Hydration properties and microstructure characteristics of alkali–activated steel slag

Abstract

The large quantity of steel slag generated from the iron and steel industry needs to be disposed. Using alkali–activated steel slag as a building material is a good method of comprehensive utilization. In this paper, the properties of steel slag activated by liquid sodium silicate with a silicate modulus of 1.5 as clinker–free cement were investigated. Portland cement with the same water–binder ratio of 0.45 was used as the reference sample. Detailed comparisons including the hydration properties, microstructure characteristics and mechanical strength of the two binders were conducted. The results show that alkali–activated steel slag and cement have similar hydration processes and products. However, the hydration of alkali–activated steel slag has a shorter dormant stage, an earlier and smaller second exothermic peak, a lower cumulative heat, less and poorer crystallization of Ca(OH)2, a lower Ca–Si ratio and a similar Al–Si ratio in gels. Compared to the morphology of cement paste, looser microstructure, unhydrated particles and Ca(OH)2 crystals in a single sheet can be seen in alkali–activated steel slag paste, forming a weak link in the matrix and damaging the strength development. The compressive strengths of alkali–activated steel slag hardened pastes are only 30–40% of the strengths of cement pastes due to poor hydration of former. Therefore, although its efficacy is lower than that of cement, steel slag could replace cement as a building material for certain engineering applications due to its more economic and environmental benefits.