Mechanical properties of eco-cement mortar containing MgO-modified phosphorous slag

Phosphorous slag (PS), is currently utilized in concrete industries with a low cement replacement rate of 20-30%, due to its low pozzolanic activity. This study was thus performed to enhance the pozzolanic activity of PS by chemical modification with MgO additions of 0 %, 5 %, and 10 %. The addition of MgO facilitates the conversion of structural units from Q4 to Q3 and Q2, favoring the depolymerization of silicate glass. Composite cements were prepared by replacing 40 wt% of Portland Cement (PC) with modified PS (MPS). Significant retardations in setting time were observed for the S-CMPS mortars, due mainly to the low early reactivity and presences of phosphate and fluoride, which can be mitigated by increasing the MgO content in MPS. The MPS richer in MgO released more active Si and Al to take part in pozzolanic reaction, contributing to intensified CH consumptions and C-S-H and C-A-H formations. The paste of S-CMPS10, acquired higher compressive strengths than those for SPC and other S-CMPS pastes at both 28 and 90 days of curing, with SEM images showing a compact microstructure of more gel phases and less pores. These results indicate that the modification with MgO addition is an effective way to enhance the pozzolanic activity of PS, which can be potentially utilized as an economic and environmental cementitious material. However, there are also some limits in this study, for instance, effects of temperature and leaching behavior on the mechanical properties of S-CMPS pastes, which are planned in the future research to enhance the sustainable utilization of PS.