Abstract
The addition of calcium sulfate in the preparation of autoclaved aerated concrete (AAC) leads to improved mechanical properties but the precise mechanism is not well constrained. To study this, the hydrothermal curing process was monitored quantitatively with and without sulfate addition using ex situ and in situ X-ray diffraction. Higher concentrations of sulfate ions delayed the formation of the intermediate C-(A)-S-H phase and the dissolution of quartz. Once the additional sulfate ions were depleted by hydroxylellestadite formation, C-(A)-S-H formation and quartz dissolution started simultaneously at higher rates. This delayed reaction is suggested to enhance the mechanical strength by i) improving the diffusion of silicate ions in hydrothermal solution, ii) improving the distribution of C-(A)-S-H and tobermorite crystallites, and iii) closing larger capillary pores in the range of 1–5 μm. This could explain some of the ambiguous results from previous studies and provide an opportunity to optimize the AAC production process.
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