Al/Si

Pos –65.9 –73.0 –77.7 –81.6 –85.0 BFS + (ppm) a7. 9 a0.26 68.0 b NaOHN Area 12.5 ---- 9.4 22.5 21.7 36.0 10.2 (%)

Pos. –67.1 –74.0 –78.0 –82.0 –85.5 –92.0 –97 BFS + (ppm) a12.3 0.11 69.7 b b Wt Area 25.7 0.16 8.2 22.0 12.6 15.8 24.0 11.3 5.8 (%)

(a) Values found using the Richardson (2014) equations (see Table 6.1). (b) Values found using the Myers (2013) equations (see Table 6.1).

Many papers can now be found in the literature that confirm the presence of Q 3(mAl) units in the C–A–S–H gels forming during slag alkaline activation (Schilling et al., 1994, Fernández-Jiménez 2000, 2003, Puertas et al., 2011). One prominent article, published by Puertas et al., in 2011, reported that in C–A–S–H gels forming in slag activated with an NaOH solution, the presence of tetrahedral Al in bridging sites in the silicate chains prompts a considerable increase in the number of Q 2(1Al) units and a small proportion of Q 3(mAl) units. When waterglass is used as an activator, the aluminium content in tetrahedral bridging sites, raising the percentage of Q 3(mAl) units significantly. That favours chain cross-linking and with it the formation of lay- ered structures in certain regions. This gel is also observed to contain a small amount of alkalis (normally Na), that neutralise the charge imbalance created when an Si is replaced by an Al tetrahedron, forming what is actually a calcium-sodium aluminosilicate hydrate [C–(N)–A–S–H] gel. In 2013 Mayer et al., proposed a series of equations to determine the mean chain length and Al/Si ratio of the C–(N)–A–S–H gel forming when blast furnace slag is alkali-activated. This model, in which the equations are somewhat different from the ones proposed by Richardson (see Table 6.1), was denominated by its authors as the cross-linked substituted tobermorite model (CSTM). Unlike earlier models for the C–(N)–A–S–H phase, the CSTM is not based primarily on non-cross-linked tobermorite structures. It is therefore more consistent with recent findings in connection with the density and structure of the C–(N)–A– S–H product in alkali-activated slag. It is, moreover, the first model to address a mix of cross-linked and non-cross-linked C–(N)–A–S–H structures across the full range of compositions observed in AAS systems. Applying the CSTM equations to the deconvolution data for the spectra shown in Figure 6.13 (see Table 6.2) yields longer mean chain length values for the C–A–S–H gels than obtained with the Richardson equations. These discrepancies in the interpretation of the results attest to the need

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for considerable further research to thoroughly understand these cementitious gels and their formation. NMR will obviously play a vital role in the future development of more precise models better that depict twenty-first century cements more accu- rately. The aforementioned models for C–A–S–H gels assume that aluminium can only be taken up into silicon tetrahedral chains by replacing a silicon in a bridging tetrahedron (Figure 6.11). Nonetheless, recent theoretical studies (Pardal et al., 2012) show that even if Al uptake is favoured, energetically speaking, in bridging sites such as Q 3 or Q 2B, replacement is also likely to occur in pairing sites also, especially at higher Ca/Si ratios (in C–S–H). Using ab initio calculations to determine the uptake of tetrahedrally coordinated Al ions in dreierketten- configured silicate chains, those studies (Pardal et al., 2012, Manzano et al., 2009) show that energy is lower when Al is in bridging sites Q 3 or Q 2B. Given that the difference between the bridging and pairing sites is small (0.17 eV per pentamer unit), however, the two positions can compete even at room temperature. In other words, aluminium has been hypothesised to be taken up in dreierketten silicate chain tetrahedra in three distinct sites: an Al[Q 3] bridging site across the interlayer (Sun et al., 2006, Bach et al., 2012) or between two dreierketten chains adjacent to the same calcium oxide plane (Pardal et al., 2012); an Al[Q 2p] bridging site charge-balanced by interlayer Ca2+; and an Al[Q 2] paired site (Pardal et al., 2012, Bach et al., 2012). Further support for that hypothesis was furnished by Pardal et al., in 2012 in 29Si MAS NMR analyses of synthetic C–A–S–H gels, who observed that Al replaces Si primarily in bridging sites and less frequently in pairing sites under certain conditions. It is found in the latter only with Ca/(Si + Al) ratios of over 0.95 (equivalent to 4 mmol · L–1 of calcium hydroxide). The chemical shift intervals in this model are: –77/–76 ppm for Q 2B(1Al); –80/–79 ppm for Q 1; –82/–81 ppm for Q 2p(1Al); –83.5/–82 for Q 2B ppm; –86/–85 ppm for Q 2P and –94/–90 ppm for Q 3(1Al). The result is a wide signal on the 29Si spectrum at –74/–95 ppm, enveloping all these overlapping signals. The Spectrum deconvolution is complex in these cases. The use of techniques such as CP/MAS and suitable deconvolution criteria, such as the second derivative (Perez et al., 2014), proves helpful. Moreover, the 29Si and 27Al NMR findings must be

