Received: 18 November 2019 | Revised: 22 January 2020 | Accepted: 25 January 2020 DOI: 10.1111/jace.17046

ORIGINAL ARTICLE

Influence of water activity on hydration of tricalcium aluminate-calcium sulfate systems

Jonathan Lapeyre1 | Hongyan Ma2 | Monday Okoronkwo3 | Gaurav Sant4 | Aditya Kumar1

1Department of Materials Science and Engineering, Missouri University of Abstract Science and Technology, Rolla, MO, USA The hydration of tricalcium silicate (C3S)—the major phase in cement—is effec- 2 Department of Civil, Architectural and tively arrested when the activity of water (aH) decreases below the critical value of Environmental Engineering, Missouri 0.70. While it is implicitly understood that the reduction in aH suppresses the hydra- University of Science and Technology, Rolla, MO, USA tion of tricalcium aluminate (C3A: the most reactive phase in cement), the depend- 3 Department of Chemical and Biochemical ence of kinetics of C3A hydration on aH and the critical aH at which hydration of Engineering, Missouri University of C3A is arrested are not known. This study employs isothermal microcalorimetry and Science and Technology, Rolla, MO, USA complementary material characterization techniques to elucidate the influence of aH 4Department of Civil and Environmental Engineering, Department of Materials on the hydration of C3A in [C3A + calcium sulfate (C$) + water] pastes. Reductions Science and Engineering, California in water activity are achieved by partially replacing the water in the pastes with iso- NanoSystems Institute, Institute for Carbon propanol. The results show that with decreasing aH, the kinetics of all reactions as- Management, University of California Los Angeles, Los Angeles, CA, USA sociated with C3A (eg, with C$, resulting in ettringite formation; and with ettringite, resulting in monosulfoaluminate formation) are proportionately suppressed. When Correspondence a ≤0.45, the hydration of C A and the precipitation of all resultant hydrates are Aditya Kumar, Department of Materials H 3 Science and Engineering, Missouri arrested; even in liquid saturated systems. In addition to—and separate from—the University of Science and Technology, experiments, a thermodynamic analysis also indicates that the hydration of C3A does B49 McNutt Hall, 1400 N. Bishop, Rolla, a a MO 65409-0340, USA. not commence or advance when H ≤0.45. On the basis of this critical H, the solu- −20.65 Email: [email protected] bility product of C3A (KC3A) was estimated as 10 . The outcomes of this work

articulate the dependency of C3A hydration and its kinetics on water activity, and Funding information a Division of Civil, Mechanical and establish—for the first time—significant thermodynamic parameters (ie, critical H Manufacturing Innovation, Grant/Award and KC3A) that are prerequisites for numerical modeling of C3A hydration. Number: 1661609 and 1932690 KEYWORDS calorimetry, hydration, thermodynamics, tricalcium aluminate, water activity

1 | INTRODUCTION humidity (RH) of concrete—for example, via drying-induced removal of liquid water—suppresses the kinetics of cement Conventional concrete is a mixture of ordinary portland ce- hydration, and, consequentially, slows down all processes ment (OPC; subsequently referred to as cement), water, sand, linked with cement hydration (eg, development of proper- and large aggregates. Herein, the reaction between cement ties). Powers1 and, later, Spears2 suggested that below a criti- and water (ie, hydration) results in precipitation of hydration cal RH of 80%, the hydration of cement is arrested. Flatt et al3 products, evolution of the microstructure, and development showed that the aforesaid critical value of RH (ie, 80%) was of properties (eg, strength). Reduction in internal relative applicable to tricalcium silicate (Ca3SiO5; written as C3S in

J Am Ceram Soc. 2020;00:1–20. wileyonlinelibrary.com/journal/jace © 2020 The American Ceramic Society | 1 2 | LAPEYRE et al. * 4 cement chemistry notation ), the major phase in cement. hydration, wherein αC3A ≈1.0) for samples cured in saturated 3 Flatt et al estimated that when the internal RH of C3S paste conditions (ie, RH ≈ 100%) to ≈60% (ie, αC3A ≈ 0.60) when [ie, C3S + water (H)] decreases below 80%, the chemical po- the curing is conducted at RH of 49% or below. Although tential of water is significantly reduced. This is due to the the authors did not quantify a critical RH for C3A hydration, development of immense negative capillary stress (ie, their results clearly showed that as RH decreases—and, pre- ≈−30 MPa, as calculated from the Laplace-Young equation) sumably, water is gradually removed from the paste's micro- within the pore-network of the microstructure, and emptying structure, thus resulting in decline of aH—the hydration of of all pores with diameters larger than ≈5.4 nm. Interestingly, C3A is progressively suppressed. These inferences of Patel even at such low RH, a thin layer (thickness of approximately et al,6 were corroborated by Jensen,7 who showed that, in 0.6 nm) of liquid water remains adsorbed on the pores’ walls. [C3A + C$H2 + H] pastes, the hydration of C3A did not However, as the chemical potential of the liquid layer is progress to a significant degree (ie, αC3A < 0.02) when it was equivalent to that of C3S, the hydration of C3S—in spite of exposed to moisture maintained at RH of ≤43% for an entire being in contact with liquid water—is infeasible, thereby en- year. However, as the RH was increased to 66% and 75%, forcing equilibrium of C3S with respect to the contiguous liq- αC3A increased to 0.08 and 0.28, respectively, after a year. uid water layer as well as the hydration products (ie, CH: On the bases of these results, Jensen7 estimated that RH of portlandite; and C–S–H: calcium–silicate–hydrate). 60% was the limiting value for cessation of C3A hydration. In the work of Flatt et al3—which was broadly analyti- In another study conducted by Dubina et al,16 the critical RH cal, rather than experimental—chemical potential of water for C3A hydration was estimated as 55%—which is in good was manipulated via vapor pressure (ie, ambient/internal agreement with the value reported by Jensen.7 RH) reductions; that is, a vapor-phase route. Nonetheless, Although the studies cited in the preceding paragraph do changes in water's chemical potential can also be induced show that the hydration of C3A is limited, or entirely arrested, via the liquid-phase route. For example, Oey et al 5 altered at RH 60%,6,7 none of them describe the influence of RHor the chemical potential of water—specifically, the activity of aH, which, like RH, is linked to the chemical potential of water (aH)—by partially or fully replacing it with isopropa- water—on the temporal rate of C3A hydration (in the pres- nol (IPA). The authors reported that—much like RH—with ence of a soluble sulfate compound). Importantly, prior stud- decreasing aH, C3S hydration is suppressed, and, conse- ies do not shed any light on the influence of aH on hydration quently, the rates of growth of the hydrates (ie, C–S–H and of C3A in water-abundant systems, wherein C3A particulates CH) decline progressively. As aH regresses to a value of have unremitting access to water and capillary stress develop- 0.70—which is achieved when 63%mass of water is replaced ment is effectively avoided. Such knowledge, of dependency with IPA—equilibrium is achieved between C3S, water, and of C3A hydration on aH, is a prerequisite for: quantification the hydration products. Even in water-abundant suspensions of rate controls on C3A (and, therefore, cement) hydration; (ie, [C3S + H] mixtures prepared at high liquid-to-solid determination of thermodynamic parameters relevant to C3A mass ratios), below the critical aH of 0.70, the hydration of hydration (eg, solubility constant of C3A, which is needed to 17‒22 C3S, despite its unhindered access to liquid water, is effec- develop and/or validate cement hydration models ); and tually arrested. accurate estimation of critical aH at which hydration of C3A Several other studies6‒9 have reported that below the is arrested. The last point is expressly significant, because, critical RH of 80%, hydration of cement or of C3S neither in practice, to arrest the hydration of cement (for example, initiates nor advances. Conversely, for the other anhydrous to characterize the microstructure at a given degree of hy- phases present in cement, the dependency of their hydration dration), cementitious specimens are typically immersed in 23,24 kinetics on RH (or aH) and the critical RH (or aH) at which [water + IPA] mixtures for prolonged periods (NB, com- their hydrations are arrested are not well established. Among mercially available IPA is generally impure, containing finite these phases, perhaps the most important is tricalcium alumi- amount of water). Proper knowledge of critical aH for C3A nate (Ca3Al2O6; or C3A, as per cement chemistry notation), hydration would enable the estimation of appropriate amount 4,10,11 the most reactive phase in cement . In typical cements of impure IPA that would ensure that hydration of C3A is no (ie, OPC), a highly-soluble sulfate compound—for example, longer feasible. Knowledge of critical aH for C3A hydration calcium sulfate (CaSO4; or C$, as per cement chemistry nota- would also enable the selection of appropriate curing con- tion) or gypsum (CaSO4. 2H2O; or C$H2)—is added to avert ditions (eg, ambient RH, that is high enough to ensure that 4,12‒15 6 flash-setting. In a classic study, Patel et al showed that, aH is above the critical value at all times) of cementitious 25,26 in cement pastes, the ultimate degree of hydration of C3A systems—including calcium sulfoaluminate systems — (αC3A; Unitless) decreases from ≈100% (ie, near-complete that would warrant faster (or slower) kinetics of reactions, and, therefore, swift (or gradual) development of physical * As per cement chemistry notation: C = CaO; S = SiO2; A = Al2O3; properties. Lastly, such knowledge is also expected to aid H = H2O; and $ = SO3. in development of strategies to mitigate delayed ettringite LAPEYRE et al. | 3 formation,27 a degradation phenomenon that negatively af- oxide-trisulfate (AFt) phase] precipitate in a nucleation fects the durability and service life of concrete infrastructure. burst; the crystals subsequently grow into the capillary pore To address the abovementioned knowledge-gaps, this space.12,14,15 The chemical reaction for ettringite precipita- study employs a combination of experimental techniques (eg, tion is shown in Equation 2. While the change in enthalpy isothermal microcalorimetry; X-ray diffraction; and thermo- (ΔH) of this reaction, as reported in literature, has varied gravimetric analyses) and thermodynamic calculations to from −452 kJ per mole C3A to −600 kJ per mole C3A, the elucidate the effect of water activity on the hydration of C3A value used in this study is −522.04 kJ per mole of C3A; this 4,33‒35 in [C3A + C$ + H] pastes. All pastes were prepared at high value was drawn from Cemdata18 database. liquid-to-solid mass ratios (l/s ≥ 2.0) such that majority of → the pores within the pastes remain saturated with liquid at all C3A+3C$ +32H C6A$ 3H32 (2) times; this is important because partial desaturation of the pores could result in development of large capillary stresses. The rate of ettringite precipitation (and, thus, the kinetics To regulate the activity of water in the mixing solution, IPA of C3A hydration and rate of heat evolution during stage I) is was used to replace 0-to-100%mass of the water, hence en- rapid for a short period (following the initial nucleation burst), compassing a wide range of water activities (1.0 ≥ initial aH and thenceforth very slow, proceeding at a near-constant rate 12,34,36 (aH0) ≥ 0.0). IPA—as opposed to other organic alcohols such for several hours until C$ is the system is exhausted. as ethanol or methanol—was chosen to achieve desired water As such, the duration over which ettringite continues to form, activities because of its: miscibility with water; inertness (ie, increases as C$ content in the paste increases. The underlying inability to react with C3A or C$ or any of the hydration prod- mechanisms that explain the highly nonlinear and nonmono- ucts in the paste); and small molecular size, which permits its tonic variations in the hydration kinetics during the precipi- access to large as well as very small pores within the micro- tation of ettringite (Equation 2) have long been debated by structure.5,23,28‒32 Overall, outcomes of this work clearly ex- researchers. Some studies have postulated that the kinetics of press the dependency of the C3A hydration kinetics on water ettringite precipitation is dictated exclusively by morphology activity, and establish, for the first time, significant thermo- and size of C3A particulates, whereas others have suggested dynamic parameters relevant to hydration of C3A—that is, that the low, near-constant rate of C3A hydration (after nu- solubility constant of C3A and critical water activity below cleation of ettringite crystals) is due to the formation of a 12,34 which hydration of C3A is arrested. barrier-layer that prevents C3A from dissolving rapidly. The aforesaid barrier-layer hypothesis, however, appears im- plausible in light of recent findings. Notably, X-ray synchro- 37 2 REVIEW OF HYDRATION OF tron analysis and other examinations of C3A particulates' | 38,39 C 3A IN [C3A + C$ + H] PASTES surfaces have shown that a metastable calcium aluminate hydrate phase does form on C3A surface; however, this phase The section presents a brief overview of the three distinct does not appear to inhibit dissolution of C3A or the transport stages of C3A hydration in [C3A + C$ + H] pastes. The re- of ions from C3A surface to the contiguous solution. In other actions pertaining to each stage, and mechanisms associated studies,13,34,37‒40 a surface passivation mechanism has been with the reactions, are described for pastes in which the mix- theorized to be at the origin of retardation of C3A hydration ing solution is pure water (ie, aH at the time of mixing is during the period when ettringite crystals grow at a slow rate. 1.00). The influence of aH on the various reactions are dis- Though, there is still no consensus on which chemical species cussed toward the end of this section. or surface complex is responsible for passivation of the C3A surface. Some studies have argued that the passivation layer 2− 12,34 comprises of SO4 ions ; although, results contradicting 2.1 | Stage I this argument have been presented in a study conducted by Collepardi et al.41 Other studies 37‒39, have reported that Ca– After initial contact with water and wetting of the surface, Al–SO3 complexes, that preferentially adsorb onto the Al- C3A begins to dissolve; the consequent release of ions is de- rich C3A surface, are responsible for passivating it. scribed by Equation 1.

