The effect of salt freeze-thaw cycles coupled with carbonation on the mechanical performance of quick hardening sulphoaluminate cement-based reactive powder concrete combined with basalt fibers was investigated. The ratios of basalt fibers in sulphoaluminate cement-based reactive powder concrete (SAC-RPC) were 1%, 2%, 3% and 4% by the volume of concrete. The mechanical strengths (compressive strength, flexural strength and bonding strength) of SAC-RPC were investigated after curing for 5 h, 1 d, 14 d and 28 d, respectively. Meanwhile, the mechanical strengths of resultant concrete were detected, when different NaCl freeze-thaw cycles and carbonation were adopted. Results showed that the addition of basalt fibers could effectively improve the mechanical strengths, especially the flexural strength of SAC-RPC. The dosage of 3.0% was the threshold value affected mechanical strengths. The flexural, compressive and bonding strengths of SAC-RPC were higher than 8.53 MPa, 34 MPa and 3.21 MPa, respectively. The mass loss and mechanical strengths loss of SAC-RPC increased in the form of quadratic function with the increasing number of NaCl freeze-thaw cycles and varied in the form of quadratic decreasing function. Meanwhile, the effect of carbonation on the mechanical strengths of SAC-RPC can be ignored. Additionally, the coupling effect of salt freeze-thaw cycles and carbonation could accelerate the attenuation of concrete strength. The mechanical strengths loss demonstrated a decreased quadratic function with the increasing volume of basalt fibers.

Phosphogypsum is a kind of solid waste which pollutes the environment without reasonable treatment. The application of phosphogypsum in the field of building materials provides an ideal method to solve this problem. Meanwhile, the phosphogypsum mixed with foam can be used for the thermal insulation of wall materials. This paper aims to study the influence of foam volume and cement content on the thermal conductivity, water resistance coefficient and the mechanical strengths of foamed phosphogypsum. The volume of foam in this study ranged from 0% to 60%. Moreover, the influences of humidity and cement content on the mechanical strengths of phosphogypsum are investigated. Scanning electron microscope and the mercury intrusion porosimetry are used for the microscale research and analysis. Results indicate that the relationships between the parameters (thermal conductivity, water resistance coefficient and mechanical strengths) and foam volume fit well with a negatively correlated linear function. The addition of cement can improve the compactness of phosphogypsum and decrease the diameter and volume of pores. Therefore, the thermal conductivity, water resistance coefficient and mechanical strengths of phosphogypsum are increased by the addition of cement.

This paper aimed to investigate the coupling effects of NaCl freeze–thaw cycles and cyclic loading on the mechanical performance and permeability of sulphoaluminate cement reactive powder concrete (RPC). Firstly, the compressive and flexural strengths of sulphoaluminate cement RPC were investigated. Then, the chloride ion permeability, mechanical strengths (compressive and flexural strengths) and mass loss were determined. Results indicated that the increased steel fibers content and curing age played positive roles in the mechanical strengths. The threshold values of steel fibers and curing age were 3.0% and 14 days. Sulphoaluminate cement RPC with early curing age (5 h) showed relatively high mechanical strengths: flexural strength (8.69~17.51 MPa), and compressive strength (34.1~38.5 MPa). The mass loss, the chloride migration coefficient, and the compressive strength loss increased linearly with NaCl freeze–thaw cycles. Meanwhile, the flexural strength loss increased with the exponential function. The relative dynamic modulus of elasticity of specimens decreased linearly with the increased freeze–thaw cycles. Finally, it was observed from this paper, cyclic loading demonstrated negative roles on the mechanical strengths and resistance to chloride penetration.