consistent. The presence of AlT in two positions in the gel structure should gener- 27 ate different AlT signals in the Al MAS NMR spectra. Signal intensities must also be fairly proportional: each Al substituting for an Si in a Q 3 or Q 2B site has two Si Q 2paired(1Al) neighbours and each Al substituting for an Si in a Q 2P site has an Si Q 2paired(1Al) and an Q 1(1Al) neighbour. Whilst the three aforementioned replacement sites are possible, most of the papers published deem the first to be the most likely. As a rule all the 29Si MAS NMR studies conducted indicate that the higher the Ca/Si ratio in C–S–H or C–A–S–H gels, the less polymerised are the gels, and therefore the shorter their chains. In contrast, as the Ca/Si ratio declines, more polymerised gels form, aluminium uptake is favoured, primarily in bridging tetrahedra, and the number of cross-linked units rises, as predicted by the cross-linked substituted tobermorite model, CSTM.

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4.3. N–A–S–H gels: Ca-low alkaline cements

In cementitious systems where the Ca content is very low (Ca/Si ratios < 0.3) (Fernández-Jiménez et al., 2005; Provis et al., 2009, Shi et al., 2011, Garcia-Lodeiro et al., 2010b, Palomo et al., 2014), three-dimensional alkaline aluminosilicate hydrates form, including N–A–S–H and K-A-S-H gels (depending on the alkali). Such gels are also called alkaline inorganic polymers or geopolymers (Provis et al., 2009). These gels form in the binders obtained by hydrating fly ash, metakaolin or other calcium-low aluminosilicates in the presence of a highly concentrated alkaline solution. Alkaline solutions NaOH, KOH or sodium silicate with molarities ranging from 8 M to 12 M are normally used. Whilst these reactions proceed at ambient temperature, the materials are generally cured at 65 °C to 85 °C (Fernández-Jiménez et al., 2005; Provis et al., 2009, Shi et al., 2011, Palomo et al., 2014). In the nineteen eighties, Davidovits (Davidovits J. et al. (1985) US Patent n.º 4509985) pioneered the nuclear magnetic resonance-based study of the structure of the inorganic polymers resulting from the alkali activation of metakaolin, which he dubbed “geopolymers”. He speculated that the wide signal located at around 85 ppm to 100 ppm on 29Si MAS NMR spectra is due to the overlap of five possible resonances associated with Q 4(mAl) silicon species, where m = 0, 1, 2, 3 or 4. That identification was supplemented with 27Al MAS NMR spectra, on which the sharp