Ca Al O +6H O → 3Ca2+ +2Al (OH)− +4OH− 3 2 6 2 4 (1) 2.2 | Stage II

Akin to C3A, dissolution of C$ commences shortly after After complete depletion of C$ in the paste, hydration of C3A 2+ 2− contact with water, resulting in the release Ca and SO4 (if still left) is invigorated. In an isothermal calorimetry profile into the contacting solution. Within a few minutes of dis- (ie, heat evolution profile), this is observed as rapid increase solution, crystals of ettringite [C6A$3H32; an alumina-ferric in heat flow rate up to an intense exothermic peak, followed 4 | LAPEYRE et al. by a sharp decline, and then an exponentially decaying shoul- rate of C3A decreases as the aH in the contacting solution 12,34,36,42 der that lasts for several hours. Depending on the C3A decreases (N. B., aH was controlled by partially replacing and C$ particulate sizes, and their contents, characteristics of water with ethanol). Therefore, in [C3A + C$ + H] paste, this peak (ie, approach to, departure from, and intensity of the reduction in aH is expected to decelerate C3A dissolution ki- peak) are subject to change.12,33,43 During this stage (ie, stage netics (Equation 1), and, consequentially, retard the release 2+ − ¯ II), ettringite reacts with C3A and water to form monosul- of Ca , OH , and Al(OH)4 ions into the contacting solu- foaluminate [C4A$H12; an alumina-ferric oxide-monosulfate tion. Slower release of ionic species is expected to suppress (AFm) hydrate phase as described in Equation 3]; ΔH of the the kinetics of all succeeding reactions: the precipitation of 4,33‒35 reaction is −237.64 kJ per mole C3A. ettringite (Equation 2), monosulfoaluminate (Equation 3), and hydrogarnet (Equation 4). Although the influence of a → H C3A+0.5C6A$ 3H32 +2H 1.5C4A$ H12 (3) on the kinetics of these reactions has not been established yet, the effect of RH reductions on the stability of ettringite 45,51‒56 The kinetics of C3A hydration during stage II—albeit, and monosulfoaluminate has been extensively studied. not fully understood—is assumed to be dictated by the nu- Ettringite (C6A$3H32) loses a maximum of two moles of 12,34,36,44 cleation-and-growth of monosulfoaluminate. As water (resulting in stabilization of C6A$3H30) as RH is re- with other nucleation-and-growth processes (eg, precipita- duced from 100% to 10%; this loss of loosely bound water tion of ice from liquid water), the rate of C3A hydration does not induce any perceptible distortion of ettringite's crys- varies in a nonlinear and non-monotonic manner with re- talline structure. Below 10% RH, however, ettringite begins spect to time. to destabilize, and decompose into calcium sulfates and cal- cium aluminate hydrates.51‒53 Owing to its stability across a wide range of RHs, in this study, the stoichiometry of ettring- 2.3 | Stage III ite is assumed to remain constant at C6A$3H32, irrespective of the mixture design of the paste or the aH at the time of After ettringite in the paste is exhausted, the anhydrous C3A mixing. Unlike ettringite, the stability of monosulfoalumi- (if still left) reacts directly with water to form a solid-solu- nate (C4A$H12) is strongly dependent on RH. As RH reduces tion45‒47 of metastable calcium aluminate hydrate phases (eg, from 100% to lower values, the moles of water per mole of C2AH8 and C4AH13) and poorly-crystalline aluminum hy- the hydrate change drastically (ie, 16, 14, 12, 10.5, or 9 in 48 droxide (AH3). Over time, these metastable hydrates trans- relation to decreasing RH), thus inducing substantial changes 49 55,56 form into a singular stable phase—hydrogarnet (C3AH6). in its crystalline structure. In all [C3A + C$ + H] systems As the formation of hydrogarnet is predicated upon the ex- described in this study, C4A$H12 is assumed to be predom- haustion of ettringite (unless C3A is exhausted before com- inant AFm phase that forms during stage II, since it is the plete depletion of ettringite), in sulfate-rich [C3A + C$ + H] most stable one (among other forms of monosulfoaluminate) pastes, the formation of hydrogarnet is wholly negated. at RHs between 97%-and-23% (at room temperature).55,56 Contrastingly, in sulfate-deficient [C3A + C$ + H] pastes, hydrogarnet does form; its formation, however, begins tens- to-hundreds of hours after mixing.12,34 Such delayed forma- 3 | CALCULATION OF WATER tion of hydrogarnet is attributed to two reasons: (a) the slow, ACTIVITY IN [C3A + C$ + H] exponential decay of the rate of monosulfoaluminate precipi- SYSTEMS tation (and ettringite consumption) during stage II; and (b) slow kinetics of transformation of metastable calcium alu- To study the effect of water activity (aH) on hydration of minate hydrates to hydrogarnet. For the sake of simplicity, C3A, this study emphasizes achieving reductions in aH via in this study, the formation of metastable hydrates and their the liquid-phase route—that is, by partially replacing water, subsequent transformation to hydrogarnet during stage III in the mixing solution, with IPA aH represents the net differ- have been combined into a single reaction with C3AH6 as the ential in the chemical potential of liquid water as it transitions singular hydration product (Equation 4); ΔH of this reaction from a pure state (ie, when solute concentration in the water 4,33‒35 is −261.02 kJ per mole C3A. is zero) to that of a solution (ie, when a finite amount of sol- ute is dissolved in the water). The mathematical formalisms C A+6H → C AH 3 3 6 (4) used to calculate aH in [water + IPA] mixtures are identical to those implemented in the study of Oey et al.5 Therefore, Readers are reminded that reactions described in Equations to avoid replication of information, descriptions of the prin- 1-4 pertain to [C3A + C$ + H] pastes, in which the initial cipal equations used for estimation of aH are kept succinct aH = 1.00. In a prior study conducted on [C3A + H] suspen- in this section; for further details, readers are requested to sions, Brand and Bullard50 demonstrated that the dissolution consult references3,5 and the Supplemental Material section. LAPEYRE et al. | 5 Likewise, the calculations accounting for the extraneous fac- tors not accounted for in the van Laar equations (ie, effects of ionic strength of the pore solution; and gradual consump- tion of water during hydration on aH) are discussed in the Supplemental Material. In this study, the van Laar equations (Equations 5-6) were used to calculate aH and aIPA as the IPA replacement increases in the mixing solutions.5,57,58

