With the accelerating pace of global urbanization, the generation of concrete waste has reached enormous levels and is expected to remain high in the foreseeable future. Converting concrete waste into recycled concrete powder (RCP) through crushing and grinding and using it as a partial cement replacement in recycled mortar/ concrete represents a key pathway for the high-value utilization of concrete waste. This approach not only mitigates excessive reliance on cement but also contributes to significant reductions in CO2 emissions. RCP primarily consists of hydration products such as Ca(OH)2 and C-S-H gel, along with inert phases including SiO2 and CaCO3, resulting in relatively low reactivity. Direct substitution of cement with RCP generally compromises the microstructure, mechanical strength, and durability of cementitious materials, with performance deterioration becoming particularly pronounced at high replacement ratios. In recent years, researchers worldwide have employed CO2 mineralization (carbonation) technology to enhance the RCP reactivity and improve the micromacro performance of RCP-blended cementitious materials, yielding a wealth of encouraging findings. This review focuses specifically on the utilization of CO2 mineralization for enhancing the performance of RCP and its blended cementitious materials. It provides a systematic overview of RCP and carbonated recycled concrete powder (CRCP) production, carbonation technology, carbonation mechanisms, with particular emphasis on the influence of CRCP on the microstructure, hydration kinetics, mechanical properties, and durability of cementitious materials. This review aims to consolidate existing advanced research and provide a robust theoretical foundation and practical guidance for the application of CRCP in low-carbon cementitious materials.

With the rapid development of structural health monitoring, intelligence has become an important trend in cement-based materials. Recycled carbon fibers (RCF) feature favorable electrical conductivity. This study investigates the effects of varying fiber contents on the piezoresistive properties of cement mortar. The relationship between electrical signals and stress under failure and cyclic loading was analyzed. Piezoresistive performance was evaluated in terms of linearity, sensitivity, repeatability, and hysteresis, and the damage-sensing capacity was explored via acoustic emission tests. The results reveal that the incorporation of RCF significantly enhances the piezoresistive properties of cement mortar, exhibiting a favorable linear correlation between the fractional change in resistance and stress. Acoustic emission signals and resistivity variations can effectively characterize the internal damage evolution of mortar. Under cyclic loading, recycled carbon fiber cement mortar (RCFCM) presents good repeatability and stability. With rising loading amplitude, the irreversible damage of the conductive network aggravates, and the hysteresis effect increases.

Strain-hardening geopolymer composites (SHGC) exhibit low-carbon characteristics and excellent mechanical performance, but the conventional precursors used in SHGC preparation are limited in supply and expensive. Using recycled brick powder (RBP) ground from calcined clay brick waste as a green precursor for fabricating ultra-high ductility SHGC promotes the high-value utilization of calcined clay brick waste while decreasing the reliance on conventional mineral precursors such as fly ash (FA) and slag. This investigation systematically studied the microstructural characteristics and mechanical performance of RBP-SHGC. Incorporating RBP as a moderate substitute for FA and slag had negligible influence on the microstructure and geopolymeric products of SHGC, but substituting RBP for high-dosage slag negatively affected the microstructure. The mechanical strength initially rose and then decreased with the increasing substitution ratios of RBP for FA and slag, though RBP-blended SHGC maintained considerable strength even at 100% replacement rates. Substituting RBP for an appropriate proportion of FA and slag improved both the uniaxial tensile strength and strain capacity. The SHGC in which RBP simultaneously replaced 50% of FA and 50% of slag exhibited an ultimate tensile strain comparable to that of the reference SHGC. Raising the alkali-activator modulus and alkali content effectively improved the microstructure and mechanical behavior of RBP-blended SHGC.