signal at 60 ppm is associated with AlT surrounded by four silicons in an arrangement similar to that detected in zeolites. Davidovits proposed a number of three-dimensional models for the structure of the inorganic polymer formed, depending on the Si/Al ratio (Davidovits, J., 2008). The improvements in NMR that deliver higher resolution spectra have contributed to a better understanding of these cementitious products. In 2005 Fernán- dez-Jiménez et al., introduced the term N–A–S–H gel to designate the main reaction product formed in fly ash activation, based on the nomenclature previously established for portland cement. That denomination is now generally accepted worldwide. Fernández-Jiménez et al. (2006), using NMR and FTIR data, showed that the gel forming in alkali-activated fly ash is three-dimensional, with the Si in a variety of environments. They also showed that the gel evolves over time. First an Al-high metastable gel with a predominance of Q 4(4Al) and Q 4(3Al) units, known as Gel 1, forms. It subsequently evolves into Gel 2, an Si-high product with a predominance of Q 4(3Al) and Q 4(2Al) units (Figure 6.12), to which these materials owe their high mechanical strength. The foregoing is illustrated in Figure 6.14, which shows the changes in 29Si and 27Al MAS NMR in the early stages of fly ash activation with 8 M NaOH at 85 °C. The 29Si MAS NMR spectrum for the anhydrous ash differs substantially from the 2, 5, 8 and 20 hour and 7 day post-activation spectra. The most prominent changes are observed during the early reaction stages (2 h-8 h). The most intense signal, located at around –87 ppm, is associated with the formation of an aluminium-rich N–A–S–H gel with a predominance of Q 4(4Al) environments. The signals detected around –80 ppm, –77 ppm and –72 ppm, in which intensity declines as

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the reaction progresses, are attributed to the presence of less condensed units (monomers and dimers). Assigning signals located at chemical shifts more negative than –88 ppm is complex because they overlap with the signals generated by the unreacted ash. The spectra change substantially as the reaction progresses (20 h and 7 d), with five signals clearly visible at around –110 ppm, –104 ppm, –98 ppm, –93 and –88 ppm. These resonances are respectively attributed to the presence of silicon tetrahedra surround by zero Q 4(0Al), one Q 4(1Al), two Q 4(2Al), three Q 4(3Al) or four Q 4 (4Al) aluminium tetrahedra, indicative of a N–A–S–H gel with a high silicon content (Gel 2). The 27Al MAS NMR patterns for anhydrous fly ash (Figure 6.14) contain two

wide signals, one centred at + 53.3 ppm associated with tetrahedral aluminium (AlT) and a second small signal centred at + 2.5 ppm, attributed to octahedral aluminium

(AlO). The latter component is associated primarily with the presence of mullite in the starting fly ash (Palomo et al., 2004, Fernández-Jiménez et al., 2006). During alkali activation, the tetrahedral aluminium signal is observed to shift first from + 53.3 to + 60.0 ppm, and then from + 60.0 ppm to + 58.4 ppm, indicating that the aluminium always remains tetrahedrally coordinated. This component has been related to aluminium surrounded by four silicon tetrahedra, which is characteristic of Al in zeolite precursors and N–A–S–H gels. These data clearly show that the structure of the N–A–S–H gels forming in the alkaline activation of calcium-low aluminosilicate materials differs substantially from the structure observed in C–S–H and C–A–S–H gels. The former is three-dimensional (3D), with the Si in a variety of environments and a predominance of Q 4(mAl) (m = 0, 1, 2, 3 or 4) units. The Si4+ and Al3+ cations are tetrahedrally coordinated – and joined by oxygen bonds. The negative charge on the AlO4 group is neutralised by the presence of alkaline cations (typically Na+ and K+). Nonetheless, gel structure may vary significantly depending on the degree of reaction, curing temperature and particularly the presence of soluble silica in the activator (Engelhardth and Michel 1987, Palomo et al., 2004, Fernández-Jiménez et al., 2006). Duxson et al. (2005) analysed the 23Na spectra generated in gels formed in the alkaline activation of metakaolin to determine the role of alkalis in these gels. They concluded from the two signals observed at around –4 ppm and 0 ppm that sodium can neutralise the excess negative charge in two ways. The –4 ppm signal is attributed to sodium associated with aluminium in the gel structure (offsetting the excess negative charge). The signal at 0 ppm, which only appears in spectra for alkaline aluminosilicate gels with Si/Al ratios of under 1.4, is associated with the sodium – present in the pore solution, neutralising the negative charge on the Al(OH)4 groups. Criado et al. (2007) also studied the position of sodium in the N–A–S–H gel formed during fly ash alkaline activation. In dehydrated zeolites they found the sodium ions to be coordinated directly with oxygen anions, translating into signals located at very negative chemical shifts (around –20 ppm). GarciaLodeiro et al. (2010b), likewise analysing the 23Na MAS NMR spectra generated by synthetic N–A–S–H gels, confirmed the presence of a sole signal located at around –10 ppm, + which they attributed to partially hydrated sodium (Na(H2O)x ).

