Early hydration of blast furnace slag in the presence of sodium chromate

Abstract

Blast furnace slag (BFS) is a potential candidate for the immobilization of chromium-bearing wastes. This paper investigated the effect of soluble hexavalent chromium (Na2CrO4) on the early hydration behavior of BFS by means of isothermal calorimetry, X-ray diffraction (XRD), thermogravimetry (TG), scanning electron microscopy (SEM) and pH measurement of pore solution. Meanwhile, the compressive strength of BFS pastes in the presence of different dosage of chromate was tested for the structural study. The primary aim was to verify the alkaline activation of Na2CrO4 on the BFS. It was found that the BFS in water had scarcely hydrated in the first 7 days, whereas the addition of Na2CrO4 greatly accelerated the hydration of BFS and promoted the hardening. Increasing the Na2CrO4 dosage could significantly improve the compressive strength, which was sufficient to meet the requirement for immobilization of wastes. The hydration products of BFS in Na2CrO4 solutions were mainly C-(A)-S-H and CrO4-U phase. The CrO4-U phase played a key role in Cr(VI) binding due to its layered structure. During the formation of CrO4-U phase, a large number of alkalis was released, which largely increased the pH of pore solution, thus greatly promoting the BFS hydration. The results indicated that the Na2CrO4 had a strong alkaline activation on blast furnace slag.