The enhancement of waste concrete powder (WCP) which occurs within alkali-activated systems is aimed to improve its utilization. Therefore, a multiscale framework integrating molecular dynamics (MD) simulations and experimental study was developed in the article. The study aims to elucidate the interaction of alkali content and silicate modulus on activation kinetics, gel formation, and pore structure evolution. MD simulations revealed that an 8 % alkali content led to synchronized jumps in the Mean Square Displacement (MSD) of Na+ ions and OH- ions at approximately 1.8 x 10(6) timesteps, indicating rapid depolymerization and channel connectivity, followed by accelerated polycondensation. Although 10 % alkali content promoted faster initial dissolution, excessive ion strength caused reaction passivation in later stages. Combined Radial Distribution Function (RDF) and Qn analysis confirmed that enhanced Na-O inner-sphere coordination and increased Q(0) species signify more favorable activation at 8 % alkali content. Replacing 25 % of metakaolin (MK) with WCP led to a 19.54 % improvement in 28-day compressive strength and 14 % improvement in 28-day flexural strength. Notably, thermal curing resulted in a substantial increase in compressive strength ranging from 45.36 % to 152.96 %, accompanied by a marked decrease in water absorption and porosity. However, higher WCP content or over-alkalization increased drying shrinkage and dehydration risks. The optimum mixture was identified as 75 % WCP activated with 8 % alkali and 1.5 silicate modulus under thermal curing. This study offers mechanistic insights and practical guidance for the performance optimization and sustainable use of WCP in alkali-activated binder systems.

Incorporating recycled carbon fiber (RCF) into cement-based materials not only alleviates the environmental burden associated with carbon fiber waste disposal but also improves the mechanical performance of cement-based composites. Nevertheless, the low surface chemical reactivity and poor hydrophilicity of RCF often lead to weak fiber-matrix interfacial bonding and non-uniform fiber dispersion in recycled carbon fiber cement mortar (RCFCM). In this study, the surface of RCF was physically modified, and the optimal ratio of the modification solution was determined through fiber characterization and mechanical testing of RCFCM. The effects of fiber length and dosage on mortar performance were also evaluated. The results show that the chitosan hydrochloride-sodium dodecyl sulfonate (CHI-SDS) modification introduced hydroxyl groups onto the RCF surface, reducing the contact angle by 44.73%, strengthening fiber-matrix adhesion, and increasing flexural and tensile strengths by 33.23% and 19.09%, respectively. Polycarboxylate superplasticizer (PCE) modification decreased the contact angle by 36.88% and improved fiber dispersion through electrostatic repulsion and steric hindrance; at a solution concentration of 20%, the flexural and tensile strengths increased by 29.11% and 10.27%, respectively. Incorporating 12 mm fibers achieved the greatest improvement in mechanical properties, while 6 mm fibers enhanced electrical conductivity. Overall, CHI-SDS modification improved interfacial bonding and significantly enhanced mechanical and electrical properties at higher dosages, whereas PCE modification promoted better dispersion and yielded notable improvements even at lower dosages.

Ultra-high-performance concrete (UHPC), owing to its outstanding mechanical strength and durability, is increasingly used in structural engineering. This study systematically investigates the size effect of small UHPC cylinders under uniaxial compression. The experimental program covers five strength classes (UC120-UC200), three diameters (30, 40 and 50 mm), and aspect ratios H/D = 1.0-3.0. The influence of strength class, specimen size and aspect ratio on compressive strength is quantified by the size-effect degree (eta) and the size-effect conversion coefficient (C). Test results show that compressive strength decreases with increasing specimen size, with this reduction amplified at higher aspect ratios; small-diameter specimens (especially 30 mm) exhibit the greatest size sensitivity. Bazant's size effect law (SEL) is employed to fit the data across strength classes and specimen sizes and to derive strength conversion relationships. Finite-element simulations in ABAQUS reproduce the observed failure patterns and confirm the dependence of the size effect on aspect ratio. The proposed sizeeffect indices, SEL-based laws and conversion coefficients provide a practical basis for converting strengths between UHPC cylinders of different sizes and for calibrating structural design using small-scale specimens.

