Research and Engineering Application of Salt Erosion Resistance of Magnesium Oxychloride Cement Concrete

Study on the Deterioration Mechanism of Magnesium Oxychloride Cement under an Alkaline Environment

The deterioration process and deterioration mechanism of magnesium oxychloride cement (MOC) in an alkaline environment were studied using a scanning electron microscope (SEM), an X-ray diffractometer (XRD), a Fourier transform infrared spectrometer (FT-IR) and a micro-electro-hydraulic servo pressure testing machine to investigate the effects of soaking time in 10 wt.% NaOH solution on the macro- and micro-morphology, phase composition and compressive strength of MOC samples. The results show that the deterioration of MOC samples under an alkaline environment is mainly caused by the alkaline environment providing more OH- ions, which can react with 5Mg(OH)2·MgCl2·8H2O (P 5) in the sample. The resulting reaction gives rise to a faster decomposition of 5Mg(OH)2·MgCl2·8H2O (P 5) and a substantial reduction in the strength of the sample, and finally leads to a gradual deterioration of MOC samples. Meanwhile, immersion time exhibits a significant effect on MOC samples. The extension of immersion time coincides with more OH- ions entering the sample, and the greater presence of OH ions increases the likelihood that more P 5 will produce a hydrolysis reaction, further resulting in the increased deterioration of the sample. After soaking for 6 h in alkaline media, the main phase composition of the surface layer of an MOC sample changes to MgO and Mg(OH)2, and its microscopic morphology is also dominated by round sheets, giving rise to a sharp decrease in its compressive strength (52.2%). When the immersion time is prolonged to 72 h, OH- ions have already immersed into the inner core of the sample, causing the disappearance of P 5 from the whole sample. At the same time, both the surface and inner core of the sample exhibit a disc-shaped morphology, and chalking phenomena also appear on the surface of the sample. This reduces the compressive strength of the sample to 13.5 MPa, only 20% of its compressive strength in water. The compressive strength of the sample after 120 h of immersion is as low as 8.6 MPa, which is lower than that of the sample dipped in water for 21 days (9.5 MPa). As a result, the MOC samples studied in alkaline environments exhibit a faster deterioration rate, mainly because of a faster hydrolysis reaction by P 5, caused by more OH- ions.

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

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.

Study on Deterioration Law and Mechanism of Gray Brick Due to Salt Crystallization

Salinization has an important impact on the degradation of ancient masonry buildings, and systematically mastering the law of salt migration and degradation of ancient masonry buildings is an important part of the protection of ancient buildings. In this paper, the damage law of gray bricks under the action of salt crystallization is studied. The orthogonal test method is used to carry out cyclic degradation tests on gray bricks. The nominal strength is proposed as a mechanical parameter to measure the structural damage of grey bricks, and the change in compressive strength and crystallization pressure of the samples after the test is measured and analyzed. The results show that the damage of different salts in the gray bricks shows a certain difference. Magnesium sulfate and sodium chloride cause significant damage to the surface of the gray bricks, while calcium chloride does not cause significant damage to the surface of the gray bricks. When the concentrations of sodium chloride solution, calcium chloride solution and magnesium sulfate solution are less than 13.73 mol/L, 11.47 mol/L and 17 mol/L, respectively, the nominal strength of gray brick samples increases; In the range of 9.9 mol/L and 4.73–8.94 mol/L, the crystallization pressure began to appear inside the sample. The research results provide an important scientific basis for evaluating the damage caused by salting to the damage of porous ancient building materials such as masonry.

Effect of Calcination Temperature on Mechanical Properties of Magnesium Oxychloride Cement

In order to make full use of magnesium chloride resources, the development and utilisation of magnesium oxychloride cement have become an ecological and economic goal. Thus far, however, investigations into the effects on these cements of high temperatures are lacking. Herein, magnesium oxychloride cement was calcinated at various temperatures and the effects of calcination temperature on microstructure, phase composition, flexural strength, and compressive strength were studied by scanning electron microscopy, X-ray diffraction, and compression testing. The mechanical properties varied strongly with calcination temperature. Before calcination, magnesium oxychloride cement has a needle-like micromorphology and includes Mg(OH)₂ gel and a trace amount of gel water as well as 5 Mg(OH)₂·MgCl₂·8H₂O, which together provide its mechanical properties (flexural strength, 18.4 MPa; compressive strength, and 113.3 MPa). After calcination at 100 °C, the gel water is volatilised and the flexural strength is decreased by 57.07% but there is no significant change in the compressive strength. Calcination at 400 °C results in the magnesium oxychloride cement becoming fibrous and mainly consisting of Mg(OH)₂ gel, which helps to maintain its high compressive strength (65.7 MPa). When the calcination temperature is 450 °C, the microstructure becomes powdery, the cement is mainly composed of MgO, and the flexural and compressive strengths are completely lost.