Effects of NaOH and gypsum contents on the properties of cemented paste backfill using NaOH/gypsum-activated slag binders

In this study, the early age (<= 28 days) unconfined compressive strength (UCS) values of cemented paste backfill (CPB) samples with five different NaOH contents (i.e. 2 %, 4 %, 6 %, 8 % and 10 % by slag weight) and four different gypsum contents (i.e. 0 %, 10 %, 20 % and 30 % by binder weight) are investigated. The UCS development of several typical CPB samples from 3 days up to 180 days of curing was also examined, with the CPB sample using ordinary Portland cement (CPB-OPC) as the reference sample. The results demonstrate that the effects of NaOH and gypsum contents on the UCS of CPB samples using NaOH/gypsum-activated slag (NGAS) as binders depend on the NaOH-gypsum-slag interactions and their respective contents. CPB-N6AS-G20 samples with 6 wt% NaOH and 20 wt% gypsum achieved the highest UCS values at 3 days (2.1 MPa), 28 days (6.2 MPa) and 180 days (7.3 MPa) among all the samples studied, which is 1.4, 2.3 and 2.6 times higher than that of the reference sample (CPB-OPC), respectively. Therefore, the optimum NaOH and gypsum contents (ONG) in CPB matrices were found to be 6 wt% and 20 wt%, respectively. The presence of gypsum in CPB matrices was found to accelerate the hydration process of slag by facilitating the complete formation of ettringite (AFt) and promoting amorphous phase development at early ages (<= 28 days), resulting in a denser microstructure and increased strength, and vice versa. The higher compressive strengths of CPBs was attributed to the higher content of amorphous hydration products. However, the combined effects of higher NaOH content (6 wt%) and high gypsum content (30 wt%) result in long-term strength loss of CPB samples. In conclusion, using NaOH/gypsumactivated slag binders with ONG in CPB demonstrates the potential to achieve higher early age strengths (at 3 days and 28 days) and long-term strength development while also offering significant economic and environmental benefits compared to NaOH-activated slag binders and OPC.