B a = X exp H H 2 (5) ⎡ 1+ BXH ⎤ ⎢ AXIPA ⎥ ⎢� � ⎥ ⎢ ⎥ ⎣ ⎦ A a = X exp IPA IPA 2 (6) ⎡ 1+ AXIPA ⎤ ⎢ BXH ⎥ ⎢� � ⎥ ⎢ ⎥ ⎣ ⎦ The variables utilized the van Laar equations are the mole fraction of water and of IPA (XH and XIPA, respectively; both unitless) in the mixing solution as well as the unitless co- efficients A and B. These were previously determined to be 1.000 and 0.483 through regression analysis of liquid-vapor FIGURE 1 Activities (a) of water (H) and isopropanol (IPA) equilibrium data, at standard temperature and pressure condi- in [water + IPA] mixtures, as calculated from the van Laar equations tions, by Wilson and Simmons.57 As can be seen in Figure 1, (Equations 5-6). Dashed lines represent ideal behavior, as described a = X a X aH monotonically reduces with increasing IPA replacements; by Raoult's Law (ie, H H and IPA = IPA). All calculations are deterministic; as such, there is no uncertainty associated with them sharp reductions in aH occur when the IPA content in the mixture exceeds 35%.

shown in Equations 2, 3, and 4, respectively. Hydration of C3A 4 | ESTIMATION OF in such pastes terminates if one or both of the following condi- CRITICAL WATER ACTIVITY tions are met: (a) C3A is exhausted; or (b) equilibrium between AND SOLUBILITY CONSTANT water and all solidus phases (ie, C3A, C$, and whichever hydra- BASED ON THERMODYNAMIC tion products that are present in the paste at the point in time) is CONSIDERATIONS reached. When equilibrium is indeed reached, the implication is that the Gibbs free energy of reactions (ΔGRxn), associated 3 5 In the works of Flatt et al and Oey et al, a common with ettringite precipitation (ie, C3A-AFt; Equation 2), mono- thermodynamic approach was used to estimate important sulfoaluminate precipitation (ie, C3A-AFm; Equation 3), and thermodynamic constants associated with C3S hydration: hydrogarnet precipitation (ie, C3A–C3AH6; Equation 4), are solubility constant of C–S–H; and equilibrium constant of all 0 at the same time. In this study, for calculation of thermo- C3S hydration (ie, C3S–C–S–H reaction). In both studies, dynamic parameters (ie, critical aH and KC3A), the thermody- the critical chemical potential of water—estimated on the namic approach only considers the equilibrium of C3A-AFt and 3 5 bases of critical RH or critical aH, below which hydra- C3A-AFm reactions; the C3A–C3AH6 reaction is disregarded. tion of C3S is arrested—was used as the primary input. In This is for two reasons: (a) for calculation of thermodynamic this study, a modification of the aforementioned thermo- parameters, two reactions (ie, Equations 2-3) involving C3A dynamic approach is used to estimate two significant ther- are sufficient, hence, making the inclusion of the third reaction modynamic parameters associated with the hydration of (ie, Equation 4) redundant; and (b) assessing the conditions of C3A in [C3A + C$ + H] systems: critical aH below which equilibrium of C3A–C3AH6 reaction is difficult, because the hydration of C3A is arrested, and solubility constant of C3A precipitation of C3AH6 does not occur directly, but rather indi- (KC3A). rectly through two separate stages—precipitation of metastable In [C3A + C$ + H] pastes, the hydration of C3A advances se- hydrates (eg, C2AH8 and C4AH13) followed by transformation 45‒47 quentially through stages I, II, and III, as delineated by reactions of the metastable hydrates to C3AH6. 6 | LAPEYRE et al. 3,5 Using the same approach as that described in Ref. , the constants of the reactants (ie, C3A and C$) and the hydration equilibrium constants (Krxn; Unitless) for both C3A-AFt and product (ie, the AFt phase: ettringite). This is shown in Equation C3A-AFm reactions, at pressure (p) = 1 bar and temperature 8. Likewise, if hydration of C3A is terminated during stage II, (T) = 25℃, can be estimated by solving Equation 7. and it reaches a state of equilibrium with water and the hydra- tion products (ie, AFt; and AFm phase: monosulfoaluminate), K VH n V the equilibrium constant of C3A-AFm reaction ( C3A–AFm) can ln RH = ln K + H H ln a K V rxn V H (7) be estimated using Equation 9. Δ Δ +1    3 KC AKC$ K = 3 (8) C3A-AFt In Equation 7: RHK is the ambient relative humidity that KAFt would correspond to chemical equilibrium of water in the vapor phase with pure liquid water in a partially saturated pore; ΔV 0.5 3 −1 KC AK (cm . molC3A ) is the chemical shrinkage associated with the 3 AFt KC A-AFm= (9) reaction; n (mole) is the number of moles of water per mole 3 1.5 H KAFm of C3A required for completion of the chemical reaction; and V 3 −1 H (cm . molwater ) is the molar volume of water. It should be noted that Equation 7 can be concurrently applied to both C3A- By solving Equations 7-9 simultaneously, the equi- AFt and C3A-AFm reactions, as long as both reactions have librium constants for the aforementioned pair of reac- reached equilibrium, RHK ≤ 100%, and 0.0 ≤ aH ≤1.0. It is also tions (ie, KC3A–AFt and KC3A–AFm), the solubility constant worth pointing out that Equation 7 considers the combined ef- of C3A (KC3A), and the critical aH at which hydration of fects of liquid-vapor meniscus curvature and the suppression of C3A is arrested can be estimated. First, the equilibrium aH (due to the dissolution of solute, for example IPA, in water) constant of the C3A-AFt reaction (KC3A–AFt) as a function on the state of equilibrium of the two reactions. Therefore, Eq. 7 of aH (0.0 ≤ aH ≤ 1.0) can be calculated from Equation can be applied—as is—to [C3A + C$ + H] pastes, wherein pure 7, while assuming near-liquid saturation conditions (ie, water is used as the mixing solution, or even in cases wherein RHK ≈ 100%). Second, and once again assuming near-liq- water is partially replaced with IPA. uid saturation conditions, the equilibrium constant of the Using the molar masses and densities (ρ) of rele- C3A-AFm reaction (KC3A–AFm) as a function of aH can be vant phases—as reported in the prior work of Balonis and calculated from Equation 7. Third, using Eq. 8 in conjunc- 59 Glasser —the phases' molar volumes were estimated (see tion with the functional relationship between KC3A–AFt and Table 1). On the bases of these molar volumes and stoichiom- aH, as derived from the first step, the solubility constant of etries of the chemical reactions shown in Equations 2-3, the C3A (KC3A) can be plotted as a function of aH. And, lastly, chemical shrinkage (ΔV) associated with C3A-AFt and C3A- using Eq. 9 in tandem with the KC3A–AFm vs. aH relationship, 3 −1 AFm reactions were estimated as −97.824 cm . molC3A and as derived from the second step, the solubility constant of 3 −1 −14.744 cm . molC3A respectively. C3A (KC3A) can once again be plotted as a function of aH. In Equation 7, the Krxn term is a shorthand for equilibrium For the penultimate and last steps, the solubility constants constant for each of the two reactions: C3A-AFt (KC3A–AFt) and of relevant phases must be known. Toward this, KC$, KAFt, −4.357 −44.9 −29.23 C3A-AFm reactions (KC3A–AFm), represented by Equations 2 and KAFm can be assumed as 10 , 10 , and 10 , and 3, respectively. In a [C3A + C$ + H] paste, if C3A stops respectively; these values correspond to P = 1 bar and hydrating during stage I—thereby, reaching equilibrium with T = 25℃, and were drawn from the Cemdata18 database.35 C$, water, and ettringite—the equilibrium constant of C3A-AFt By following the steps described above, KC3A was ob- reaction (KC3A–AFt) can be estimated as function of solubility tained as a function of aH (Figure 2). As can be seen, two different curves are obtained: one that is derived from the TABLE 1 Densities (ρ) and molar volumes (V) at standard equilibrium of the C3A-AFt reaction and other derived conditions for the cementitious phases utilized in equilibrium from the C3A-AFm reaction. The two curves intersect at calculations. The densities reported herein were drawn from Ref. 59 a unique value of aH, that is, at aH = 0.45. This intersec- Density Molar Volume tion point is significant because it represents the exclusive -3 3 -1 Cementitious Phases (g cm ) (cm mol ) value of aH at which both C3A-AFt and C3A-AFt reactions reach equilibrium. In other words, at the critical a of 0.45, C3A 3.030 89.17 H C$ 2.958 46.02 C3A reaches a state of equilibrium with all other constit- uents of the system—that is, water, C$, ettringite, and H 1.000 18.02 monosulfoaluminate. On account of establishment of equi- C A$ H 1.778 705.9 6 3 32 librium between all solidus phases and water, it can, thus, C4A$H12 2.015 308.9 be said that hydration of C3A is effectually arrested when LAPEYRE et al. | 7

