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.

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.

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.

In the field of construction, the promotion of prefabricated buildings has been strongly supported by the state due to its low-carbon, environmental protection and high-efficiency characteristics. The process of design, prefabrication, and installation, is restricted by factors such as unsound policy standards, insufficient technological innovation, lack of professional talents, and high costs, which have led to the slow development of prefabricated buildings in China. The main factors that restrict the development of prefabricated buildings in Yancheng are identified from the researcher’s point of view by literature review and questionnaire survey method. The degree of centrality and cause of each constraint has been analyzed by the decision-making laboratory method (DEMATEL), and the interpretation structure method (ISM) was used to build a multi-level hierarchical structure model of constraints, the logical relationship, hierarchical relationship and relative importance of each constraint are clarified. It is concluded that industry policies, imperfect standards and insufficient government publicity are the fundamental reasons to hinder the development of prefabricated buildings in Yancheng. According to the order of the centrality, the main restrictive factors are determined, which benefits the establishment of the homologous counterplan for the vigorous promotion of prefabricated buildings in Yancheng.

Waste fly ash (WFA) and secondary aluminum dross (SAD) are common solid wastes inducing environmental pollution. These materials contain certain active substances that can be used in cement-based materials. Therefore, cement concrete can be used to solidify these solid wastes. In this study, the influence of the assembly unit of secondary aluminum dross (SAD) and waste fly ash (WFA) on the properties of ultra-high-performance concrete (UHPC) is investigated. The slump flow, the plastic viscosity, the yield shear stress, and the initial setting time of fresh UHPC are measured. Moreover, the flexural and compressive strengths and the dry shrinkage rate (DSR) are determined. The electrical resistance and reactance are tested. The electron microscopy spectroscopy (EDS), thermogravimetric analysis (TG), and X-ray diffraction spectrum (XRD) curves are obtained for revealing the mechanism of macroscopic performances. Results show that due to the optimal specific surface area and the volcanic ash effect, the UHPC with the assembly unit of 50% SAD and 50% WFA provides the highest slump flow, DSR, and mechanical strengths, while the corresponding plastic viscosity, yield shear stress, and electrical resistance are the lowest. The SAD can delay the setting time of UHPC. The relationship between the electrical resistance or the electrical reactance and the mass ratio of SAD accords with the quadratic function. The corresponding electrical resistance is the lowest. The relationship between the mechanical strengths and the electrical resistance fits with the cubic function. The leaching amounts of Z_n and C_r increase in the form of cubic function with the immersing time. Meanwhile, the SAD can decrease the Z_n and C_r by 0%–46.3% and 0%–45.2% respectively. As obtained from the EDS results, the element of Al is increased by adding SAD. The XRD curves show that the crystals of Al₂O₃ are increased and the SiO₂ crystals are decreased by the added SAD. UHPC with 50% SAD exhibits the highest compact microstructures and the least Ca(OH)₂ and 3CaO·S_iO₂ hydration products. The TG results show that UHPC with 50% SAD shows the lowest TG values of all the groups. This research will provide new UHPC materials and techniques applied in solidifying the WFA and SAD in the future.

To investigate the dynamic performance of self-compacting concrete (SCC), the dynamic uniaxial compression tests at eight different loading strain rates were performed on the ordinary concrete and SCC cubic specimens. Based on the tests, the compression failure patterns and stress–strain curves of both kinds of concrete were obtained. The results show that SCC performs more brittle than ordinary concrete by showing the diagonal crack failure pattern of SCC at a high strain rate. Besides, with the increase of loading strain rate, the peak compressive stress of SCC is slightly lower than that of ordinary concrete, but the increase of elastic modulus is slightly higher than that of ordinary concrete. The peak compressive strains of the two kinds of concrete are discrete under the influence of loading strain rate, thus putting forward the relation equation for the loading strain rate and peak compressive stress increase coefficient of the two kinds of concrete. Besides, based on the theory of elastic–plastic damage and considering the dynamic extension of damage, the dynamic constitutive relation with good applicability between ordinary concrete and SCC was established.