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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 nucle- us) 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 res- onance (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]. 06_applications.indd 203 3/6/19 14:15 [ 204 ] AUTOR 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 indisput- ably 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 hex- agonal 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 gyp- sum. 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 ce- ment, 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 06_applications.indd 204 3/6/19 14:15 CEMENTITIOUS MATERIALS [ 205 ] 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 prima- ry 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 conse- quently 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, anhy- drite...) 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 hy- droxide (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 hydra- tion. 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 con- stituents 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 06_applications.indd 205 3/6/19 14:15 [ 206 ] AUTOR 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 nano- structural 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.
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