Use of air-cooled blast furnace slag as supplementary cementitious material for self-compacting concrete production

Abstract

The manufacturing process of pig iron generates large amounts of slag. The slow cooling of this waste generates the air-cooled blast furnace slag (ACS). Since it is less reactive than granulated blast furnace slag (GBFS), ACS finds little use as supplementary cementitious material (SCM) and the existing studies use it only as aggregates in concrete. This work investigated the use of finely ground ACS as SCM for self-compacting concrete (SCC) production. Pastes and SCCs with GBFS, ACS in two grinding levels and limestone filler (LF) were produced, and their fresh state performance, hydration kinetics and compressive strength from 3 to 91 days were evaluated. ACS increased the yield stress and viscosity of the pastes compared with LF, also increasing the SCC’s superplasticizer content required to reach the target slump flow of 650 ± 50 mm. Nonetheless, this increase was from 0.95 up to 1.36 kg/m3 of concrete, and all the mixes presented V-funnel time of 4–6 s and were highly stable in the fresh state (VSI = 0, with no segregation/bleeding). Isothermal calorimetry indicated that ACS slightly shortened the induction period compared with LF, but led to equivalent peak values (6.7–6.9 mW/g cement). Concretes with ACS and LF had similar strengths at all ages, reaching 32 ± 1 MPa at 3 days and 41 ± 2 MPa at 91 days, indicating that no significant reaction was observed for ACS up to the later age. Furthermore, blending ACS with LF in equal proportions resulted in a SCC with fresh performance equivalent to that containing only LF, in addition to increasing the strength by about 5% at all ages. In general, ACS had comparable performance to that of LF and can be used as SCM in SCC.