Reactive rice husk ash (RHA) is used as a supplementary cementitious material (SCM) to prepare cement composite pastes. The impact of RHA content and the internal curing effect on the hydration process of the cementitious system was studied. The hydration heat, degree, and product content of the cement–RHA composite system at 3, 7, and 28 d were analyzed using hydration microcalorimetry, thermogravimetry, and XRD (Rietveld) analysis. The results show that with the increase in RHA, the main exothermic peaks move forward, and the values increase. The induction period is prolonged, and the acceleration period is shortened. The induction period of 15% RHA is extended to 3 h. The hydration heat of cement composite pastes is mainly divided into three stages. Namely, the first stage (0–18 h) is the superposition of the RHA nucleation effect and chemical effect, the second stage (18–51 h) is the superposition of the dilution effect and internal curing effect, and the third stage (51–72 h) is the internal curing effect with the water-release. The internal curing effect of RHA has a certain periodicity, which is related to its content. The water-release age in the early stage (24 h) advances with the increase of content, and the water-release effect in the later stage (7–28 d) is also significant with the increase of content. The higher the content, the more significant the promotion of the internal curing effect on cement hydration and the pozzolanic reaction of RHA.

Low strength and low impact toughness are two of the main issues affecting the use of lightweight aggregate concrete in harsh cold environments. In this study, the strength of concrete was improved by adding high-strength fibers to bear tensile stress and organize crack propagation. Four sets of comparative experiments were designed with freeze–thaw cycles of 0, 50, 100, and 150 to study the mechanical properties of fiber-reinforced lightweight aggregate concrete under freeze–thaw conditions. A detailed study was conducted on the effects of freeze–thaw on the compressive strength, flexural strength, impact toughness, and microstructure of concrete with different fiber contents (3, 6, and 9 kg/m³). The results show that for ordinary lightweight aggregate concrete, under the freeze–thaw cycle, the internal pore water of the concrete froze and generated expansion stress, resulting in tensile cracks inside the concrete. The cracks gradually accumulated and expanded, ultimately leading to cracking and damage of concrete structures. After 150 cycles, the strength loss rate exceeded 25%. When adding a reasonable amount of fiber (6 kg/m³), the fiber took on the tensile stress and hindered the development of internal cracks, significantly enhancing the splitting tensile strength, flexural strength, and impact toughness of lightweight aggregate concrete. And the failure pattern of concrete was significantly improved. At the beginning of the freeze–thaw cycle, the internal tensile stress was less than the fiber tensile strength and the fiber–matrix bonding strength, and the strength reduction rate of the concrete was slow. Relying on the friction absorption capacity between the fiber and the matrix, the fiber used its own deformation to resist the tensile stress. In the late stage of the freeze–thaw cycle, due to the destruction of the fiber–matrix transition zone structure, the bond strength decreased, the crack resistance and toughening effect decreased, and the strength of the concrete decreased rapidly. Moreover, the reduction in impact toughness was greater than the compressive strength and flexural strength under static load.

Adding expansion agents to compensate for concrete shrinkage is a common crack resistance technique, but excessive expansion can also increase the porosity of concrete and reduce its strength. The addition of fibers can reduce expansion and improve the compactness of concrete. However, too little fiber will not be effective in inhibition, while too much fiber will cause aggregation. In this study, steel fiber and MgO expansive agent were used at the same time, and the effect of fiber on the mechanical properties of MgO concrete was studied. The results showed that the appropriate amount of MgO (8%) could compensate for the shrinkage of concrete and slightly improve the strength of concrete. When the content reached 10%, MgO produced excessive expansion under free conditions, which reduced the strength of the concrete. After using MgO and steel fiber at the same time, steel fiber could restrain the expansion of MgO, improve the compactness of concrete, produce a “super superposition” benefit, and increase the strength of concrete by 20%. In addition, the reinforcing effect of steel fiber on MgO was closely related to its distribution. In the composite system, steel fiber not only played a “bridge role” but also needed steel fiber to effectively restrain the expansion of MgO and produce self-stress. Only when the steel fibers were evenly distributed could reliable bonding be ensured between the fibers and the matrix, and at this time, the fibers could restrain the expansion of MgO. Considering the uniformity of steel fiber distribution and construction cost, adding 8% MgO and 1% steel fiber has the maximum benefit.

