Long-term mechanical properties and micro mechanism of magnesium oxychloride cement concrete

According to the C20 Portland cement concrete (PCC) batching standard, magnesium oxychloride cement concrete (MOCC) is made using bischofite from salt lakes and light-burned magnesium oxide as the main raw materials. In this work, the mechanical properties of MOCC (setting time and compressive and flexural strengths) were tested. The rules governing the phase composition and microstructure changes of the hydration products were analysed using X-ray diffraction and scanning electron microscopy. The results showed that MOCC solidifies rapidly, has high early compressive and flexural strengths and exhibits excellent long-term mechanical properties, far superior to those of the same grade of C20 PCC. The main phase at different curing times was found to be 5Mg(OH)₂.MgCl₂.8H₂O (5.1.8). With hydration from 7 to 365 d, the amount of 5.1.8 fluctuated between 30% and 37% and the microscopic crystal morphology of the MOCC changed significantly. Importantly, fibre-like and rod-like crystals were observed to transform into their respective gel phases from 180 to 270 d. This phenomenon explains the fact that the content of 5.1.8 was the lowest but the mechanical strength of MOCC was the highest at 180 and 270 d. It was also confirmed that 5.1.8 gel is one of the main strength phases.