Basic magnesium sulfate cement products exposed to air at various exposure ages: Phase composition, microstructure, and mechanical characteristics

Basic magnesium sulfate cement (BMSC) is a kind of air hardening cementitious material with the advantages of high strength, light weight, fire resistance, low alkalinity. However, the durability of the main binding phase (phase 5) of BMSC should be verified. In this work, the flexural strength of BMSC boards with ages between 0.25 years and 11years was investigated. The phase composition and microstructure of BMSC samples, including BMSC boards and the sample of BMSC beam-column joint that hydrates at low temperature were studied. The results reveal that the flexural strength of glass fiber-reinforced BMSC boards is higher than that of ordinary BMSC specimens, and the higher the content of phase 5 is, the greater the flexural strength is. Phase 5 within BMSC boards exposed to the air shows good environment durability, and the change of its content shows little relationship with the service age of BMSC. The phase composition of BMSC samples with long ages still predominantly consists of phase 5, and the microstructure of BMSC remains dense. Besides, the incorporation of air entraining agent can effectively improve the pore distribution of BMSC and generate numerous capillary pores, which may benefit the frost resistance of BMSC. Additionally, even at low temperatures (around 0 °C), BMSC concrete can be hardened to achieve the design strength and fly ash can be dissolved in the matrix of BMSC to form new gel phases.