Modelling the dissolution and precipitation process of the early hydration of C3S
Introduction
Tricalcium silicate (Ca3SiO5 or C3S) is the main clinker phase of Portland cement and is often used in model systems to study the hydration of cement [1,2]. The hydration of C3S includes its dissolution in water, followed by the nucleation and growth of calcium silicate hydrate (CSH) and calcium hydroxide (CH) [3]. Modelling the nucleation and growth process of hydration is important in investigating the early hydration kinetics of cement.
The classical Avrami equation assumes that the nucleation and growth of small nuclei occur randomly in space [[4], [5], [6]]. This equation has been widely used to simulate the early hydration of cementitious materials [2,[7], [8], [9]]. The commonly used form of the Avrami equation is shown in Eq. (1).where X is the transformed volume fraction, k is a rate constant, and n is a parameter related to the dimensionality of products and nucleation condition [10]. The nucleation of CSH mainly takes place on the surface of C3S particle [2,11]. Therefore, a new boundary nucleation (BN) model has been developed from Cahn's model [12] (Eq. (2)) supposing that nuclei are randomly distributed on the surface of solid:where S is the surface area per unit volume, G is the growth rate of nuclei, N is the producing rate of nuclei per unit uncovered surface and y is a dummy variable. This model was also applied to describe the early hydration of C3S [11,[13], [14], [15], [16], [17]]. Both Avrami equation and Cahn equation supposed that the nucleation rate and growth rate of nuclei are constants. But the nucleation rate and the growth rate of nuclei are related to the saturation index of crystallization phases according to the classical nucleation theory [18].
In this research, the early hydration of C3S is modelled according to the boundary nucleation theory considering the influence of the saturation index of C3S and CSH on dissolution kinetics and precipitation kinetics. The simulated result was compared with the exothermal data reported in the literature.
Section snippets
Basic thermodynamic model
The dissolution-precipitation process that occurs during the hydration of C3S can be approximated as follows [19]:
C3S dissolution:
CxSHy precipitation:
CH precipitation:
Experimental
The exothermic rate of C3S with a specific surface area of 300 m2/kg was measured using an isothermal calorimeter (TAM Air from TA Instruments) at 20 °C. The water/solid ratio was 0.4. To avoid a considerable temperature difference between the paste and the isothermal environment, the binder and water were weighed separately and placed in the chamber in advance. After 10 h of balance and gain calibration, water was injected into the sample from a microsyringe, and the paste was stirred in situ
Comparison to the exothermal data for C3S in this study
The exothermal heat rate is shown in Fig. 6. The experiment was repeated twice; the results show that the first hydration heat peak is insignificant. The small first hydration peak is mainly attributed to the wetting heat and the dissolution heat of a small amount of C3S. It can be calculated by PHREEQC that the maximum content of C3S dissolved in 1 mL deionized water is approximately 5.875 μmol according to the equilibrium constant. Thus, the maximum amount of C3S dissolved initially is
The main hydration peak
The discussion of the mechanism governing the acceleration and deceleration periods of the hydration is important for a newly constructed kinetic model. In the past, some hypotheses have been used to explain the transition from acceleration period to deceleration period, e.g., the diffusion barrier theory, nucleation and growth with perpendicular impingement, confined-growth theory and the CSH growth hypothesis. And some related models have been reported for every hypothesis. The main arguments
Conclusion
A thermodynamic and kinetic model is developed in this study to simulate the early hydration of C3S. Both the dissolution and precipitation processes control the early hydration kinetics of C3S, and they are connected via the aqueous properties of the solution. The dissolution rate is characterized by an empirical formula that clearly describes the dissolution kinetics that applies to various saturation conditions. The precipitation kinetics of CSH is characterized by a developed boundary
CRediT authorship contribution statement
Zhang Zengqi:Conceptualization, Methodology, Writing - original draft.Han Fanhui:Investigation, Visualization, Data curation.Yan Peiyu:Conceptualization, Methodology, Writing - review & editing.
Declaration of competing interest
The authors state that there is no competing interest.
Acknowledgments
The authors would like to acknowledge the National Key Research and Development Program of China (2017YFB0310101), National Natural Science Foundation of China (No. 51878381) and the China Postdoctoral Science Foundation (2019M660037).
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