Stability and phase transition of 5·1·7 phase in alkaline solutions. (20th October 2020)

• Record Type: Journal Article
• Authors: Zhao, Jiuye; Xu, Jiahui; Cui, Chunyi; Yu, Chunyang; Chang, Jun; Hu, Zhiqi; Bi, Wanli
• Is Part Of: Construction & building materials. Volume 258(2020)
• Publication Date: 2020-10-20
• Subjects: 5·1·7 phase -- Magnesium oxysulfate cement -- Compressive strength -- Microstructure -- Rietveld method; Building materials -- Periodicals; 624.18
• DOI: 10.1016/j.conbuildmat.2020.119683
• Languages: English
• ISSNs: 0950-0618

Abstract

Highlights: Stability of 5·1·7 phase in alkaline solutions of different pH values is investigated. 5·1·7 phase is stable when immersed in distill water and alkaline solutions of pH ≤ 11. 5·1·7 phase decomposes to be Mg(OH)2 and MgSO4 when the pH values of immersing solutions exceed 12. Decomposition of 5·1·7 phase leads to increasing porosity of the harden pastes. Abstract: 5Mg(OH)2 ·MgSO4 ·7H2 O (5·1·7 phase) is considered to be the main strength phase of magnesium oxysulfate (MOS) cement. In this paper, effect of alkaline solutions on stability of 5·1·7 phase is investigated. Harden pastes containing nearly pure 5·1·7 phase are synthesized and characterized. Compressive strength of 5·1·7 phase, immersed in sodium hydroxide solutions of different pH values for different ages, are compared. X-Ray diffraction (XRD) test and Rietveld quantitative phase analysis (RQPA) are employed to determine phase transition in the harden pastes, meanwhile ion chromatograph (IC) and inductively coupled plasma (ICP) were conducted to confirm ionic concentrations of immersing sodium hydroxide solutions. Moreover, through scanning electron microscope (SEM) and mercury intrusion porosimetry (MIP) test, influence of alkaline solutions on microstructure of 5·1·7 phase is analyzed. The results show that, immersed in distill water and relatively low alkaline (pH ≤ 11) solutions, 5·1·7 phase keeps to be stable and the compressive strength tends to be approximately constant with immersing ages prolonging, meanwhile contents of 5·1·7 phase in the harden pastes tended to be almost constant during the immersion and still accounted for over 90 wt% until 28d. In condition of pH ≥ 12, alkaline solutions show negative effects on the mechanical property of 5·1·7 phase, which appears to be more obvious with pH value increasing. Moreover, 5·1·7 phase decomposes to be Mg(OH)2 and MgSO4 when the pH values of immersing solutions exceed 12, which can be reflected by decrement of 5·1·7 phase contents in harden pastes and obvious increasement for concentrations of Mg 2+ and SO4 2- in immersing solutions. Additionally, decomposition of 5·1·7 phase would lead to increasing porosity of the harden pastes.