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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. [ABSTRACT FROM AUTHOR]

The application of CO2 curing on sludge ceramsite may improve its mechanical properties, and then increase the corresponding corrosion resistance. In this study, the influence of CO2-cured sludge ceramsite on the strength and long-term properties of cement concrete is investigated. CO2 curing time ranges from 0 h to 2 d. The cylinder compressive strength and water absorption rate of CO2-cured sludge ceramsite are first determined. Additionally, the flexural and compressive strengths, the chloride permeability and the freeze—thaw damage, as well as the corresponding thermal conductivity of cement concrete, are tested. Furthermore, the corrosion resistance of reinforcement inner-sludge-ceramsite cement concrete is measured. Finally, the scanning electron microscope photos of sludge ceramsite are obtained. Results show that the cylinder compressive strength of CO2-cured sludge ceramsite is 15.1, ~34.2% higher than that of sludge ceramsite. Meanwhile, the water absorption rate of CO2-cured sludge ceramsite is 39.6, ~82.4% higher than that of sludge ceramsite. The compressive strength and the flexural strength of cement concrete with CO2-cured sludge ceramsite are 11.4 and 18.7, ~21.6% and ~31.5% higher than the cement concrete with sludge ceramsite, respectively. The resistance of NaCl freeze—thaw cycles, determined by comparing the mass loss rate and the loss rates of mechanical strengths, is effectively improved by CO2 curing, while the thermal conductivity of cement concrete is decreased by CO2 curing. The corrosion resistance of inner reinforcement is improved by the application of CO2 curing on sludge ceramsite. [ABSTRACT FROM AUTHOR]

Carbon nanotubes (CNTs) are very effective in improving the performance of cement-based materials. Mechanical properties and pore structure were investigated for cement mortar with CNTs. Meanwhile, the composite morphology of CNT–cement material and the evolution of hydration products were observed by scanning electron microscope (SEM), and the quantitative relationship between mechanical properties and pore structure was analyzed. The results indicated that the strength of mortar increased with the addition of 0.05% CNTs and decreased when the fraction of CNTs increased to 0.5%. The porosity of mortar with dispersed CNTs increased significantly, as these pores may be introduced by the dispersant. The quantitative relationship between porosity and strength proved that the increased porosity is the reason for the decreased strength of mortar with 0.5% CNT content, while mortar matrix strength with 0.05% and 0.5% CNTs increased by 44.03% and 71.18%, respectively. SEM images show that CNTs are dispersed uniformly in the mortar without obvious agglomeration and that the CNTs and hydration products form a meshwork structure, which is the mechanism by which CNTs can enhance the strength of the cement matrix. [ABSTRACT FROM AUTHOR]