The Early Age Hydration Reactions of a Hybrid Cement Containing a Very High Content of Coal Bottom Ash

A study examining the complex hydration chemistry of a hybrid alkaline cement containing a high content of coal bottom ash (BA) (>70%) and a low content of portland cement clinker in the presence of an alkaline activator is presented. The use of a water reducing additive was found to significantly delay the overall hydration process, allowing an opportunity to more clearly distinguish the hydration reactions that take place. The results presented showed that both the cement clinker phases and the ash glassy phases are highly reactive for the first 3 d of hydration. In situ formed reaction products portlandite and gypsum were shown to be metastable and had disappeared within 3 d of hydration. Ettringite stability was limited in the hybrid system but unlike gypsum and portlandite, remained detectable for the first 3 d of hydration at least. SEM‐EDX and subtracted Fourier transform infrared evidence suggest the development of three different gel bond environments, tentatively attributed to C–(A)–S–H, C–A–S–H, and (N,C)–A–S–H type gels.