The synergistic effect of AFt enhancement and expansion in Portland cement-aluminate cement-FGD gypsum composite cementitious system

Abstract

In this research, the synergistic effect of AFt enhancement and expansion in Portland cement-aluminate cement-FGD gypsum ternary binder was systematically analyzed. The following conclusions can be drawn from the interaction of AFt formation rate, compressive strength and free swelling rate. In appropriate proportion, the strength of the ternary composite binders was significantly improved compared with the reference sample. The highest strength of ternary binder which was composed of 90% Portland cement and 10% mixed binder of FGD gypsum and aluminate cement reached 117% of the reference sample at 3d and 107% at 60d. The highest strength of ternary binder composed of 80% Portland cement and 20% mixed binder of FGD gypsum and aluminate cement reached 136% of the reference sample at 3d and 110% at 60d. Most of the free swelling ratios of the ternary binder were lower than 0.6% except high-substitution samples such as cracking samples of ternary binder composed of 70% Portland cement and 30% mixed binder of FGD gypsum and aluminate cement. Similar to hardened Portland cement, hydration products such as AFt, C-A-H, Ca(OH)2 and C-S-H gel existed in the ternary composite binder. The AFt contents of the ternary composite binder increased with the increase of substitution amount of FGD gypsum and aluminate cement. When the mass ratio of aluminate cement to FGD gypsum was higher than 4:1, the formation of AFt in the ternary binder with free expansion ratio less than 0.6% reached the highest at around 7d, which hence results in the highest mechanical strength. Bycontrast, when the mass ratio of aluminate cement to FGD gypsum was lower than 4:1, the formation of AFt in the ternary binder reached the highest at around 28d. The delayed formation of massive AFt has been found responsible for the deleterious expansion.