Magnesium-based cements, which combine MgO and nesquehonite (MgCO₃·3H₂O), are promising low-carbon binders which sequester CO₂ and gain strength through the formation of hydrous carbonate-containing brucite (HCB). This study analyses the evolution of solid and liquid phases in MgO-nesquehonite suspensions (0 to 60% nesquehonite by mass, w/s = 10) over a year using XRD, TGA, FTIR, Raman spectroscopy and liquid analysis (pH, ionic composition), and compares them with previously studied paste systems (w/s = 1.25). The results show that HCB is the main hydrate after 28 days, while artinite forms in addition in blends with more than 30% nesquehonite after 91 days, consuming CO₂ from HCB. Calculated saturation indices and the evolution of dissolved Mg and inorganic carbon confirm the dissolution of nesquehonite and the subsequent formation of HCB and artinite. Compared with paste systems, suspensions show faster phase evolution and artinite formation due to the higher water availability and continuous mixing. The findings confirm the metastability of HCB and provide data for the thermodynamic modelling of durable MgO-based binders.

Deciphering the fracture mechanisms of concrete at the mesoscale is pivotal for predicting the structural integrity of quasi-brittle materials. This study develops a high-fidelity 3D discrete element method (DEM) framework, characterized by real-shaped, crushable aggregates and a sophisticated contact constitutive model designed to address the inherent limitations of traditional DEM, such as insufficient compression-to-tension ratios and the lack of softening effects. Implemented on the open-source platform MUSEN with GPU-accelerated computing, the model enables large-scale simulations of wedge splitting tests (WST) with unprecedented efficiency. Validation against laboratory experiments confirms the model’s capability to capture the macroscopic mechanical response and the intricate F–CMOD relationship. Through the evolution of internal principal stress fields, the study clarifies how boundary-induced end effects and mesoscopic heterogeneity govern crack initiation and propagation. Furthermore, a parametric analysis of aggregate and interfacial transition zone (ITZ) strengths reveals a fundamental transition from transgranular to intergranular failure modes. The results quantify the crack-bridging and toughening mechanisms provided by aggregates, providing a robust computational tool for the multiscale design and safety assessment of concrete structures.

Magnesium oxide-magnesium carbonate cements are promising alternatives to Portland cement due to their ability to sequester CO₂. This study investigates the effect of sodium orthophosphate on workability, hydration kinetics and hydrate assemblage in MgO/nesquehonite binder to assess its possible use as an admixture. Sodium orthophosphate decreases the yield stress of pure MgO pastes. In case of MgO/nesquehonite blends, the yield stress is much lower than for the pure MgO system and is not significantly further decreased by sodium orthophosphate. Isothermal calorimetry results indicate a delay in the hydration of MgO pastes, increasing with the sodium orthophosphate content, while for MgO/nesquehonite blends, only a moderate retardation is observed. Mortars containing up to 1 mass-% of sodium orthophosphate show similar flexural and compressive strength values as mortars without sodium orthophosphate. Phosphate species sorb onto the surface of MgO resulting in a strongly negative surface charge and retarding the reaction of MgO to brucite. During hydration, magnesium phosphate phases precipitate and phosphate is incorporated into brucite-like phases. Thus, despite a mixed effect of sodium orthophosphate on MgO/nesquehonite cement, it is an encouraging additive to control the kinetics and workability times of MgO-only systems.