Behaviour of autoclaved magnesium oxysulfate cement pastes

This study investigates the influence of autoclave curing on Magnesium Oxysulfate Cement cement compared to acid addition effects, especially on microstructure and mechanical properties. For this purpose, compressive and flexural strength, water resistance and volume stability were analysed. In addition, X-ray diffraction and Scanning Electron Microscopy were applied to verify the phase composition and microstructure of these matrices. Results indicate that the autoclave process affects the mechanical properties. The increase in the molar ratio reduces flexural strength and slightly increases the compressive strength. The autoclave also reduces water resistance, especially with increasing curing time. Longer autoclaving time (4 h) increases expansion in the pastes, which stabilize in around 30 days. In the microstructure, autoclaving reduces or inhibits the formation of the 517 phase, which is responsible for increased mechanical strength, and provides the formation of the Mg(OH)2 and 512 or 513 phase. Thus, in MOS cement, the autoclave process mainly promotes the formation of 512 or 513 phases. Therefore, the material can be improved to be submitted to curing in an autoclave and present an increase in mechanical properties, which will provide the large-scale production of lightweight boards in the construction materials industry and may influence the increase in the use of this material in construction.