Investigation on environmentally friendly PFA based expansive agent

In order to make use of Al₂O₃ in fly ash as a substitute for natural aluminate materials to manufacture concrete expansive agent, a kind of pro-hydrated fly ash (PFA) was made by chemical activation and calcination. Cementitious minerals of PFA were leached and quantificationally analyzed, thereby the mixture of PFA based expansive agent (PBEA) was made up according to principles of cement chemistry, and dosage of PBEA was optimized through optimum seeking method. Based on characteristics of cement, recipe of combined PBEA was experimentally researched, and microstructures of hydration products of cement pastes containing combined PBEA were observed by SEM. It is indicated that cementitious minerals of PFA are C₁₂A₇ and C₂S, and that the appropriate mass ratio of PFA to anhydrite in PBEA is 85:15. Properties of PBEA can meet the needs of current standard when its dosage in concrete is more than 5.6%-6.0%. Combined PBEA can not only compensate shrinkage of concrete but also enhance slump retention ability of concrete. As sulfoaluminate type expansive agent, combined PBEA may hydrate with cement hydration products to form plenty of ettringite crystals. Therefore, PBEA can be a substitute for expansive agent made of natural aluminate materials to compensate shrinkage of concrete.