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Figure 6.14. 29Si MAS NMR (left) and 27Al MAS NMR spectra (right) for: anhydrous fly ash and fly ash activated with 8 M NaOH at 85 °C for 2 h, 5 h, 8 h, 20 h and 7 d. 29Si and 27Al MAS-NMR spectra were performed with an MSL-400 Bruker apparatus. The resonance frequencies used in this study were 79.5 and 104.3 MHz, with spinning rates of 4 kHz and 12 kHz respectively (adapted from Fernández-Jiménez et al., 2006).

4.4. Cementitious gel compatibility: hybrid alkaline cements

As noted in the introduction to this chapter, a relatively new line of research has arisen around the development of environmentally sustainable cements containing a substantial SCM and much smaller clinker fraction. These so-called hybrid alkaline cements are the result of combining the existing knowledge of the basics of both portland and alkaline cement chemistry. The Eduardo Torroja Institute and more specif- ically the authors of this chapter pioneered this research, publishing their first article on the subject in 2007 (Palomo et al., 2007). Much progress has been made over the last 10 years in the understanding and development of hybrid cements, which have now even been tested on an industrial scale. NMR has indisputably played an in- strumental role in such swift development.

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In these cements, the type of gel forming depends largely on the chemical composition of the precursors and the type and concentration of the alkaline products used. According to the studies published to date, the reaction products comprise not a single but a mix of gels (Garcia-Lodeiro et al., 2010a, 2010b, 2013, 1016). In blends containing 30 % OPC clinker and 70 % fly ash, for instance, the reaction product has been proven to be an Al- and alkali-bearing C–S–H gel yielding a (C, N)–A–S–H– like gel, and a calcium-carrying N–A–S–H gel yielding (N–(C)–A–S–H (Palomo et al., 2007, Garcia-Lodeiro et al., 2013, 2016). The study of such complex systems has been broached by addressing gel compatibility using synthetic gels (Garcia-Lodeiro et al., 2010a, 2010b, 2012) and actual samples (prepared with low clinker and high ash, pozzolan or slag content) (Palomo et al., 2007, Garcia-Lodeiro et al., 2013, 2016, Fernández-Jiménez et al., 2013, Alah- rache et al., 2016). In real hybrid cements, the chemical composition may vary substantially depending on the type of SCM used in their manufacture. By way of example, the results for two hybrid cements with the same clinker content (20 %) and different types of SCM [80 % fly ash (BFA) or 80 % blast furnace slag (BSLAG)] are discussed hereunder. Table 6.3 shows the strength values when hybrid alkaline cements are water-hydrated in the absence (H) or presence (C) of a solid-state alkaline activator,

4 % Na2CO3. Higher 2 and 28 day mechanical strength is observed in the presence of the activator as a result of the significant microstructural changes taking place in the gel or gels formed.