FIGURE 2 Plots of the solubility constant of C3A (KC3A) as function of water activity (aH), as derived from the equilibrium of C3A → AFt and C3A → AFm reactions. The unique point of intersection between the two curves is highlighted by means of the dashed lines. The point of -20.65 intersection corresponds to aH = 0.45, that is, when IPA content in the [water + IPA] mixture is 87.1%mass. At aH = 0.45, KC3A = 10 . All calculations are deterministic; as such, there is no uncertainty associated with them

aH reaches the critical value of 0.45. It should be noted that the formation of hydrates was forbidden, thereby allow- in a [water + IPA] mixture, aH of 0.45 corresponds to IPA ing C3A to dissolve until it reached a state of equilibrium replacement level of 87.1% (see Figures 1 and 2), thereby with the ionic species released from its dissolution (ie, 2+ − − implying that the same or higher percentage of water ought Ca , Al(OH)4 , and OH ). At equilibrium, the activities to be replaced in a [C3A + C$ + H] paste to ensure that the of the ionic species (denoted as a) were calculated using 62 hydration of C3A is ceased. Importantly, at the critical aH the extended Debye-Hückel equations (embedded within (=0.45), the solubility constant of C3A (KC3A)—as derived GEMS), and then inserted in Eq. 10 to calculate KC3A (see from equilibrium of either of two reactions (ie, C3A-AFt Table 2). −20.65 and C3A-AFm)—is 10 . By inserting this value of KC3A into Equations 8-9, equilibrium constants of the C3A-AFt As can be seen in Table 2, the values of KC3A determined reaction (KC3A–AFt) and the C3A-AFm reaction (KC3A–AFm) from the GEMS simulations are within two orders of mag- 11.180 0.742 2 were estimated as 10 and 10 , respectively. a 3 a 4 a KC A = Ca2+ OH− Al(OH)− (10) Given the sheer number of equations, parameters, and 3 4 their interrelations that have been described thus far in this     section, is worth reiterating the two significant thermody- nitude of 10−20.65, the value estimated from thermodynamic namic parameters that were estimated on the basis of ther- considerations described earlier in this section. It is acknowl- modynamic considerations of C3A hydration in near-liquid edged that, within the GEMS software, assumptions have saturation conditions: critical aH at which hydration of been made to simplify the simulations (eg, precipitation of C3A is arrested = 0.45; and the solubility constant of C3A hydrates is disallowed); therefore, it is possible that the dis- −20.65 (KC3A) = 10 . These are the first estimates of either solution reaction of C3A may not have reached equilibrium, parameter; hence, their corroboration from literature is not especially at higher l/s. This is likely the reason why the feasible. Notwithstanding, in section 6.0, results obtained GEMS-derived values of KC3A are altered with increasing l/s from a series of experiments are analyzed to verify the (as opposed to remaining unchanged), and different (albeit, critical aH of C3A hydration in [C3A + C$ + H] pastes. by less than 2 orders of magnitude) from the value derived And, to corroborate the aforesaid value of KC3A, additional simulations in GEMS, the thermodynamic modeling soft- TABLE 2 The solubility constant of C3A, calculated via GEMS, 35,60,61 l/s ware package, were implemented. Specifically, in in relation to varying these simulations, a single gram of C3A was allowed to Simulated l/s KC3A log (KC3A) dissolved in water, under extremely dilute conditions (ie, −20 1000 9.04 × 10 −19.043 water-to-C3A mass ratio (l/s) = 1000-to-3000) and exposed 2000 4.71 × 10−22 −21.327 to an inert (nitrogen) atmosphere maintained at 1 bar of −23 pressure and temperature of 25℃. In these suspensions, 3000 1.95 × 10 −22.709 8 | LAPEYRE et al.

powders (ie, C3A and C$) at two distinct mass proportions: [85% C3A + 15% C$] and [70% C3A + 30% C$]. The mixing solutions were comprised of mixtures of water and reagent-grade (99.9% pure) isopropanol (IPA; sourced from Fisher Chemical). IPA was used to replace 0-to-100%mass of water in the mixture, to encom- pass a wide range of water activities (0.0 ≤ aH ≤ 1.0). These solutions were added to the powder mixtures at a fixed l/s of 2. A fairly high value of l/s was chosen for this study to ensure that complete hydration of C3A could be achieved in majority of the pastes (with the exception of the few prepared at IPA replace- ment levels of ≥80%; see Supplemental Material). Although IPA and water are completely miscible, the [water + IPA] mixtures were shaken to ensure homogeneity before introducing them to the anhydrous powder mixtures, followed by hand-mixing of the paste for 1 minute. Hydration kinetics of C3A in each paste was monitored for 72 hours using a TAM IV (TA Instruments) isothermal conduction microcalorimeter, programmed to main- tain a constant sample temperature of 20°C (±0.1°C). In each calorimetry experiment, the logging of calorimetric data (ie, FIGURE 3 Particle size distributions (PSDs) of C3A and C$ powders used in this study. Uncertainty in the median particle diameter heat flow rate and heat release) was delayed by 15 minutes to ensure that the signal was stable. Calorimetry profiles of the (d50) of each PSD is within ±6% pastes, as obtained from isothermal microcalorimetry, were pro- −1 cessed—using enthalpies of −522.04 kJ moleC3A for stage I; −1 1 −237.64 kJ moleC3A for stage II; and −261.02 kJ moleC3A- for from the above-described thermodynamic approach. In prior stage III of C3A hydration (see section 2.0)—to estimate time-de- 3,63 studies, the dissolution rate of C3S, at very low undersatu- pendent evolutions of degree of reaction (αC3A) and the rate of −1 ration, was measured and subsequently processed to estimate reaction (dα/dt; units of h ) of C3A. its solubility constant. It is expected that similar experimental X-ray diffraction (XRD) and thermogravimetric analysis protocols could be applied to obtain more accurate estimates (TGA) experiments were carried out to identify anhydrous of the solubility constant of C3A. phases and hydration products in [C3A + C$ + H] pastes after 72 hours of hydration. For such investigations, hydration of C3A in the paste was arrested by submerging the crushed (or 5 | MATERIALS AND METHODS powdered) paste in “fresh” IPA for 12 hours or longer (in which case, IPA was periodically replenished). Prior to testing, the Both the C3A (cubic polymorph) and C$ powders were sourced samples were oven-dried at 65°C for 4 hours. XRD was con- from commercial suppliers. Quantitative X-ray diffraction ducted in continuous scan mode using a Phillips Panalytical (QXRD) revealed that the C3A (sourced from Kunshan Chinese X`pert MPD diffractometer; the diffractometer uses CuKα ra- Technology New Materials Co., Ltd) is >98% pure, with <2% diation (λ = 1.5418 Å) with fixed divergence and anti-scatter of free lime (CaO). Likewise, the AR-grade anhydrous C$ slit-sizes of 0.5° and 0.25°, respectively. The X-ray tube was (sourced from Alfa Aesar) was found to be >99% pure, with operated at a voltage of 45 kV and current of 40 mA. The pow- trace amounts of impurities. The particle size distributions dered sample was placed within the sample holder, and X-ray (PSDs) of C3A and C$ powders, as obtained from static light diffractograms were obtained for the scanning range of 4.995°2θ scattering (Microtrac S3500 particle size analyzer), are pre- -to-89.989°2θ, with a step size of 0.026°2θ. For the TGA ex- sented in Figure 3. Prior to PSD measurements, each powder periments, the dried powders were heated at 10°C min−1 from specimen was suspended in IPA, and subsequently agitated at room temperature to 1000°C in Al2O3 crucible, with inert gas ultrasonic frequencies for 4 minutes to alleviate the effects of flowing over the specimen at a flow rate of 100 mL min−1. particulate agglomeration that may have occurred during stor- age of the materials. The median particle sizes (d50, µm) of the PSDs of C3A and C$ powders are 6.51 µm and 9.99 µm, re- 6 | EXPERIMENTAL RESULTS spectively; their specific surface areas (SSA, cm2 g−1) were esti- AND DISCUSSION mated to be 3554.21 cm2 g−1 and 5130.81 cm2 g−1, respectively. To monitor the rate and extent of C3A hydration, Figure 4 presents heat evolution profiles of [C3A + 15% [C3A + C$ + H] pastes were prepared by mixing the two anhydrous C$ + H] pastes, prepared by replacing 0%-to-85% of water LAPEYRE et al. | 9

