Investigations of anhydrite in CFBC fly ash as cement retarders

Circulating fluidized bed combustion (CFBC) ashes are different from conventional coal ashes for good pozzolanic and hydraulic properties. However, the ashes are undesirable as cement admixture due to excessively high SO3 content. The aim of this paper is to investigate the possible displacement of natural gypsum in cement with anhydrite in CFBC ash in terms of setting time, strength development, volume stability and hydration process. Experimental results indicated that anhydrite in CFBC ash can be used as an efficient setting retarder but lead to lower mortar strength. The effect is harmless to volume stability of cement paste with appropriate anhydrite addition. Compared with natural gypsum, anhydrite in CFBC ash makes ettringite formation slower and more transformable.