Table 6.3. Compressive strength in hybrid cement mortars (Fernández-Jiménez et al., 2013)

Without activator With solid 4 % Na2CO3 Binder (H) (C) 2 days 28 days 2 days 28 days BFA (20 % clinker+80 % FA) 4.25 MPa 21.59 MPa 20.45 MPa 34.63 MPa BSLAG (20 % clinker+80 % slag) 9.46 MPa 30.02 MPa 16.73 MPa 45.27 MPa

Figure 6.15 shows the 27Al and 29Si spectra for the aforementioned binders. The 27Al MAS NMR spectrum for OPC clinker exhibits a sharp signal centred

over + 80.3 ppm, associated with the AlT in the C3A and C4AF present in these materials. The fly ash spectrum contains a wide resonance at around + 56 ppm, associated with tetrahedral Al [Figure 6.15(a)]. The spectrum for the blast furnace slag (BFS) exhibits a very wide, asymmetric signal centred over + 58.4 ppm [Fig-

ure 6.13(b)] associated with the presence of AlT, which acts as a vitreous phase network former in slag. The 27Al MAS NMR spectra for the water-hydrated BFA materials in the absence (BFA-H) and presence (BFA-C) of the activator contains a wide signal at

around + 57 ppm attributed to AlT. A substantial part of this resonance may be gen-

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erated by the presence of unreacted fly ash. Nonetheless, the sharper, slightly nar- rower signal on the 28 day spectra for the hydrated pastes (particularly BFA-C) and its shift to higher δ (ppm) values suggest that much of the Al is taken up in the gel and surrounded by four silicons. The signal observed at around + 10 ppm in BFA-H,

associated with octahedral aluminium (AlO), may be attributed to the calcium aluminate hydrates formed during hydration of the C3A present in the clinker (in the absence of gypsum).

An intense signal at + 9.5 ppm, associated with AlO, is detected on the spectra for both BSLAG-H and BSLAG-C [Figure 6.15(b)]. That signal is much more intense in BSLAG-C, possibly due to the presence of Al in calcium carboaluminate

hydrates, a phase detected in this material by XRD. An AlT signal at + 60/ + 64 ppm in both the absence and presence of the activator indicates that the post-reaction Al is slightly less polymerised than the aluminium in the BFA cements. The 29Si MAS-NMR spectrum for clinker [Figures 6.15(c) and 15(d)] contain a sharp, symmetric signal with a linewidth of approximately 15 ppm. This signal has a resonance centred at –71.38 ppm and a shoulder at –73.99 ppm, respectively re-

flecting the tetrahedral Si monomers present in C3Sss (alite) and C2Sss (belite). The fly ash (FA and BFA) spectra [Figure 6.15(c)] exhibits a wide signal centred at around –100 ppm with peaks at –88 ppm, –100 ppm, –103 ppm and –107 ppm, associated with the various forms in which silicates appear in fly ash (mullite, vitreous phase, quartz (Fernández-Jiménez et al., 2006). The wide and asymmetric signal centred at –76.5 ppm on the blast furnace slag (SLAG and BSLAG) spectra [Figure 6.15(d)] is associated with the akermanite-like vitreous phase of this material. Blast furnace slag, which is primarily vitreous, generally comprises a solid solution of akermanite

(Ca2MgSi2O7) and gehlenite (Ca2(Al2Si)O7). Akermanite consists of Ca-O laminas interconnected to tetrahedral layers of Si and Mg. The SiO4 units in these tetrahedral layers are organised into chains of dimers, in which the Mg occupies tetrahedral sites, bonding the dimers (Fernández-Jiménez, 2000). In the 28 day 29Si MAS NMR spectra for hydrated binder BFA [Figure 6.15(c)] (spectra BFA-H and BFA-C), the signal at –71.3 ppm is smaller, confirming that the calcium silicates in the clinker react. Two new signals observed in both spectra at around –85 ppm and –79 ppm may be associated with the Q 2 and Q 1 units typical of a CSH gel. The high intensity of the component associated with unreacted fly ash in binder BFA-H (no alkaline activator), which reveals the existence of a large amount of unreacted ash, would explain the lower mechanical strength observed for this material. On the BFA-C spectrum, the slight shift in the signal at –85 ppm to –86 ppm and the existence of signals at –92 ppm and –96 ppm (possibly associated with Q 3(mAl) or Q 4(mAl) units) are indicative of greater fly ash reactivity in this cement. The presence of Al in positions at around +58 ppm in Figure 6.15(a) is particularly suggestive of the possible presence of Q 4(4Al), Q 4(3Al) and Q 4(2Al) units. Nonetheless, the possibility of overlap between Q 3(mAl) and Q 4(mAl) units cannot be ruled out. The intensity of the signals on the 28 day 29Si spectra for the BSLAG system [Figure 6.15(d)] located at –72 ppm, associated with the Q 0 units in the clinker, and –74 ppm, attributed to the Q 1 units in the slag (Fernández-Jiménez et al., 2000, 2003), is substantially lower than in the unhydrated material. New signals appear on these