FIGURE 4 Isothermal microcalorimetry based determinations of heat evolution profiles of [C3A + 15% C$ + H] pastes, prepared at different levels of replacement of water with IPA. (A) shows the heat flow rates in linear scale; (B) shows the heat flow rates in semi-log scale; and (C) shows the cumulative heat releases in linear scale. l/s = 2 in all pastes. For a given system, uncertainty in the peak heat flow rate (ie, the intensity of the stage II peak)—as determined from 3 repetitions—is within ±2% with IPA. Readers are reminded that in pastes prepared with At high IPA replacement levels (ie, ≥85%), the delay in oc- 0% and 85% of water replaced with IPA, the initial water activi- currence of the stage II peak is substantial, so much so that ties (aH0) are 1.00 and 0.50, respectively (see section 3.0). To it does not even appear within the 72 hours of hydration. better understand the influence of water activity reductions on All things considered, these results suggest that reductions hydration kinetics of C3A in such pastes, it is important to de- in aH0 result in proportionate suppression of C3A hydration, scribe the various stages of C3A hydration that are revealed in and retardation of all processes (ie, precipitation of hydration the heat flow rate profile of the control paste (IPA replacement products and consumption of C$) that are derived from hy- level = 0%). As described earlier in section 2.0, during stage I, dration of C3A through the different stages. In a prior study, 5 crystals of ettringite precipitate during a nucleation burst, which Oey et al reported that in [C3S + H] pastes, reductions in occurs shortly after mixing; the crystals then grow slowly, at a aH0 (achieved via IPA addition) result in sustained inhibition near-constant rate, until C$ in the system is depleted.12,34,42 As of growth of the main hydrate (ie, C–S–H) as soon as it nucle- such, in the control paste, the heat flow rate rapidly increases ates. Likewise, it is hypothesized that in [C3A + 15% C$ + H] during stage I and then declines swiftly to stable low value pastes, IPA-induced reductions in aH0 result in suppression (Figure 4A,B). The onset of stage II is marked by rapid increase of post-nucleation growth of ettringite, monosulfoaluminate, in the heat flow rate up to an intense exothermic peak, followed and hydrogarnet (and/or metastable hydrates) during stages by a sharp decline, and then an exponentially decaying shoul- I, II, and III of C3A hydration, respectively. This hypothesis der that lasts for several hours. During this period, C3A reacts will be revisited later in this section, where its veracity will with water and ettringite to form monosulfoaluminate. Stage II be further examined. IPA-induced suppression of C3A hydra- ends—and stage III begins—once ettringite is the system is de- tion, as observed in heat flow rate profiles (Figure 4A,B), are pleted. This transition from stage II to stage III; however, can- expectedly reflected in the cumulative heat release profiles not be inferred directly from visual observation of the heat flow as well (Figure 4C). In general, with decreasing values of rate profile of the control paste. During stage III, C3A reacts aH0, the cumulative heat released from the pastes at 72 hours with water to form metastable hydration products, all of which decreases. As an example, at 72 hours, the cumulative heat ultimately transform to hydrogarnet (C3AH6); see section 2.0 released from hydration of the control paste (aH0 = 1.00) for more details. amounted to ≈810 J per gram of C3A; whereas, in the paste Going back to Figure 4, it can be seen that in [C3A + 15% prepared with 85% IPA (aH0 = 0.50), the cumulative heat re- C$ + H] pastes, as the IPA content increases (and consequen- lease was relegated to ≈175 J per gram of C3A. tially, aH0 decreases), the stage II peak—corresponding to The effects of aH0 reductions—resulting from partial replace- monosulfoaluminate precipitation—is increasingly delayed ment of mixing water with IPA—on hydration kinetics of C3A and broadened. Delay in occurrence of the stage II peak in [C3A + 30% C$ + H] pastes (Figure 5) are analogous to those entails slower kinetics of ettringite precipitation, therefore, in pastes prepared with 15% C$. In the heat flow rate profile inhibits the hydration kinetics of C3A and slows the con- of [C3A + 30% C$ + H] paste prepared with 0% IPA, the first sumption of C$ during stage I. Broadening of the stage II two stages of C3A hydration are clearly noticeable. Owing to its peak—including a decline in its peak intensity—entails de- higher C$ content, stage I of the control paste is longer—lead- celeration of kinetics of monosulfoaluminate precipitation. ing to precipitation of larger amount of ettringite—compared to 10 | LAPEYRE et al.

FIGURE 5 Isothermal microcalorimetry based determinations of heat evolution profiles of [C3A + 30% C$ + H] pastes, prepared at different levels of replacement of water with IPA. (A) shows the heat flow rates in linear scale; (B) shows the heat flow rates in semi-log scale; and (C) shows the cumulative heat releases in linear scale. l/s = 2 in all pastes. For a given system, uncertainty in the peak heat flow rate (ie, the intensity of the stage II peak)—as determined from 3 repetitions—is within ±2%

FIGURE 6 Relationship between initial water activity (aH0) and the corresponding calorimetric parameters pertaining to the stage II peak: (A) inverse of time to the stage II peak; (B) heat flow rate at the stage II peak; and (C) slopes of acceleration and deceleration regimes of the stage II peak. The corresponding IPA replacement levels are shown in the secondary x-axis. Results pertaining to all [C3A + C$ + H] pastes evaluated in this study are included. l/s = 2 in all pastes. Uncertainty in each calorimetric parameter is within ±2%

