Mechanistic study of the synergistic regulation of strength and microstructure in low-grade-diatomite-based cementitious materials by sodium sulfate and sodium dodecylbenzenesulfonate

Clinker-free cementitious materials (CFCMs) derived from low-calcium solid wastes are promising alternatives to energy-intensive cement owing to their resource efficiency and environmental benefits. However, their application is limited by insufficient early-age strength. Although sodium sulfate (hereafter called SS) and sodium dodecylbenzenesulfonate (SDS) have been used in cement hydration systems, their applications and mechanisms of action in pozzolanic-reaction-dominated, low-calcium CFCMs remain insufficiently studied. In this work, a low-grade diatomite-based cementitious material (LCM) was used as a representative system to investigate the synergistic effects of SS and SDS on the strength development and the microstructural evolution of LCM. The results indicated optimal dosages of 1.5% SS and 1% SDS (mix LCM-N1.5S1). At these concentrations, the compressive strengths at 3, 7 and 28 days were 7.76, 16.34 and 30.26 MPa, respectively, and the flexural strengths were 1.90, 3.64 and 5.58 MPa, respectively. These values indicate early-age strength improvements of 34–42%. LCM-N1.5S1 showed enhanced binding energy of Si2p and S2p on day 3 and day 28, increasing the calcium/silicon ratio to 1.91–3.15 and the aluminium/sulfur ratio to 1.03–3.12. These compositional shifts generated calcium silicate hydrate gel and calcium aluminate phases, producing products with more spherical morphologies and a denser microstructure. Consequently, early- and late-age strengths were improved.