Influence of Mineral Fillers and Curing Conditions on the Hydration, Microstructure, and Mechanical Performance of MgO-Based Cements

Abstract

Magnesium oxysulfate (MOS) cement, also known as basic magnesium sulfate cement (BMSC), is a low-alkalinity binder with potential for sustainable construction due to its lower calcination requirements and compatibility with alternative magnesium oxide sources. This study investigated the effects of partially replacing MgO with limestone filler and the addition of citric acid (CA) on hydration phase formation, physical properties, and mechanical performance of MOS cement. The influence of curing conditions was evaluated using mixtures containing calcitic limestone to isolate the curing effect, while filler comparison was conducted under ambient curing (23 ± 2 °C and 60% relative humidity). Results showed that mixtures without CA mainly formed the 318 phase, Mg(OH)₂, and residual periclase, together with carbonate phases such as dolomite, magnesite, calcite, and aragonite. The addition of CA promoted the formation of the 517 phase, leading to an increase of up to 78% in compressive strength compared to the reference MOS cement. The highest strengths were obtained under curing at 80% relative humidity, whereas thermal curing (60 °C and 90% RH) resulted in lower mechanical performance due to increased brucite formation.