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Figure 6.15. 27Al MAS NMR spectra of (a) FA and BFA (b) SLAG and BSLAG; and 29Si MAS NMR spectra

of (c) FA and BFA (d) SLAG and BSLAG. The registration conditions were 9 T, and νR = 10 kHz. (Reprinted with permission from authors Palomo et al., 2013, copyright Romanian Journal of Materials).

spectra, at –79 ppm, –83 ppm and –85 ppm, associated with Q 1, Q 2(1Al) and Q 2 units, respectively, in C–A–S–H-like gels. The absence of signals for more polymerised units attests to the formation of a C–A–S–H-like gel in the system. In short, there is every indication that essentially two types of gels form in this system: i) as a result of the clinker reaction, a C–S–H gel that takes aluminium up in its structure → C–(A)–S–H; and ii) another gel with a composition that may vary with the precursor. For systems with a high silica and alumina content (i. e., BFA hybrids), it is a N–A–S–H gel that takes Ca up in its structure → (N,C)–A–S–H; while for Ca- high systems (i. e., BSLAG hybrids) a C–A–S–H-like gel is the main product generated. In light of the behaviour of synthetic materials, with time these gels may be expected to interact and evolve into a new single (and more thermodynamically stable) gel that could be denominated (N)–C–A–S–H (García-Lodeiro et al., 2011). These findings further confirm that the activators used by those authors intensify the reaction of the precursors, enhancing the formation of a larger amount of cementitious gel or gels to which hybrid alkaline cements owe their good mechanical performance. In 2016, Garcia-Lodeiro et al., based on NMR as well as XRD, FTIR, BESEM and TEM data proposed a nanostructural model for gel formation in hybrid alkaline cement. The various stages of that model are illustrated in Figure 6.16. The process begins with the dissolution of the source of silicoaluminates and calcium silicates in the alkaline solution, with the release of a wide variety of dissolved species (Figure 6.16a). The medium becomes saturated with ions that are not uniform- ly distributed, but rather exhibit local concentrations of the various species, depending on the nature of the nearest particle. When these local concentrations reach saturation, C–S–H and N–A–S–H gels precipitate simultaneously (competitive reactions), although which of the two precipitates more rapidly has yet to be determined (Figure 6.16b). As the reaction progresses, more Si–O groups dissolve out of the initial aluminosilicate (fly ash) and the calcium silicate in the cement, raising the silicon concentration in the reaction medium and with it silicon uptake in both gels (Figure 6.16c).

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At the same time, the Ca2+ and Al3+ ions present in the aqueous solution begin to diffuse through the hardened cementitious matrix. A small number of Ca2+ ions (not taken up in the C–S–H gel) interact with the N–A–S–H gel to form an (N,C)– A–S–H gel. Inasmuch as Na+ and Ca2+ share similar ionic radius and electronegative

Figure 6.16. Nano-structural mechanism for gel formation in hybrid alkaline cements. (Adapt form reference García-Lodeiro et al., 2016).

