Influence of the availability of calcium on the hydration of tricalcium aluminate (C₃A) in seawater‐mixed C₃A–gypsum system

Abstract

This work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C3A) in C3A–gypsum and C3A–gypsum–Ca(OH)2 systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C3A and solubility of gypsum, which accelerated the early hydration of C3A and the formation of ettringite (AFt), leading to a higher hydration degree of C3A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)2, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO4–AFm) and the formation of hydrogarnet (C3AH6), which would further reduce the hydration degree of the C3A at the later ages. However, in the presence of Ca(OH)2, the hydration degree of C3A–gypsum–Ca(OH)2 at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW‐mixed cement concrete can affect the hydration degree of C3A in cement.

Volume Deformation and Hydration Behavior of Ordinary Portland Cement/Calcium Sulfoaluminate Cement Blends

“…AFt crystals from the CSA cement hydration grew gradually from the supersaturated solution, and the expansion was caused by the confined growth of ettringite in the available porosity [ 49 , 50 ]. In addition, the growth rate of the crystals was proportional to the reciprocal of the radius of the curved meniscus, and the smaller size micropores with a high curvature could easily contribute to the growth of AFt crystals due to the low free energy of crystallization [ 30 ]. Therefore, the slight expansion in the OPC/CSA cement blends was beneficial in decreasing the porosity and improving the compressive strength when less than 6% CSA cement was used.…”

“…Based on the measured concentrations of Ca, S and Al, the saturation index of the ettringite was calculated using GEMS to evaluate the driving force of the expansion in the cement blends. The detailed description of the saturation index is given in Ref [ 30 ].…”