Durability of Magnesium Oxychloride Cement in Application: Phase Composition Transition and Microstructure Characteristics

Abstract

To study the durability of magnesium oxychloride cement (MOC) in practical applications, samples from 6 to 80 years in different regions of China, including both north and south China, were collected. Phase composition and microstructure of MOC were analyzed, and phase transition patterns and microstructure characteristics of MOC were explored. The results show that, for MOC material, the main hydration product is 5Mg(OH)2·MgCl2·8H2O (phase 5·1·8), and the carbonation products are Mg(OH)2·MgCl2·2MgCO3·6H2O (phase 1·1·2·6) and 4MgCO3·Mg(OH)2·4H2O (phase 4·1·4). For MOC samples with ages less than 20 years, the content of phase 5·1·8 and the total content of carbonized phases (phase 1·1·2·6 and phase 4·1·4) increases and decreases in turn. The change of the total content of the carbonized phase is opposite to that of the phase 5·1·8 and is affected by phase 5·1·8. A calculation formula for the carbonation degree of MOC material system is proposed. The calculation results show that when the age is less than 14 years, the carbonation rate of MOC samples in the dry environment from north China is slow, but obviously increases when the age exceeds 14 years. Carbonation degree of the MOC samples at 6 years in the high humid environment of south China is close to that of the samples at about 20 years in north China. The crystal morphologies of different phases in MOC are varied. There are needle rod-like and flocculent crystals of phase 5·1·8, short rod-like crystals of phase 1·1·2·6, flaky crystals of phase 4·1·4, and nubbly crystals of MgCO3 in MOC. Phase 5·1·8 ensures the integrity of the glass fiber, while sawdust shows disadvantages on the long-term service of MOC.