Fully utilizing solid waste as supplementary cementitious materials (SCMs) while ensuring the mechanical properties of cement-based materials is one of the pathways for carbon reduction in the cement industry. Understanding the effects of the two solid wastes-limestone powder (LP) and rice husk ash (RHA) on the mechanical properties of cement-based materials is of great significance for their application in concrete. This study investigates the impact of LP and RHA on the strength of cement mortar at various ages and the microhardness of hardened cement paste. The results suggest that two materials have a certain synergistic effect on the mechanical properties of the cementitious materials. The addition of RHA effectively addresses the issues of slow strength development, insufficient late-stage strength of the cementitious material, and the low strength blended with a large amount of LP, while a suitable amount of LP can promote the strength increase in the cement-RHA system. Based on the comprehensive analysis of compressive strength and microhardness, the optimal solution for achieving high mechanical properties in composite cementitious materials is to use 10% each of LP and RHA, resulting in a 9.5% increase in 28 d strength compared to a pure cement system. The higher the content of LP, the greater the increase caused by 10% RHA in compressive strength of the composite system, which makes the strength growth rate of cementitious material mixed with 10% LP at 3–56 d 62.1%. When the LP content is 20% and 30%, the addition of 10% RHA increases the 28 d strength by 44.8% and 38.8%, respectively, with strength growth rates reaching 109.8% and 151.1% at 3–56 d.

This study investigates the effects of the combined addition of MgO expansive agent (MEA) and rice husk ash (RHA) on the performance of concrete. Results show that MEA absorbs water and competes with superplasticizers for adsorption, reducing early-age fluidity. In the later stages, its reaction with RHA generates M-S-H gel, accelerating slump loss. At early ages (up to 7 days), due to the slow hydration of MEA and partial replacement of cement, fewer hydration products are formed. Additionally, the pozzolanic reaction of RHA has not yet developed, resulting in the low early strength of concrete. In the later stages, Mg(OH)₂ fills pores and enhances compactness, while the pozzolanic reaction of RHA further optimizes the pore structure. The internal curing effect also provides the moisture needed for continued MEA hydration, significantly improving later-age strength. Moreover, in the post-cast strip of a tall building, the internal curing effect of RHA ensures the effective shrinkage compensation by MEA under low water-to-cement ratio conditions. The restraint provided by reinforcement enhances the pore-filling effect of Mg(OH)₂, improving concrete compactness and crack resistance, ultimately boosting long-term strength and durability.

This study comprehensively evaluates the mechanical properties, shrinkage behavior, and durability of concrete prepared with limestone- and granite-manufactured sands under standard-curing and steam-curing conditions. The results indicate that limestone-manufactured sand concrete consistently exhibits superior compressive strength and splitting tensile strength across all curing ages, outperforming granite-modified counterparts. The introduction of granite-manufactured sand significantly degrades these mechanical properties, with deterioration intensifying as granite content increases. Dynamic elastic modulus and damping ratio analyses reveal that limestone-based concrete maintains the highest dynamic stiffness and lowest energy dissipation under both curing regimes, suggesting fewer internal defects. In contrast, granite incorporation reduces the dynamic elastic modulus and increases the damping ratio, reflecting structural deterioration and enhanced energy loss. Drying shrinkage tests demonstrate that limestone concrete achieves the lowest shrinkage deformation throughout the testing period, even under steam-curing conditions. Conversely, granite addition markedly elevates shrinkage, particularly under steam-curing conditions, leading to compromised volumetric stability. Durability assessments highlight that manufactured sand concrete exhibits higher capillary absorption, electrical flux, and porosity, attributed to inherent material defects and the surface characteristics of manufactured sand. Granite-modified concrete further weakens interfacial shear strength between aggregates and cement paste, indicating poor interfacial bonding. Steam curing exacerbates microstructural defects, emphasizing the need to optimize curing protocols. The findings propose strategies for enhancing manufactured sand concrete performance: improving interfacial adhesion between aggregates and cement paste, rationalizing supplementary material dosages, and refining steam curing regimes. These measures offer potential pathways to develop high-performance manufactured sand concrete with balanced mechanical and durability properties.