Ultra-high-performance concrete (UHPC) is recognized for its excellent mechanical properties and durability, but its high carbon footprint and cost limit large-scale use. Alkali-activated UHPC (A-UHPC) has emerged as a greener alternative, yet its performance, especially under elevated temperatures, remains underexplored. This study provides a multi-scale comparison of cement-based UHPC (C-UHPC) and A-UHPC, focusing on mechanical properties, shrinkage, thermal resistance, microstructural changes, and simplified CO2 and cost assessments. At ambient conditions, fiber-free prismatic (40 × 40 × 160 mm) and cubic specimens (70.7 mm) were tested for compressive and flexural strength, ultrasonic pulse velocity, porosity, and capillary absorption. At elevated temperatures, hybrid fiber-reinforced specimens were used to assess thermal resistance. Microstructural analyses of PP-fiber-reinforced paste samples were conducted using SEM, MIP, TGA, FTIR, and XRD. Results indicate that C-UHPC exhibits higher strength, lower shrinkage, and a denser microstructure at room temperature due to C-(A)-S-H gel formation, but experiences rapid deterioration above 600℃ from matrix cracking and hydration loss. In contrast, A-UHPC, though initially weaker, shows more gradual degradation due to the formation of stable crystalline phases, such as nepheline and augite. Microstructural analysis confirms distinct transition pathways and damage patterns between the two systems. Environmentally, A-UHPC reduces CO2 emissions by over 40

Developing damage self-sensing intelligent concrete to achieve real-time monitoring of structural health is crucial for ensuring the long-term and safe operation of major infrastructure. Recycled carbon fibers (RCFs) from pyrolysis of waste carbon fiber composites were introduced into concrete matrix to prepare conductive recycled carbon fibers concrete (RCFC) in this study. The self-sensing performance based on electrical characteristics of RCFC under compressive and bending loads was evaluated. The influence of factors such as RCFs volume dosage, loading modes (cyclic and monotonic), loading rate, and loading amplitude on self-sensing performance was considered. Scanning electron microscopy (SEM) observation was used to explain self-sensing behavior of RCFC. The results showed that incorporating RCFs enhanced the self-sensing performance of concrete under cyclic compression. However, this enhancement was affected by the RCFs dosage, loading rate, and loading amplitude. The stress sensitivity of RCFC exhibited a non-monotonic trend, initially increasing and then decreasing with rising RCFs content. When the volume fraction of RCFs was between 0.4% and 0.6%, RCFC achieved good self-sensing performance. The effect of loading amplitude on the stress sensitivity was not significant. When the cyclic loading rate was 3kN/s, the loading amplitude was 4 MPa, and the RCFs volume fraction was 0.4%, the stress sensitivity was 3.18%/MPa, which was 16.7 times higher than that (0.18%/MPa) of the reference concrete without RCFs. SEM results showed that RCFs constructed a conductive network in concrete by overlapping with each other. When a load was applied, significant and regular changes in the conductive network were triggered by changes in fiber contact state or fiber breakage, thereby achieving good self-sensing ability.

The utilization of recycled powder (RP) as an alternative to conventional binders in foam concrete constitutes a viable method for realizing sustainable construction. However, systematic comparative studies on RP foam concrete across different matrix systems under identical environmental conditions remain scarce. Therefore, this study innovatively utilized the matrix type as a key variable to systematically investigate the multi-scale performance evolution of recycled powder foam concrete. The results indicate that the foam concrete in both systems exhibits respective advantages and disadvantages in terms of macroscopic properties. The alkali-activated recycled powder foam concrete (A-RPFM) exhibits a higher specific strength; at the A06 density grade without the incorporation of recycled powder, the specific strength is 148.05% of that of the cement-based group. However, it is accompanied by greater shrinkage and thermal conductivity. In terms of the mesoscopic pore structure, A-RPFM features finer and more regular pores with smooth pore walls. At the same time, there are also some large pores and microcracks on the pore walls. Finally, microstructural analysis shows that the reaction products of A-RPFM are dense and continuous amorphous gels, including calcium aluminosilicate hydrate (C-A-S-H) and sodium aluminosilicate hydrate (N-A-S-H). In contrast, cement-based recycled powder foam concrete (C-RPFM) produces calcium silicate hydrate (C-S-H) as well as numerous crystalline phases such as ettringite and portlandite. RP has only a slight adverse effect on the strength of A-RPFM; at a 30% RP replacement rate, the 28-day compressive strength retention of A06-grade A-RPFM remains at 75.49%, compared to a sharp decline to 56.36% in C-RPFM, and it improves the drying shrinkage performance and reduces the formation of microcracks.

