Effectiveness of alkali-activated slag as alternative binder on workability and early age compressive strength of cemented paste backfills

A full understanding of the workability and unconfined compressive strength (UCS) of cemented paste backfill (CPB), made of alkali-activated slag (AAS) as an alternative binder, is critical for effectively applying the AAS technology to most modern mines. In this paper, the effects of solid content, binder dosage, activator to binder (Al/Bi) ratio, sodium silicate to sodium hydroxide (SS/SH) ratio, and curing temperature on fresh and hardened properties of AAS-CPB mass were experimentally investigated after curing ages of 3, 7, 14 and 28 days. Results have shown that for a given backfill recipe and curing time, AAS-CPB matrix provides far better workability and UCS (2.2-3.3 fold) performance than OPC-CPB matrix. The workability of fresh AAS-CPB matrix decreases with solid content and SS/SH ratio, and increases with an increase in binder dosage and Al/Bi ratio. The strength increases with solid content and binder dosage, but at a decreasing rate. At low levels, an increase in the Al/Bi ratio leads to higher UCS, while extremely high Al/Bi ratio hinders the hydration of Slag, resulting in lower UCS. It is also found that the SS/SH ratio has a major effect on UCS of AAS-CPB matrix, depending on curing age. At early ages (3 days), AAS-CPB matrix experiences a gradual increase in UCS up to a SS/SH ratio of 1, followed a small decrease with a further increase in the SS/SH ratio. Moreover, elevated temperature gives higher 3-day UCS, but notably inhibits strength gaining beyond 3 days. The UCS performance of AAS-CPB matrix subjected to expose curing at 3 days is higher than sealed curing sample, and then almost levels out. The findings of this study will be useful for the final design of more cost-effective, durable and environmental friendly AAS-CPB matrix at ambient conditions. (C) 2019 Elsevier Ltd. All rights reserved.