12,33,34,36 control [C3A + 15% C$ + H] paste. Akin to the trends of C3A hydration dynamics, the calorimetry profiles of all shown in Figure 4, IPA-induced reductions in aH0 result in pro- [C3A + C$ + H] pastes were processed to extract four distinct portionate suppression of C3A hydration across all stages in calorimetric parameters: inverse of time to the stage II peak −1 −1 [C3A + 30% C$ + H] pastes (Figure 5A,B). With increasing IPA (h ); heat flow rate (mW gC3A ) at the stage II peak; and replacement levels, the stage II peak is progressively delayed, slopes of acceleration (ie, approach to the peak) and decelera- and becomes less intense and broader. Owing to these alterations tion (ie, departure from the peak) regimes of the stage II peak −1 18,64‒66 in heat flow rates, [C3A + 30% C$ + H] pastes prepared with [mW. (h · gC3A) ]. Upon extraction, the calorimetric higher IPA contents feature lower cumulative heat release, at any parameters of all [C3A + C$ + H] pastes were plotted against given age, compared to pastes prepared with lower IPA contents. the corresponding aH0 (primary x-axis) and IPA replacement The inferences made thus far, regarding the influence of level (secondary x-axis). aH0 on hydration kinetics of C3A, have been based on visual As can be seen in Figure 6, with increasing IPA replace- observations of the pastes' heat evolution profiles, and, thus, ment levels, as aH0 reduces, all four calorimetric parameters are are qualitative in nature. For more quantitative description progressively reduced in monotonic manner. First, the delay in LAPEYRE et al. | 11 occurrence of the stage II peak with respect to decreasing val- ues of aH0 (Figure 6A) signifies prolongation of stage I. This decelerates the ettringite precipitation kinetics, thereby delay- ing the nucleation of monosulfoaluminate. Second, the pro- gressive decrease in intensity of the stage II peak (Figure 6B) with respect to decreasing values of aH0 entails deceleration of kinetics of monosulfoaluminate precipitation. Lastly, the reduc- tions in slopes of acceleration and deceleration regimes of the stage II peaks (Figure 6C) with decreasing values of aH0—once again—suggest that the kinetics of monosulfoaluminate precip- itation are retarded in pastes with higher IPA contents. In Figure 6, it should be noted that any given value of aH0 (including when aH0 = 1), all four calorimetric parameters of [C3A + 30% C$ + H] pastes are consistently lower than those of [C3A + 15% C$ + H] pastes. This—as stated in section 3.0—is because in the [C3A + 30% C$ + H] pastes, as the C$/C3A molar ratio is higher compared to that in [C3A + 15% C$ + H] paste, stage I of the hydration process (ie, Equation 2: ettringite precipitation reaction) is lengthier. Consequently, in all [C3A + 30% C$ + H] pastes: (a) the “nucleation event,” when crystals of monosul- foaluminate precipitate for the first time is delayed; and (b) fol- FIGURE 7 Cumulative heat released after 72 h of hydration of lowing the nucleation event, the kinetics of monosulfoaluminate C3A in [C3A + C$ + H] pastes plotted against aH0 (primary x-axis) precipitation is suppressed due to the lack of free-space in the and IPA replacement level (secondary x-axis). Data pertaining to 5 microstructure (which, compared to equivalent pastes prepared [C3S + H] pastes have been adapted from Ref. . The dashed lines are l/s with 15% C$, contain more ettringite). Regarding the latter exponential fits to the datasets. = 2 in all [C3A + C$ + H] pastes. 12,33,34,36,44 Uncertainty in each heat release value is within ±2% point, prior studies have shown that in [C3A + C$/ C$H2 + H] pastes, the rate of growth of monosulfoaluminate crystals decreases proportionately with respect to decreasing volume of free-space in the microstructure at the time of the the pastes, it can be said that reductions in aH0 suppress crystals' nucleation. On account of suppressed kinetics of mo- the hydration of C3A throughout stages I and II (as shown nosulfoaluminate precipitation in [C3A + 30% C$ + H] pastes, in Figure 6) and even stage III (if it occurs), thus resulting as compared to their counterparts prepared using 15% C$, the in lower values of αC3A at later ages. It must be pointed effects of aH0 on the former set of pastes are more severe. As out that the exponential relationship between cumula- can be seen in Figure 6, in [C3A + 30% C$ + H] pastes, that tive heat and aH0 in [C3A + C$ + H] pastes is strikingly were prepared at IPA replacement levels of 30% or above (ie, similar to that observed in [C3S + H] pastes (see Figure 5 aH0 ≤ 0.90), the stage II peak does not even occur within the 7). In their study, Oey et al reported that lower cumula- 72 hours of hydration. In contrast, calorimetry profiles of all tive heats (and, thus, lower degree of C3S hydration) in [C3A + 15% C$ + H] pastes prepared at IPA replacement levels [C3S + H] pastes prepared at lower aH0 were due to the of 70% or below (ie, aH0 ≥ 0.70) do feature the stage II peak suppression of growth rate of C–S–H nuclei. Likewise, in within the same period of hydration. [C3A + C$ + H] pastes, it is hypothesized that reductions Calorimetric parameters featured in Figure 6 pertain in aH0 result in suppression of post-nucleation growths of primarily to kinetics of C3A hydration during stage II. ettringite, monosulfoaluminate, and hydrogarnet (and/or To better understand the overarching influence of aH0 on metastable hydration products) during stages I, II, and III hydration of C3A—across all three stages—the cumula- of the C3A hydration process, respectively. This hypothesis tive heat released from the pastes at 72 hours of hydration gains support from another study,36 wherein it was shown were extracted, and plotted against the corresponding aH0 in [C3A + C$H2 + H] pastes, the disparities in kinetics (primary x-axis) and IPA replacement level (secondary of ettringite and monosulfoaluminate precipitation with re- x-axis). As can be seen in Figure 7, in both sets of pastes spect to differences in C$H2/C3A molar ratio (at the time (ie, [C3A + 15% C$ + H] and [C3A + 30% C$ + H] pastes), of mixing) are predominantly due to differences in post-nu- the cumulative heat (at 72 hours) declines monotonically, cleation growth rates of the crystals. In Figure 7, it is also and broadly in an exponential manner, with respect to de- interesting to note that while hydration of C3S is arrested creasing values of aH0. Importantly, as cumulative heat is at critical aH of 0.70 (cumulative heat at 72 hours is 0), an explicit indicator of degree of C3A hydration (αC3A) in the hydration of C3A—albeit suppressed—is not entirely 12 | LAPEYRE et al. ceased at the same aH. In fact, even at aH0 of 0.50, which on the C$ content, the lengths of stage I (ettringite pre- corresponds to IPA replacement level of 85%, hydration cipitation) and stage II (monosulfoaluminate precipitation) of C3A is not arrested. This suggests that the intrinsically of C3A hydration are different. Conversely, in pastes pre- higher reactivity of C3A—compared to that of C3S—en- pared at IPA replacement levels of ≥87% (ie, aH0 ≤ 0.45), ables the commencement and/or advancement of its hydra- the influence of C$ content on the heat release profiles is tion even at very low water activities (0.50 ≤ aH ≤ 0.70). imperceptible. As can be seen in Figure 8A, at any given In the above discussions, emphasis has been given to de- IPA replacement level, heat release profile of [C3A + 15% scribe the kinetics of C3A hydration in pastes prepared at C$ + H] paste overlaps with that of its counterpart pre- IPA replacement levels of 85% or below (ie, aH0 ≥ 0.50). pared using 30% C$. This result is significant because it However, it is important to examine C3A hydration in pastes indicates that in pastes prepared at aH0 ≤ 0.45, the presence prepared at higher IPA replacement levels, especially consid- of C$ is redundant in that it is unable to exert any percep- ering that in section 4.0, it was concluded that the hydration tible influence on the exothermic reactions. This implies of C3A would be arrested at the critical aH of 0.45, which that when the pastes are prepared with a mixing solution corresponds to IPA replacement level of 87.1%. Therefore, with aH0 ≤ 0.45, the reaction of C$ with C3A and water to additional calorimetry experiments were conducted to exam- form ettringite (ie, stage I of C3A hydration) does not take ine the hydration of C3A in [C3A + C$ + H] pastes prepared place—at any rate, not to an appreciable degree. As stage with IPA contents ≥87% (ie, aH0 ≤ 0.45). Results obtained I of C3A hydration is broadly annulled, it can be deduced from such experiments are shown in Figure 8A. that the lack of ettringite in the system also prevents the As can be seen in Figure 8A, even at high IPA replace- commencement of subsequent stages of C3A hydration (ie, ment levels, the cumulative heat released from all pastes are stage II: monosulfoaluminate precipitation; and stage III: positive (ie, always >0, after mixing), and monotonically precipitation of metastable calcium aluminate hydrates and, increasing with time. This suggests that even at very low ultimately, C3AH6). Simply put, results shown in Figure 8A initial water activities (ie, aH0 ≤ 0.45), one or more exother- indicate that in pastes prepared at aH0 ≤ 0.45 (IPA replace- mic processes—for example, the dissolution of C3A and ment level ≥87%), the precipitation of all hydration prod- C$—are able to commence and advance. Notwithstanding, ucts—including ettringite (during stage I)—is precluded, it is important to note that the heat release profiles shown and the hydration of C3A (ie, reaction of C3A with C$ and in Figure 8A are different from those of pastes prepared at water) is effectually arrested. Validation of this assertion is IPA replacement levels of ≤85% (shown in Figure 8B). As provided later in this section by means of results obtained can be seen in pastes prepared at IPA replacement levels from additional calorimetry experiments and material char- ≥ of ≤ 85% (ie, aH0 0.50), the temporal release of heat is acterization techniques (ie, XRD and TGA). significantly altered in relation to the initial C$ content (or If we accept the premise that in pastes prepared at C$/C3A molar ratio). This is expected because depending aH0 ≤ 0.45, the precipitation of hydration products is

FIGURE 8 Cumulative heat release profiles of [C3A + C$ + H] pastes, prepared at: (A) high IPA replacement levels (≥87%); and (B) low IPA replacement levels (≤60%). The solid lines correspond to [C3A + 15% C$ + H] pastes, whereas the dashed lines represent [C3A + 30% C$ + H] pastes. l/s = 2 in all pastes. Uncertainty in the cumulative heat release, at any given time, is within ± 2% LAPEYRE et al. | 13 disallowed (as discussed above), there still needs to be an is significant because it suggests that in [C3A + C$ + H] explanation for the heat released in such pastes (see Figure pastes, prepared at low initial water activities, the release 8A). It is hypothesized that in the pastes, the heat release is of heat is exclusively due to the dissolution of C3A and exclusively due to the (exothermic) dissolution of C3A and C$; between the two phases, C3A is the major contribu- C$—which initiate and advance even at very low values tor of heat release. When combined, the results shown in of aH0, thereby releasing heat that monotonically increases Figures 8,9 corroborate the hypothesis presented above with time. As aH0 in the paste decreases, the kinetics of C3A – that in [C3A + C$ + H] pastes, prepared at low initial 50 and C$ dissolution are retarded ; consequently, the cumu- water activities (ie, aH0 ≤ 0.45), while the dissolutions of lative heat release, at any given point in time, also reduces C3A and C$ do commence and advance, the precipitation (Figure 8A). To verify this hypothesis, additional calorime- of hydration products (ie, ettringite and monosulfoalumi- try experiments were conducted to monitor the heat release nate, resulting from C3A-C$ interactions), and, therefore, in single-phase systems: [C3A + H] paste and [C$ + H] the hydration of C3A is arrested. It could still be argued paste. In either paste, as there is no chemical interaction that in [C3A + C$ + H] and single-phase [C3A + H] pastes, between C3A and C$, the precipitation of ettringite or mo- prepared at aH0 ≤ 0.45, C3A could react directly with water nosulfoaluminate is infeasible. Both pastes were prepared to form metastable calcium aluminate hydrates (eg, C2AH8 48 at l/s of 2.0, and, in each paste, 92%mass of the water was and C4AH13), which would subsequently transform to replaced with IPA (ie, aH0 = 0.33). As can be seen in Figure hydrogarnet (C3AH6). Nevertheless, results obtained from 9A, even at such low value of aH0, both C3A and C$ are able XRD and TGA—that are described next—clearly show to dissolve, and, thus, release heat. After 72 hours, the cu- that the formation of such hydration products is also disal- mulative heats released from C3A's and C$'s dissolution are lowed at aH0 ≤ 0.45. −1 −1 ≈85 J gC3A and ≈1 J gC$ , respectively; this difference Based on results obtained from thermodynamic calcula- in heat release is expected to arise due to differences in en- tions (section 4.0) and analyses of experimental results (de- thalpies and kinetics (eg, differences in SSA of the particu- scribed thus far in section 6.0), it has been posited that the lates) of dissolution of the two phases. Next, the cumulative hydration of C3A in [C3A + C$ + H] pastes is arrested at heat release profiles of [C3A + H] and [C$ + H] pastes aH0 ≤ 0.45 (ie, when 87% or more of mixing water is replaced were compared against those of [C3A + 15% C$ + H] and with IPA). This entails that precipitation of all hydration [C3A + 30% C$ + H] pastes (Figure 9B); all pastes were products (eg, ettringite, monosulfoaluminate, and hydrogar- prepared at the same l/s of 2 and IPA replacement level of net)—that are derived from C3A's hydration—is disallowed 92%. As can be seen, the cumulative heat release profile in pastes prepared at aH0 ≤ 0.45. To corroborate this hypoth- obtained by adding the heats released from C3A's and C$'s esis, additional numerical analyses of calorimetry results and dissolution in [C3A + H] and [C$ + H] pastes, respectively, experimental characterizations of the pastes were conducted. is very similar to the cumulative heat release profiles of For the numerical analyses, emphasis was given to uti- [C3A + 15% C$ + H] and [C3A + 30% C$ + H] pastes. lize the cumulative heat release profiles of [C3A + C$ + H] This equivalency in the cumulative heat release profiles pastes (shown in Figures 4,5) to estimate the hydration

