Study on the performance of FGD gypsum-metakaolin-cement composite cementitious system

FGD gypsum is a desulfurization by-product from the steelworks, smelting plants, thermal power stations and so on and its output grows day by day, which means that the resource utilization of FGD gypsum is of economic and social significance. In this paper, FGD gypsum dried at 40 °C and FGD gypsum thermally activated at 800 °C were used as a water hardening supplementary cementitious component, and two series of FGD gypsum-metakaolin-cement composite cementitious systems were prepared. After a series of studies, the following conclusions were obtained: The initial setting time of tested paste samples was longer and their interval between initial setting time and final setting time shortened while mortar fluidity decreased slightly. Early mortar strength of tested samples (to 28 d) was lower than the reference sample (namely Portland cement) and the strength increased obviously and some exceeded the reference sample after 28 days. Hydration products such as AFt and C-A-H existed at the same time from 1 day to 1 year in the samples with dried FGD gypsum and there was still unhydrated gypsum after a year. And crystallization of ettringite and gypsum could take place at the same time and partial crystal transformation from ettringite to AFm occurred from 40 d to 365 d with activated FGD gypsum. The volume stability of tested mortar samples kept in water was satisfied for one year in which ettringite amount was in 13-17% of harden paste and the ettringite content achieved 70-80% of the total ettringite amount at 7 d. Excessive crystallization of gypsum could also have an undesirable effect on the volume stability of harden mortar and mortar strength such as Sample B-5 from 40 d to 120 d. The volume stability of tested samples was good if the expansion stress was lower than the tensile strength.