Study on Deterioration Process of Magnesium Oxychloride Cement under the Environment of Dry-Wet Cycles

Chenggong Chang; Lingyun An; Jinmei Dong; Weixin Zheng; Jing Wen; Fengyun Yan; Xueying Xiao

doi:10.3390/ma16051817

Study on Deterioration Process of Magnesium Oxychloride Cement under the Environment of Dry-Wet Cycles

Materials (Basel). 2023 Feb 22;16(5):1817. doi: 10.3390/ma16051817.

Authors

Chenggong Chang^{1

2

3}, Lingyun An⁴, Jinmei Dong^{2

3}, Weixin Zheng^{2

3}, Jing Wen^{2

3}, Fengyun Yan¹, Xueying Xiao^{2

3}

Affiliations

¹ State Key Laboratory of Advanced Processing and Recycling of Non-Ferrous Metals, Lanzhou University of Technology, Lanzhou 730050, China.
² Key Laboratory of Comprehensive and Highly Efficient Utilization of Salt Lake Resources, Qinghai Institute of Salt Lake, Chinese Academy of Sciences, Xining 810008, China.
³ Key Laboratory of Salt Lake Resources Chemistry of Qinghai Province, Xining 810008, China.
⁴ Qinghai Provincial Key Laboratory of Nanomaterials and Technology, College of Physics and Electronic Information Engineering, Qinghai Minzu University, Xining 810007, China.

Abstract

To reveal the deterioration process of magnesium oxychloride cement (MOC) in an outdoor, alternating dry-wet service environment, the evolution of the macro- and micro-structures of the surface layer and inner core of MOC samples as well as their mechanical properties and increasing dry-wet cycle numbers were investigated by using a scanning electron microscope (SEM), an X-ray diffractometer (XRD), a simultaneous thermal analyser (TG-DSC), a Fourier transform infrared spectrometer (FT-IR), and an microelectromechanical electrohydraulic servo pressure testing machine. The results show that as the number of dry-wet cycles increases, the water molecules gradually invade the interior of the samples, causing the hydrolysis of P 5 (5Mg(OH)₂·MgCl₂·8H₂O) and hydration reactions of unreacted active MgO. After three dry-wet cycles, there are obvious cracks on the surface of the MOC samples, and they suffer from warped deformation. The microscopic morphology of the MOC samples changes from a gel state and a short, rod-like shape to a flake shape, which is a relatively loose structure. Meanwhile, the main phase composition of the samples becomes Mg(OH)₂, and the Mg(OH)₂ contents of the surface layer and inner core of the MOC samples are 54% and 56%, respectively, while the P 5 amounts are 12% and 15%, respectively. The compressive strength of the samples decreases from 93.2 MPa to 8.1 MPa and reduces by 91.3%, and their flexural strength declines from 16.4 MPa to 1.2 MPa. However, their deterioration process is delayed compared with the samples that were dipped in water continuously for 21 days whose compressive strength is 6.5 MPa. This is primarily ascribed to the fact that during the natural drying process, the water in the immersed samples evaporates, the decomposition of P 5 and the hydration reaction of unreacted active MgO both slow down, and the dried Mg(OH)₂ may provide the partial mechanical properties, to some extent.

Keywords: deterioration process; dry–wet cycles; magnesium oxychloride cement; mechanical properties; microstructure; phase composition.

Grants and funding

The authors would like to acknowledge Applied Basic Research in Qinghai Province (2021-ZJ-737), the Top Talents program of “Kunlun Talents High-end Innovation and Entrepreneurship Talents” in Qinghai Province, the Western Young Scholars program at the Chinese Academy of Sciences (2021), and the Youth Innovation Promotion Association at the Chinese Academy of Sciences (No: 2019423), who collectively funded this project.