Inevitable drawbacks of rapid setting and inadequate hydration, along with inherently elongated pores, would emerge in the cement mortar during 3D printing, seriously when improper additives were employed. To address this issue, the effect of renewable and non-hazardous tannin acid (TA) retarder on the workability and mechanical property of 3D printed cement mortar (3DP mortar) was expected in this study. Chemical bonding interactions triggered by TA were proposed in a calcium-rich hydrated environment of 3DP mortar, particularly with a focus on carboxyl functional groups from polycarboxylate superplasticizer (PS). Chelation and particular esterification were confirmed, yielding TA-calcium chelates and TA-PS esters, respectively. Fragmented chelates were anchored randomly, while extensively polymerized esters further established robust cross-links with lamellar crystals and reticular gel matrices in specific hydrated structure. A widespread distribution was visible via their amorphous and fluorescent morphology, leading to reduced hydration sites and extended setting. Consequently, continuous printability and stable buildability were enhanced through optimized rheological behavior with a larger thixotropic loop, despite reductions in initial fluidity and slump. After complete hydration for 90d at 25 degrees C/65%, 100% recovered mechanics were achieved in both bending and compressive strength, especially with TA-induced reinforced zones clearly identifiable at the nanoscale. This highly depended on an increased proportion of refined spheroidal pores evolved by optimally introduced TA-induced products, particularly maintaining pH and crystalline pattern. Therefore, the TA-induced products significantly contributed to enhancing workability, stabilizing pore geometry and improving energy dissipation in 3DP mortar, which is essential for broadening its practical applications and environmentally sustainable development.

This study comprehensively examines the combined effects of alkali content and silicate modulus on the mechanical properties, moisture transport behavior, and microstructural evolution of alkali-activated ultra-high performance concrete (A-UHPC) under both ambient and elevated temperatures. Unlike previous studies that focused separately on ambient or high-temperature performance, this research offers a holistic evaluation of the manner in which these parameters influence A-UHPC across varying thermal conditions. Experimental evaluations, including mechanical strength tests, moisture transport analysis, and microstructural characterization (SEM, TGA, XRD, FTIR), demonstrate that the optimal synergy of 6 % alkali content and a 1.6 M silicate modulus results in superior compressive strength (132.60 MPa), reduced porosity, and enhanced durability. At elevated temperatures, this formulation retains structural integrity up to 400 degrees C but experiences substantial deterioration at 800 degrees C due to phase transformations and microstructural degradation. These findings contribute critical insights for optimizing A-UHPC to achieve enhanced mechanical performance and fire resistance, laying a foundation for its broader application in extreme environments.

This study systematically investigates the influence of various curing regimes (including standard, single and combined methods) on the macroscopic and microscopic performance of alkali-activated ultra-high performance concrete (A-UHPC) under both ambient and elevated temperatures. The objective is to address the challenge of optimizing curing conditions for A-UHPC, particularly in fire-exposed environments where thermal stability and durability are critical yet insufficiently studied in previous works. In this work, standard, single (dry heat, steam, hot water), and combined (dual-stage) curing methods were investigated. Mechanical behavior was evaluated through compressive and flexural strength tests, ultrasonic pulse velocity, and water absorption, while microstructural features were examined using SEM, XRD, FTIR, TGA and MIP analyses. The results suggest that combined curing methods, especially the sequence of hot water followed by steam curing (CC3), tend to enhance early alkali activation, improve matrix compactness, and support better mechanical performance at room temperature. Under high-temperature exposure (up to 800 degrees C), CC3-cured samples generally showed more favorable results in terms of structural stability, mass loss, and residual density, although performance varied depending on specific conditions. In contrast, specimens cured by dry heat alone displayed relatively high porosity and lower strength, likely due to insufficient early alkali activation. Overall, the findings highlight that the choice and sequencing of curing regimes may play an important role in improving both early-age and high-temperature behavior of A-UHPC. By demonstrating how single and combined curing enhances both mechanical performance and microstructural stability, this work provides insights for developing durable, fire-resistant alkali-activated materials suited for infrastructure operating under elevated temperature conditions.

