In this study, strength and microstructural development of full (FT) and coarse sulphidic tailings (CT) cemented paste backfill (CPB) produced from sodium-silicate activated slag (SSAS) at different silicate modulus (Ms) were investigated in the short- and long-term. SSAS samples (SSASs) with varying Ms for both FT and CT produced 1.5-3.5 fold unconfined compressive strengths in the long-term compared to ordinary Portland cement samples (OPCs). Optimum Ms values were 1.0-1.25 for FT and 1.25-1.50 for CT considering the short- and long-term strength gain and microstructural properties of CPBs based on the polymerization degree and balance of C-S-H gel. Strength losses were observed in OPC-CT and in SSASs at Ms = 0.75 for FT and CT in the long-term. Formation of secondary expansive minerals such as ettringite, decalcification of C-S-H gel and the weakening of microstructure were found to be the main reasons for the strength losses due to the coupled effect of acid and sulphate attack. The use of CT and SSAS together significantly improved the microstructure of CPB. Increase in Ms decreased the porosity, refined the pore structure providing more compact microstructure and alleviated the decalcification of C-S-H gels in consequence of higher rate of polymerization. (C) 2018 Elsevier Ltd. All rights reserved.