Relationships between microstructural properties and compressive strength of consolidated and unconsolidated cemented paste backfills

Few studies have been so far done on the pore structure behaviour of cemented paste backfill (CPB) samples cured under pressure, which might be more representative of the field conditions. This is most likely due to a lack of suitable laboratory equipment and test procedures. In this paper, the effects of curing conditions on changes in CPB microstructure and corresponding unconfined compressive strength (UCS) were assessed using both an improved laboratory apparatus called CUAPS (curing under applied pressure system) and conventional plastic moulds. The CUAPS-consolidated and plastic mould-unconsolidated CPB samples were prepared with a slag-based binder which is a blend of 20 wt.% of ordinary Portland cement and 80 wt.% of blast furnace slag. CPB microstructural properties are compared for binder contents of 3, 4.5 and 7 wt.%, and curing times of 7, 14 and 28 days. Mercury intrusion porosimetry (MIP) results show that changes in intrusion porosity (and thus in microstructure) strongly depend on the water-to-cement (w/c) ratio and drainage conditions (consolidated vs. unconsolidated). Indeed, as w/c ratio decreases or curing time increases, total porosity is reduced, mainly due to the reduction in larger-sized pores by partial filling with cementitious phases. Specific surface area (SSA) measurements show good correlation with UCS values, meaning that the UCS of unconsolidated CPB increases with increasing SSA. Different regression models between UCS and MIP parameters (threshold diameter, total-, meso-, and macro-porosity) and SSA of CPB are also proposed and discussed.