Calcined alunite-modified alkali-sulphate-activated slag as a novel binder for high-performance cemented paste backfill

Abstract

The free water content and pore structure significantly affect the mechanical properties of cementitious materials. Because of high water-to-cement ratio and low binder dosage, cemented paste backfill (CPB) is characterized by high free water content and high porosity, which lead to a low strength. In this study, calcine alunite (CA) was used to modify the alkali-sulphate-activated slag (ASAS) to synthesis high-strength CPB with low porosity and low free water content. The results show that at dosage ≤ 3%, CA can significantly enhance the strength of ASAS-CPB, and the strengthening effect becomes more prominent as both the dosage and curing time rise. An addition of 3% CA leads to a 184% increase in strength at 14 days. However, excessive CA (5%) reduces 3-day strength by 65%, but provides the greatest strength at 56 days. As the dosage increases up to 3%, the promoting impact of CA on early hydration of ASAS-CPB gradually increases, and the duration of induction stage and acceleration stage gradually decreases. Nevertheless, 5% CA prolongs the duration of induction stage by 921%. Electrical conductivity and volume water content can be used to assessing the hydration process of ASAS-CPB. Besides, the addition of CA can promote the hydration of slag, resulting a greater formation of C-S-H and ettringite. The hydration products fill the pores and transform the macropores into micropores. Furthermore, mechanisms of CA-induced enhancement in strength is discussed. The results of this study can offer a reference for the synthesis of high performance CPB.