Performance of magnesium oxysulfate (MOS) cement with granite powders after combined sulfate and chloride attacks

Abstract

Salt erosion is one of the main factors affecting the durability of cement and concrete. Magnesium oxysulfate (MOS) cement is regarded as a new type of binder with many good features such as lower carbon dioxide emission, excellent fire resistance, etc. However, the salt erosion resistance of MOS cement is rarely studied. This research investigated the performance of MOS cement, with granite powders (GPs) as mineral admixture, soaked in NaCl and Na2SO4 solutions. X-ray diffraction (XRD) and simultaneous comprehensive thermal analyzer (TG-DSC) were used to detect the phase composition and hydration products of MOS cement. Scanning electron microscope (SEM) and mercury porosimeter (MIP) were used to analyze the pore structure and microstructure of MOS cement. The results showed that the value of the salt attack coefficient of MOS cement with 50 % by weight GPs reached 0.58 and 0.57 after immersion in NaCl and Na2SO4 solutions for 230 days, respectively. The compressive strength of MOS cement with 30 % by weight GPs reached the maximum value of 74.1 MPa after standard curing for 28 days by optimizing the pore structure. Both needle-like and plate-like Na2SO4·xH2O crystals were observed in the hydration products. Gypsum and Na2SO4·xH2O grew in the pores and matrix, generated expansion stress and destroyed the matrix structure of MOS cement. During the soaking period, the decomposition of the 5·1·7 phase and MgSO4·nH2O negatively affected the salt resistance of MOS cement. Physical and chemical erosion leads to a decrease in the value of the salt attack coefficient.