Microstructure evolution and strength variation of magnesium oxysulfate materials of varying thicknesses during exposure

Abstract Magnesium sulfate oxide (MOS) is an innovative lightweight material distinguished by its high strength and notable engineering potential. This study examined the influence of specimen thickness (12 mm and 15 mm) and elevated humidity conditions (relative humidity of 80%–90%) on the performance of MOS over a 28-day period. Material characterization was performed using Brunauer–Emmett–Teller (BET) surface area analysis, x-ray diffraction (XRD), and scanning electron microscopy (SEM). Reducing the thickness to 12 mm accelerated early-stage reactions, as indicated by a free-water loss of 5.5% within 3 to 7 days, compared to 3.6% for the 15 mm specimens. This faster water loss promoted more rapid formation of the 5·1·7-phase. The flexural strength of the 12 mm samples reached a peak of approximately 22 MPa at 7 days, representing a 25% increase relative to the 15 mm specimens. However, by 28 days, the thinner samples exhibited extensive microcracking. Under high-humidity conditions, the morphology of these microcracks evolved from T-shaped configurations at 7 days to I-shaped patterns at 28 days, eventually forming a network or river-like structure by 112 days. Concurrently, the pore structure underwent significant changes. The cumulative specific pore volume decreased from 0.0551 cm 3 g −1 at 7 days to 0.0288 cm 3 g −1 at 112 days, while the cumulative specific surface area increased from 4.03 m 2 g −1 to 11.04 m 2 g −1 over the same period. Flexural strength peaked at approximately 22.7 MPa at 28 days before slightly declining to around 21.4 MPa at 112 days. These findings highlight the interrelated effects of specimen thickness and humidity on both the microstructural and macroscopic properties of MOS. Overall, the results offer valuable guidance for the application of MOS in environments with high moisture levels.