FIGURE 9 Cumulative heat release profiles of: (A) [C3A + H] and [C$ + H] pastes; and (B) [C3A + H], [C3A + 15% C$ + H], and [C3A + 30% C$ + H] pastes. All pastes were prepared at l/s of 2, and IPA replacement level of 92%mass (ie, aH0 = 0.33). In (B), the symbols represent the sum of cumulative heats released from the dissolution of C3A and C$ in single-phase [C3A + H] and [C$ + H] pastes, respectively. Uncertainty in the cumulative heat release, at any given time, is within ±2% 14 | LAPEYRE et al. products that are expected to be present at 72 hours. Towards level ≥ 0%), the hydration of C3A advances to stage II; this, enthalpies of reactions corresponding to ettringite whereas, when 0.45 ≤ aH0 ≤ 0.80 (ie, 87% ≥ IPA replace- precipitation (stage I; Equation 2), monosulfoaluminate ment level ≽ 55%), the hydration of C3A is constrained to precipitation (stage II; Equation 3), and hydrogarnet pre- stage I. When aH0 ≤ 0.45 (ie, IPA replacement level ≥ 87%), cipitation (stage III; Equation 4), as described in section it is assumed that hydration of C3A does not commence and 2.0 were used. On the bases of these enthalpies, for both no hydration products are precipitated (although C3A and sets of pastes (ie, [C3A + 15% C$ + H] and [C3A + 30% C$ are expected to dissolve and release heat; see Figures C$ + H] pastes), the limiting values (ie, when aH0 = 1) of 8,9). Therefore, at 72 hours, the solid phase assemblage is −1 cumulative heat release (Q; units of J gC3A ) and the cor- expected to comprise of only C3A and C$. responding values of αC3A at the ends of stages I, II, and III Results shown in Figure 10 provide rough estimate of were estimated. Lastly, by using each paste's cumulative solid phase assemblages in [C3A + C$ + H] pastes after heat release at 72 hours (drawn from Figures 4C and 5C), 72 hours of hydration. To substantiate these findings, XRD in conjunction with the ultimate heat of C3A hydration (see was employed; the objective was to identify the phases last row of Table 3) in the paste, the degree of hydration of present in the pastes at 72 hours. As can be seen in Figure C3A (αC3A) and the corresponding combination of hydra- 11, for the [C3A + 15% C$ + H] pastes, there is good agree- tion products that are expected to be present in the paste at ment between phases predicted from the numerical analy- 72 hours were estimated. Results obtained from the analy- ses (Figure 10) and those identified from XRD patterns. In ses are shown in Figure 10. the paste prepared using pure water as the mixing solution As can be seen in Figure 10, in both sets of pastes (ie, (ie, aH0 = 1.00), metastable calcium aluminate hydrates [C3A + 15% C$ + H] and [C3A + 30% C$ + H] pastes), (C2AH8 and C4AH19) and monosulfoaluminate were found the values of αC3A (at 72 hours) decay in exponential man- confirming that the hydration of C3A had entered into stage ner with respect to decreasing aH0. Nevertheless, these III. Presence of metastable hydrates is expected since the trends are expected considering the exponential relation- transformation of these hydrates to hydrogarnet (C3AH6) ship between the 72-hour cumulative heat release and aH0 is kinetically limited. When the IPA replacement level was of the pastes (shown in Figure 9). Importantly, Figure 10 increased to 20% (ie, aH0 = 0.94), monosulfoaluminate was shows that in [C3A + C$ + H] pastes, αC3A and the cor- the only sulfate containing hydrate in the paste; this, also, responding solid phase assemblage at 72 hours are dic- is representative of C3A hydration being in stage III. When tated by aH0. Starting with [C3A + 15% C$ + H] pastes the IPA content was increased to 50% (ie, aH0 = 0.83), the 67 (Figure 10A), when 1.0 ≥ aH0 ≽ 0.85 (ie, 44% ≽ IPA re- peaks corresponding to ettringite emerged with greater placement level ≥ 0%), the hydration of C3A is expected intensity than those of monosulfoaluminate. The presence to have advanced to stage III; as such, the pastes are ex- of both ettringite and monosulfoaluminate indicates that pected to comprise of C3A, monosulfoaluminate, and the hydration of C3A is in stage II; this is in agreement with C3AH6 (and/or metastable calcium aluminate hydrates). predictions made in Figure 10. In all [C3A + 15% C$ + H] When 0.85 ≽ aH0 ≽ 0.50 (ie, 85% ≽ IPA replacement pastes, containing ≥87% IPA in the mixing solution (ie, level ≽ 44%), the pastes are expected to comprise of C3A, aH0 ≤ 0.45), no hydration products (not even metastable 68‒72 ettringite, and monosulfoaluminate; as such, the hydration calcium aluminate hydrates or C3AH6) were detected . of C3A is expected to remain in stage II. And lastly, when This confirms that the hydration of C3A in [C3A + 15% aH0 ≼ 0.50 (ie, IPA replacement level ≽85%), the hydra- C$ + H] pastes does not initiate when the initial water ac- tion of C3A does not progress beyond stage I; as such, the tivity is 0.45 or below. pastes are expected to comprise of C3A, C$, and ettringite Going back to Figure 11, for the [C3A + 30% C$ + H] (which is not expected to precipitate when aH0 ≤ 0.45). In pastes as well, good agreement was found between the phases [C3A + 30% C$ + H] pastes (Figure 10B), even at aH0 = 1, predicted from the numerical analyses (Figure 10) and the hydration of C3A at 72 hours does not advance to stage those identified from XRD patterns. In the paste prepared III. When 1.0 ≥ aH0 ≽ 0.80 (ie, 55% ≽ IPA replacement using pure water as the mixing solution (ie, aH0 = 1.00),

TABLE 3 Limiting values of Q (J. g −1) α (Unitless) C3A C3A cumulative heat (Q) released from reactions Reaction 15% C$ 30% C$ 15% C$ 30% C$ corresponding to Equation 2, 3, and 4, and C A 3C 32H → AFt limiting values of α at the ends of stage 3 + $+ 225.03 547.89 0.117 0.284 C3A C A 0.5AFt 2H → 0.5AFm I, II, and III. Results for both [C3A + 15% 3 + + 307.83 498.82 0.350 0.854 C$ + H] and [C3A + 30% C$ + H] pastes C A+6H → C AH 3 3 6 627.93 144.22 1.000 1.000 are shown Ultimate value of Q 1160.8 1190.9 LAPEYRE et al. | 15 FIGURE 10 Degree of hydration of C3A (αC3A), after 72 h of hydration, in: (A)

[C3A + 15% C$ + H]; and (B) [C3A + 30%

C$ + H] pastes, plotted against aH0. Phases, listed in the shaded areas, are expected to be present in the paste, depending on the values of αC3A and aH0. For aH0 ≤0.45, hydration of C3A is assumed to be arrested; as such, only C3A and C$ are assumed to be present

FIGURE 11 XRD patterns of (A) [C3A + 15% C$ + H]; and (B) [C3A + 30% C$ + H] pastes, prepared at different IPA replacement levels, after 72 h of hydration. For each IPA replacement level, the corresponding aH0 are indicated within parentheses. l/s = 2 in all pastes

ettringite and monosulfoaluminate were detected—thus (ie, aH0 ≤ 0.45); this confirms that the hydration of C3A in confirming that, at 72 hours (after mixing), the hydration the pastes did not commence. Cessation of C3A hydration of C3A had transitioned to stage II. As the IPA replacement in [C3A + 30% C$ + H] pastes prepared at IPA replacement level was increased from 0% to 50% (ie, aH0 = 0.83), the levels of ≥87% was also confirmed by TGA data. As can intensities of peaks corresponding to ettringite and mo- be seen in Figure 12, the characteristic differential TGA nosulfoaluminate progressively increased and decreased, (DTGA) peaks of ettringite and monosulfoaluminate73— respectively. This is indicative of C3A hydration being in that are clearly seen in the control paste (aH0 = 1.00)—are stage II, and confirms—as was stated earlier in this sec- absent in pastes prepared at IPA replacement levels of 90% tion—that the kinetics of ettringite-to-monosulfoaluminate (aH0 = 0.38) and 95% (aH0 = 0.23). transformation during stage II is progressively suppressed In Figure 11, it is worth noting that in selected pastes, with decreasing aH0. Lastly, akin to pastes prepared with traces of CO3-AFm phases (ie, hemi- and mono-carboalu- 15% C$, in all [C3A + 30% C$ + H] pastes, no hydration minate) were also found. While the exact reason for the products were detected at IPA replacement levels of ≥87% presence of these phases in the pastes is not clear, it is 16 | LAPEYRE et al.