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potential values, the ion exchange involved in the replacement of sodium with calcium resembles the mechanism observed in clay and zeolites, in which the three-dimensional structure of the (N,C)–A–S–H gel is conserved. Similarly, the C-S-H gel forming from the silicates in cement takes aluminum into its composition (preferably) in bridging sites, yielding C–(A)–S–H and subsequent C–A–S–H gels as the aluminum content rises (Figure 6.16d). Where a sufficient store of the element is available, calcium continues to diffuse through the pores of the matrix and interact with the (N,C)–A–S–H gel. The polar- ising effect of the Ca2+ (to form Si–O–Ca bonds) distorts the Si–O–Al bonds, inducing stress and ultimately rupture. To date, two hypotheses have been put forward to explain the ion exchange mechanisms between different gels generated in such cementitious systems: (a) the replacement of one Al3+ and one Na+ by two Ca2+ and (b) the replacement of two Na+ ions by one Ca2+. As aluminium is released from the N–A–S–H gel, less polymerised structures (C–A–S–H gels) form. Moreover, the previously formed C–A–S–H gel takes up more silicon and aluminium ions in bridging sites (Figure 6.16e). The alkalis released into the pore solution may also react with unreacted fly ash, thereby forming more N–A–S–H gel, which may in turn react with C–S–H gel. In other words, a significant fraction of the original alkalis may be recycled, playing a prevalent role in subsequent alkaline activation reactions. However, with time, these alkalis will become a part of the structure of the reaction products. It means that with time (with the reaction progress), the alkaline concentration will decrease until the equilibrium stage in pore solution is achieved. This process can be very slow in comparison with the very fast initial gel formation reactions. Currently authors are studying these systems by using different techniques (NMR, Electron Microscopy and Pores Solution Analysis) in order to confirm our hypothesis at long term (we are working with samples older than three years). With time and under equilibrium conditions (attainable after longer reaction times), a (N)–C–A–S–H gel prevails. In these complex cementitious blends, the products formed and their proportions depend on reaction conditions, including: the chemical composition, shape, mineralogy and particle size distribution of the prime materials (fly ash reactivity rises with its vitreous content and with declining particle size), as well as the alkalin- ity (pH) generated by the activator (García-Lodiero et al., 2016).

5. CONCLUSIONS

This chapter stresses the importance of MAS NMR in characterising binder materials. The aim has been to illustrate the key aspects of this technique and its application to both calcium aluminates and the various types of silicates and aluminosilicates forming part of cementitious materials. The focus has been on the information that this technique can deliver in ascertaining the structure and composition of cementitious gels. The complexity of such gels is constantly growing due to the use of different types of precursors to develop more eco-friendly and sustainable binders.

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Papers have been published on other factors of utmost importance as well, such as: the evolution of gel structure and composition over time, the effect of curing tempe rature (Escalante-García and Sharp 1999, Bach et al., 2012, Sáez del Bosque et al., 2014) and the presence of admixtures (Justnes et al., 1990) to name a few. Studies can also be found in the literature on the effect on cement durability of the structural change in hydration products (primarily gels) induced by carbonation (Sevelsted et al., 2015), aggre gate-alkali reactions (Tambelli et al., 2006) and other interactions with the environment. MAS NMR is particularly suitable for studies on sulfate attack, which may induce the formation of retarded ettringite in OPC concretes and mortars (Aguilera et al., 2001, Blanco-Varela et al., 2006), which can be detected by 27Al MAS NMR. In some circumstances, however, thaumasite, is formed instead. In short, applied to cementitious materials, MAS NMR yields information on many factors: i) the percentage of anhydrous and hydrated phases and the presence or absence of one or the other; ii) the formation of Si-O-Al bonds, the Al ratio, mean chain length (MCL), and the percentage of tetrahedral bridging sites that are vacant, occupied by Al or occupied by Si; iii) the degree of silicate or tetrahedral aluminosilicate polymerisation (1D, 2D and 3D structures); iv) the relative abundance of tetrahedrally and octahedrally coordinated aluminium relative to oxygen atoms; and v) the hydration or otherwise of Al or nearby Si atoms, using 29Si[1H] CP/NMR. Over the last 20 years, the use of this technique has contributed very substantially to an understanding of anhydrous and hydrated cementitious materials. But more importantly, with its ever more frequent use, it should be expected to play a significant role in the design of future cements that are more environmentally sustainable and, if possible, more durable without detracting from their technological properties.

ACKNOWLEDGEMENTS

This study was funded by the Spanish Ministry of the Economy, Industry and Com- petitive and funds FEDER, under research projects BIA 2016-76466-R.

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