As an advanced cement-based composite material, ultra-high performance concrete (UHPC) exhibits outstanding properties. However, the scope and scale of UHPC applications are limited by several issues, such as high hydration temperature rise, insufficient elastic modulus, high autogenous shrinkage (AS), and high cost. Incorporating coarse aggregate (CA) can partially overcome the limitations of traditional UHPC. The characterization of the properties, AS, and cracking risk at an early age is important for the design and application of UHPC with CA (CA-UHPC). The temperature stress test machine (TSTM) was employed to analyze the impact of CA content on the early-age temperature evolution, AS, and cracking risk of CA-UHPC under uniaxial restrained condition. The correlation between the performance of CA-UHPC and its microstructure was analyzed through the image analysis and Scanning Electron Microscope (SEM) test. Experimental results demonstrated that the AS and cracking risk of CA-UHPC decreased with an increase in the CA content. A modified model was proposed to predict the AS of CA-UHPC, considering the impact of CA content. Two critical parameters were utilized to evaluate the cost-performance ratio of CA-UHPC comprehensively. Considering the properties, AS, cracking risk, and cost-performance ratio, the optimal CA content for CA-UHPC was 400 kg/m³.

This paper mainly explores the effects of the relative ratio of tri-potassium phosphate (K₃PO₄) and potassium dihydrogen phosphate (KH₂PO₄) on the setting time, fluidity, mechanical properties and microstructure of magnesium phosphate cement (MPC) prepared by lightly burned magnesia. Results show that the composite phosphate with a higher proportion of K₃PO₄ results in an excellent retarder effect. The longest setting time of 132 min. Comprehensive consideration of setting time and mechanical properties, the superior mass ratio of K₃PO₄/KH₂PO₄ is 6/4. Under this condition, the setting time and 28 d compressive strength of MPC are 67 min and 66.4 MPa, respectively. The retarder mechanism of composite phosphate for MPC is that the higher pH value of the liquid phase is acquired with a higher proportion of K₃PO₄, which can inhibit the dissolution of MgO, leading to a decrease in the hydration heat release rate. Microscopic tests indicate that the increase in the proportion of K₃PO₄ in composite phosphate reduces the formation of hydrated product MgKPO₄·6H₂O (MKP), but more K₃PO₄ promotes the MKP crystal morphology change from prismatic to plate and increases the amount of amorphous K-struvite that fills the gaps between particles, which is conducive to the decrease in porosity and harmful pore proportion.

Cementitious capillary crystalline waterproofing (CCCW) materials are a type of rigid waterproofing materials. In this study, the effect of CCCW materials on the frost resistance of panel concrete (PC) has been investigated to solve the problem of concrete freeze–thaw damage in cold regions. Experiments were conducted to test the influence of different contents of CCCW materials on the apparent damage, compactness, strength and permeability of concrete; Results showed that after freeze–thaw cycles, the mass loss of concrete mixed with CCCW materials was relatively small, and the reduction rate of RDME, ultrasonic velocity, and splitting tensile strength of concrete decreased, indicating that its frost resistance was improved. The improvement effect was most significant when the CCCW materials content was 2%; When the content of CCCW materials becomes larger (> 2%), the microhardness of concrete decreases, and the pore structure becomes looser compared to low content, this phenomenon is consistent with the microstructure shown in the SEM images. Based on the complexation–precipitation reaction, this paper proposed the mechanism of CCCW materials on the frost resistance of concrete in freeze–thaw environment. And used the two parameters Weibull distribution, a freeze–thaw damage model was established with the splitting tensile strength of concrete as the indicator. Furthermore, this study proposed a theoretical model of ultrasonic velocity in concrete with CCCW materials, and the predicted value of this model has a small error compared to test data.

Ultra-High Performance Fiber-Reinforced Concrete (UHPFRC) effectively controls crack width, enhances tensile properties, and improves concrete toughness. This study explores the UHPFRC combination with normal concrete (NC) to form a composite beam, addressing the issues related to UHPFRC's material properties and application costs. The proposed nonlinear analytical solution, utilizing an equivalent stress block model, was employed in this study. Additionally, sensitivity tests were conducted on critical parameters such as UHPFRC thickness and NC compressive strength. Verification involved designing UHPFRC-NC composite beams with distinct steel fiber volume ratios. The results indicate that nonlinear analysis accurately reflects material behavior, contributing significantly to section design. Properly designed composite beams can meet safety requirements, economic considerations, and mechanical properties.

This study investigated the physical properties of hardened cement incorporating biomass ash (BA), considering the effect of the ratio of nonbridging oxygen atoms to tetrahedral-forming ions (NBO/T), a chemical indicator of ash. The reactivity of fly ash (FA) exhibited strong positive correlation with NBO/T, which can also serve as a reactivity indicator for other supplementary cementitious materials like slag. Chemical analysis revealed that BA possesses properties of both FA and blast furnace slag. During curing, the number of voids corresponding to small and large gel pores, formed by coagulated C–S–H, increased significantly, with sizes reaching tens of nanometers. Fourier transform infrared and aluminum nuclear magnetic resonance analyses showed that cement incorporating BA had higher compressive strength than coal ash-based cement, with higher contents of ettringite, C–A–S–H, and monosulfate after 28 days. This was attributed to the distinct compositions of coal and BA, alongside the usage of seawater and gypsum. Moreover, BA further enhanced the compressive strength owing to the influence of hydration products, including C–S–H. This positive correlation between NBO/T of BA and the compressive strength of the cured material suggests that NBO/T can serve as an indicator for estimating cement blending strength.

This paper is the English translation from the authors’ previous work [Takagi, R., Hayashi, T., Kamimura, K. and Saito, T., (2023). “Effect of NBO/T of biomass ash on compressive strength of hardened cement.” Cement Science and Concrete Technology, 77, 153-162. (in Japanese)].