Formation of Graphene Agglomerates in MOC and Their Negative Influence on Material Properties

Magnesium oxychloride cement (MOC) is a promising low-carbon alternative to Portland cement; however, its application is limited by poor water resistance and insufficient understanding of microstructure-property relationships. This study presents a multiscale investigation of graphene (G) incorporation in MOC-based composites, explicitly linking microstructural features to mechanical and fracture behavior. Composites containing 0.05-1.0 wt % of G were prepared and compared with a graphene-free reference. Microstructure was characterized using XRD, OM, SEM-EDS, mercury intrusion porosimetry (MIP), and XCT, enabling quantitative assessment of porosity and graphene agglomeration. Mechanical performance was evaluated by compressive and flexural strength tests and crack mouth opening displacement (CMOD)-controlled fracture experiments. Hygric behavior was assessed through water absorption and residual strength. At low dosage (0.05 wt %), graphene improved compressive strength and reduced 24 h water absorption from 3.01% to 2.75%. In contrast, higher graphene contents (≥0.5 wt %) led to agglomerate formation exceeding 300 μm in size, increasing total open porosity from 9.3% to 11.5% and effective transport porosity from 5.1% to 12.4%. Despite this porosity coarsening, residual compressive strength increased up to 51.0 MPa, and the softening coefficient improved from 65% to 89%, indicating partial suppression of water-induced degradation due to graphene hydrophobicity. Fracture energy remained stable at ∼290 N·mm^-1, confirming preservation of quasi-brittle behavior. The novelty lies in the integrated multiscale approach (XRD, SEM-EDS, MIP, XCT, CMOD), enabling quantitative differentiation of graphene agglomerates from air voids and direct structure-performance correlation. The results establish graphene dosage limits in MOC and demonstrate that performance is controlled by microstructural stability rather than graphene content alone.