Waste fly ash (WFA) with pozzolanic activities may be advantageous to the mechanical properties of reactive powder concrete (RPC) when WFA partially replaces cement in RPC. In this study, RPC specimens with 0–25% WFA were prepared under the curing temperatures of 0, 20, and 40 °C for 3 to 120 days. The flowability of fresh RPC, the mechanical strengths, and the NaCl freeze–thaw damage were investigated. Additionally, the following carbonation depths after different NaCl freeze–thaw cycles and the leaching amount of toxic metal elements were also determined experimentally. The results indicated that the incorporation of WFA could decrease the slump flow of fresh RPC due to the relatively smaller particle size of WFA. With an increase in the WFA content, the early-age flexural and compressive strengths first exhibited an increasing and then decreasing trend. However, WFA will always deteriorate the long-term mechanical properties, and both flexural and compressive strengths can be reduced by up to 25% when cured for 120 days. A higher temperature (i.e., 40 °C) was found to benefit the mechanical properties, especially when cured for 3 days. The RPC with 10% WFA exhibited the optimum salt-freezing resistance with an approximately 30% reduction in the mass loss rate when the NaCl freeze–thaw cycles reached 300. The improvement in durability can be attributed to a more compact microstructure of RPC with WFA through microscopic observations. The relationships between the mass and mechanical strength loss rates can be expressed through positive correlation quadratic functions. The carbonation depth decreased following a quadratic function with increasing mass ratios of WFA and NaCl freeze–thaw cycles. The leaching amounts of Cr and Zn increased with increasing WFA content over time, and the cumulative values reached equilibrium at 5 months.

CaO and MgO are both reported as effective activators for blast furnace slag. However, the synergistic effect of these two components on the mechanical properties of alkali-activated blast furnace slag remains unclear. In this study, the flexural and compressive strengths of alkali-activated blast furnace slag powder with MgO and CaO range from 0% to 30% by the mass ratio of alkali-activated blast furnace slag powder are investigated. Moreover, the dry shrinkage rate of alkali-activated blast furnace slag powder is measured. One percent refractory fibers by volume of binder materials are added in the alkali-activated blast furnace slag. Some refractory fibers are treated with water flushing, meanwhile, some refractory fibers are directly used without any treatment. Finally, the scanning electron microscope, the thermogravimetric analysis curves and the XRD diffraction spectrums are obtained to reflect the inner mechanism of the alkali-activated blast furnace slag powder’s mechanical properties. The water-binder ratios of the alkali-activated blast furnace slag powder are 0.35 and 0.42. The curing ages are 3 d, 7 d and 28 d. The measuring temperature for the specimens ranges from 20 °C to 800 °C. Results show that the flexural and compressive strengths increase with the increased curing age, the decreased water-binder ratio and the addition of refractory fibers. The water-treated refractory fibers can improve the mechanical strengths. The mechanical strengths increase in the form of a quadratic function with the mass ratio of MgO and CaO, when the curing age is 3 d, the increasing effect is the most obvious. A higher water-binder ratio leads to an increasing the drying shrinkage rate. The activated blast furnace slag powder with CaO shows a higher drying shrinkage rate. The mechanical strengths decrease with the increasing testing temperature.

This paper aims to study the electrical parameters (electrical resistivity and alternating current (AC) impedance spectroscopy) of cement paste with rice husk ash (RHA). The water to cement (Mass ratio of water to cement (w/c)) ratios of the paste in this study varied from 0.4 to 0.5. The mass ratio of rice husk ash in each w/c ratio of specimens ranged from 0% to 15% by t mass of cement. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) were used to determine the microstructures of specimens. Moreover, the slump flow and plastic viscosity of fresh paste were determined. The results indicated that with the increasing dosage of RHA, the fluidity decreased, while the plastic viscosity increased. Meanwhile, a high w/c ratio led to a low plastic viscosity and high slump flow. The electrical resistivity of RHA cement paste gradually ascended with the increasing curing period. The conduction of specimens intricately changed by mixing RHA, a reasonable equivalent circuit was selected to describe the conduction mechanism by AC impedance spectroscopy. Additionally, the results of XRD and SEM showed that RHA could effectively promote the hydration process as well as decrease the size and number of cracks in hardened cement paste.