Mechanical and hydration properties of ground granulated blastfurnace slag pastes activated with MgO–CaO mixtures

Since alkali-activated slag using conventional activators suffers from economical and technical problems, other alternative activators should be explored. This paper reports the results of an investigation into the activation of ground granulated blastfurnace slag by using 10% (by weight) reactive MgO, CaO and their mixtures with various ratios. The mechanical and hydration properties of pastes were examined up to 90 days. It was found that the strength of slag pastes activated with MgO–CaO mixtures decreased with the increasing ratio of MgO to CaO in the early age while a much steeper strength gain was observed in pastes with MgO/CaO higher than 19/1 after 28 days and longer. The addition of small amount of CaO in MgO can greatly accelerate the hydration of slag in the early age by increasing the pH of pore solution. However, pastes showed small difference in strength development at each period when MgO/CaO was less than 1. The main hydration products, analysed by XRD, TGA and SEM/EDS, were C–S–H and hydrotalcite-like phases. While CaO accelerated the formation of C–S–H in the early age, MgO induced more amount of hydrotalcite-like phases, which notably enhanced the strength of slag pastes with high MgO content after 28 days and longer. Calcite, portlandite and residual MgO were also observed, depending on the MgO/CaO ratio and the hydration time. This work indicated that the replacement of MgO by CaO can make the application of reactive MgO in slag activation more economical.