that even at aH0 ≤ 0.45, both C3A and C$ are still able to dis- solve (see Figures 8,9); however, at such low water activities, the ions released from dissolution of the anhydrous phases are unable to combine and precipitate as reaction products. While the results described thus far overwhelmingly in- dicate that the critical water activity below which C3A stops hydrating is 0.45, there is a small—yet finite—chance that the cessation of C3A hydration in the pastes (which were all prepared at a reasonably high l/s of 2) was caused due to the scarcity of water, thereby leading to the development of capillary stresses within the pore network of the pastes. Thus, it is deemed important to determine whether or not the hydration of C3A would be arrested in [C3A + C$ + H] suspensions, in which liquid saturation—even after hun- dreds of hours of hydration—would be guaranteed (thereby ensuring that capillary stresses in the pores remain insig- nificant). Toward this, [C3A + C$ + H] suspensions were prepared at l/s of 100 and allowed to hydrate for 72 hours FIGURE 12 Differential TGA traces (differential mass loss) (at 20°C), following which they were examined using XRD. of [C3A + 30% C$ + H] pastes prepared at different IPA replacement As can be seen in Figure 13, when IPA replacement level levels, after 72 h of hydration. The “pristine” system represents a solid, is 20% (ie, aH0 = 0.94), the hydration of C3A commences powder mixture of 70% C3A and 30% C$. For each IPA replacement and advances to stage II in both suspensions; monosulfoalu- a l/s level, the corresponding H0 are indicated within parentheses. = 2 in minate is detected in the [C3A + 15% C$ + H] suspension, all pastes and ettringite and monosulfoaluminate are detected in the [C3A + 30% C$ + H] suspension. However, when the IPA hypothesized that the CO3-AFm phases precipitated due replacement level is increased to 95% (ie, aH0 = 0.23), no to one or both of the following: (a) reaction of aqueous hydration products (including metastable calcium aluminate Ca2+,Al(OH)−,andOH− 4 ions, released from the dissolution hydrates and C3AH6) are detected in either suspension. This 4,35 of C3A (see Equation 2), with atmospheric CO2 ; and/or confirms that the hydration of C3A is arrested in both sus- (b) reaction of CaO impurity in C3A with IPA, water (if pres- pensions, in spite of C3A particulates having unrestricted ent), and atmospheric CO2 (likely during storage of the paste access to liquid water. Hence, in light of the above discus- specimens). The latter hypothesis is supported by a prior sions and results shown in Figure 13, it can generically be 5 study , wherein it was shown that in pastes prepared by mix- said that in [C3A + C$ + H] systems prepared at aH0 ≤ 0.45, ing C3S with pure IPA (no water), the CaO impurity in C3S the hydration of C3A (and, thus, the precipitation of all hy- reacted with IPA and atmospheric CO2 to form CaCO3. It is dration products) is arrested, regardless of the water and C$ worth noting that the CaO impurity in C3A is also detect- contents in the system. By the same logic, it can be said that able in the DTGA traces shown in Figure 12. In the DTGA even if hydration of C3A were to initiate (for example, in a traces, the mass losses in the pastes (including the pristine [C3A + C$ + H] or cement paste prepared at aH0 » 0.45), system, that is, the anhydrous mixture of 70% C3A and 30% it is expected to be arrested as soon as the water activity C$) between temperatures of 350-and-420°C are thought be declines below the critical value of 0.45. If drying—as op- linked to the formation of carbonate phase(s) resulting from posed to replacement of mixing water with IPA—is used to the carbonation of the CaO impurity present in C3A. stop C3A hydration in cementitious systems, then the inter- Overall, the results shown in Figures 11,12 clearly show nal RH would have to be reduced to ≈ 45% or below (NB, that in [C3A + C$ + H] pastes, regardless of the C$ content, the once equilibrium is reached, internal RH = aH in the solu- hydration of C3A—and, therefore, the precipitation of hydra- tion); this value of critical RH for C3A hydration is in good tion products (ie, ettringite, monosulfoaluminate, metastable agreement with those reported in prior literature.6,7,16 calcium aluminate hydrates, and C3AH6)— did not commence or advance when the initial water activity (aH0) is ≤0.45 (ie, IPA replacement level = 87%). This confirms the results ob- 7 | SUMMARY AND tained from thermodynamic considerations (section 4.0) and CONCLUSIONS inferences drawn from calorimetry experiments (Figures 4‒10)—that the critical water activity below which hydration This study examined the influence of water activ- of C3A is arrested is 0.45. Notwithstanding, it should be noted ity (aH) on the hydration of tricalcium aluminate (C3A) LAPEYRE et al. | 17 FIGURE 13 XRD patterns of (A) [C3A + 15% C$ + H]; and (B) [C3A + 30% C$ + H] suspensions, prepared at 20% and 95% IPA replacement levels, after 72 h of hydration. For each IPA replacement level, the corresponding aH0 are indicated within parentheses. l/s = 100 in all suspensions

in [C3A + calcium sulfate (C$) + water (H)] systems. 2, it was shown that even in [C3A + C$ + H] suspensions— Reductions in water activity were achieved by partially re- that were prepared at l/s of 100, to ensure that all pores re- placing water—meant to be used as the mixing liquid for main near-saturated with the liquid phase—the hydration of formulation of the pastes—with isopropanol (IPA). IPA— C3A was arrested below the critical water activity of 0.45. as opposed to other organic alcohols (eg, ethanol)—was Alongside, and independent of, the experiments, a ther- chosen because of its excellent miscibility with water, in- modynamic approach was devised and employed to estimate ertness, and small molecular size, which permits its access two significant thermodynamic parameters associated with to large as well as very small pores within the microstruc- the hydration of C3A in [C3A + C$ + H] systems: critical ture. Isothermal microcalorimetry, along with other experi- water activity below which hydration of C3A is arrested, and mental techniques, was employed to monitor the hydration solubility constant of C3A (KC3A). In excellent agreement of C3A in [C3A + 15% C$ + H] and [C3A + 30% C$ + H] with experiments, the thermodynamic calculations also in- pastes, encompassing a wide range of initial water activi- dicated that the critical water activity for C3A hydration is ties (0.00 ≤ aH0 ≤ 1.00). 0.451. On the basis of the critical aH, the solubility product −20.65 Results obtained from the experiments clearly showed of C3A (KC3A) was estimated as 10 . To the best of the that with decreasing initial water activity, the kinetics of authors' knowledge, this is the very first estimation of KC3A; all reactions associated with C3A (eg, with C$, resulting in determination of this value is expected to aid in the develop- ettringite formation; and with ettringite, resulting in mono- ment and validation of sophisticated models used for simula- sulfoaluminate formation) are proportionately suppressed. tion of hydration and property development in cementitious As the initial water activity declines below the critical value systems.17,18,20‒22,65,74‒76 of 0.45, the hydration of C3A, and the resultant precipitation Overall, this work presents cogent arguments—derived of hydration products (ie, ettringite, monosulfoaluminate, from, and supported by, rigorous experimentation and metastable calcium aluminate hydrates, and C3AH6) are es- thorough numerical analyses—that unequivocally show sentially arrested. In a [water + IPA] mixture, an initial water that the hydration of C3A is significantly affected by the activity of 0.45 corresponds to replacement of 87.1%mass of activity of water, and thus by reductions in relative humid- the water with IPA. However, even at initial water activities ity. In [C3A + C$ + H] pastes, or even in cement pastes, that are less than the critical value of 0.45, both C3A and C$ regardless of the l/s or the amount of C$ (or C$H2), the are still able to dissolve and release ions into the contiguous hydration of C3A is terminated once the water activity de- solution; however, at such low water activities, the ions are creases below the critical value of 0.45. On the basis of unable to combine and precipitate as reaction products. conclusions drawn from this study, it can be said that for While majority of the [C3A + C$ + H] pastes examined in cement-based systems, especially for those prepared at low this study were prepared at liquid-to-solid mass ratio (l/s) of l/s, it is important to maintain saturated curing conditions 18 | LAPEYRE et al. to ensure that water activity is retained well above the val- 14. Corstanje WA, Stein HN, Stevels JM. Hydration reactions in pastes C3S+C3A+CaSO4.2aq+H2O at 25°C.I. Cem Concr Res. ues of 0.45 and 0.70 to ensure the continuation of C3A and 1973;3(6):791–806. C S hydration, respectively. Failure to do so would result 3 15. Breval E. C3A hydration. Cem Concr Res. 1976;6(1):129–37. in suppression of reaction rates of anhydrous phases (ie, 16. Dubina E, Plank J, Black L, Wadsö L. Impact of environ-

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