Hydrated Portland cement as an admixture to alkali-activated slag cement

Waste concrete powder is a promising mineral admixture to Portland cements and alternative binders such as alkali-activated slag cements (AASC). The individual constituent of the waste concrete powder is mechanically activated in the ground hardened cement paste (GHCP). This influence on the properties of AASC fresh, hardened pastes and hardened concretes was assessed depending on the concentration (5–60%) and the fineness (200–800 m²/kg) of the GHCP, the nature of the alkali activator (sodium carbonate and sodium silicate) and the curing conditions (normal conditions and steam curing). The considered properties for the fresh paste were the water/binder ratio and the setting time; for the hardened paste, the water absorption, density and compressive strength after 2, 7, 14, 28, 180 and 360 d of ageing were assessed. The GHCP accelerated setting and hardening, improved the early strength and long-term strength of the AASC paste and replaced slag by up to 45%. The influencing factors affected the strength of the AASC with GHCP as an admixture in the following order: alkali activator type > curing conditions > GHCP fineness. The accelerating and strengthening effects of the GHCP were more intense under normal curing conditions than under steam curing, and the GHCP activated more AASC using sodium carbonate than sodium silicate. The compressive and flexural strength of concrete samples based on AASCs with 7·5–10% GHCP included improved up to 30–40% compared with the reference samples.