Investigation of the effects of silica source and superplasticizer type and dosage in MgO-SiO2 systems

This study established a unified mechanistic framework linking superplasticizer chemistry, silica source characteristics, and hydration regimes in MgO-SiO2 binders. Two complementary investigations were conducted: a dosage-dependent comparison of superplasticizers— polycarboxylate ether (PCE) and sodium hexametaphosphate (SHMP)—and a systematic evaluation of five different silica sources (microsilica, metakaolin, fly ash, rice husk ash, and olivine silica) under identical conditions. In low w/b systems, higher PCE (5%) dosages improved dispersion, accelerated hydration, and enhanced strength, whereas SHMP retarded early hydration at higher dosages but developed superior strength at low dosages (1%). Across silica sources, hydration followed a coupled MgO-silica-influenced continuum, governed by physical and chemical characteristics of the silica sources and superplasticiser chemistry. SEM revealed that each silica source produced a unique morphology. Distinct M-S-H structure-saponite, serpentine, and talc like were confirmed by 29Si-NMR, corelating with strength hierarchy (M-MK > M-MS > M-RHA > M − OS > M-FA). These insights provide the first dosage-dependent and source specific mechanistic insights into MgO-SiO2 hydration, guiding the design of binder-specific admixtures for efficient M-S-H formation.