To promote industrial solid waste recycling and sustainable development, this study prepared alkali-activated recycled aggregate concrete (AARAC) using ground granulated blast-furnace slag and recycled coarse aggregate. The effects of alkali content and recycled aggregate replacement ratio on AARAC performance were examined. Results indicated that while recycled aggregate can impair concrete properties, an optimal alkali content of 6% significantly mitigates these effects, improving physical properties, mechanical stability, and compactness. Microstructural analysis showed enhanced hydration product formation and a denser interfacial transition zone at this alkali level. Entropy weight-TOPSIS evaluation confirmed that 6% alkali content provided the best overall performance with low sensitivity to aggregate variation. Additionally, carbon emission analysis revealed that AARAC reduced CO2 emissions by approximately 45% compared to conventional recycled concrete. These findings suggest that alkali-activated recycled aggregate systems offer a promising approach to developing low-carbon, sustainable construction materials.

Upcycling of recycled brick powder (RBP) as an eco-friendly precursor for sustainable geopolymer is an effective pathway for the high-value recycling of clay brick waste. This study investigates the feasibility of preparing completely recycled geopolymer by substituting RBP for both FA-GGBS precursors and river sand (RS), aiming to optimize the RBP substitution method and percentages to achieve a sustainable geopolymer with high performance, as well as to reduce the demand for precursors and RS. RBP exhibits certain alkali-activation reactivity and favorable micro-aggregate filling effects, demonstrating the potential to simultaneously replace both the precursor and aggregate. Substituting a high proportion of RBP for precursors is detrimental to the geopolymerization reaction, resulting in a deterioration in the micro-macro properties of geopolymer. However, replacing an appropriate amount of RS with RBP can refine the microstructure and enhance the mechanical strength and permeability resistance of geopolymer. At an appropriate substitution rate of RBP, the positive effects of replacing RS with RBP on the geopolymer performance can offset the adverse effects of replacing precursors with RBP. By optimizing the RBP substitution percentages for precursors and RS, completely recycled geopolymer meeting various performance requirements can be prepared. When the RBP substitution rate for precursor-RS is 0 %-0 %, 30 %-0 %, 0 %-30 %, 30 %-30 %, and 50 %-50 %, the 28-day compressive strengths of the blending geopolymer are 69.0, 63.3, 78.1, 67.4, and 55.0 MPa, respectively.

Cementless alkali-activated ultra-high performance concrete (A-UHPC) offers remarkable mechanical properties, durability, and sustainability, making it a promising material for advanced construction applications. However, its use in precast concrete components has been limited due to the lack of research on curing methods. This study systematically investigates the effects of steam curing at varying temperatures (60 degrees C-90 degrees C) and durations (1, 3, and 7 days) on the mechanical properties, permeability, and microstructure of A-UHPC. The results demonstrate that steam curing significantly enhances early strength, with temperatures between 60 degrees C and 80 degrees C optimizing compressive and flexural strengths, reducing porosity, and improving impermeability, as evidenced by lower capillary water absorption. Microstructural analyses, including SEM, XRD, FTIR, TGA and BET, reveal the formation of hydration products, which contributes to a denser material with a more refined pore structure. However, curing at 90 degrees C for 7 days can lead to performance deterioration due to thermal-stress-induced cracking and uneven hydration. A comparison with standard curing at 28 days highlights the superior efficiency of steam curing in accelerating strength development and improving microstructure, offering a time- and energy- efficient solution for precast concrete applications. This study provides critical insights for the optimization of steam curing regimes, advancing the use of A-UHPC in sustainable, highperformance precast concrete construction.

Multifunctional intelligent concrete, characterized by its electrical properties, presents a broad spectrum of applications in contemporary infrastructure development. This study seeks to develop an economically viable and highly conductive recycled carbon fiber concrete (RCFC) by integrating recycled carbon fiber (RCF) into the concrete matrix. The research investigated the influence of various factors on the conductivity of RCFC, including RCF types (1# RCF and 2# RCF), RCF lengths (6 mm, 12 mm, and 18 mm), RCF volume dosages (0 %, 0.1 %, 0.2 %, 0.3 %, 0.4 %, 0.5 %, and 0.6 %), curing ages (1d, 3d, 7d, 14d, and 28d), moisture content (ranging from 0 % to 100 % relative moisture content), and temperature (spanning from 20 degrees C to 100 degrees C). The conductivity mechanism of RCFC was further examined and elucidated using scanning electron microscopy (SEM). The findings demonstrated that increasing RCF content significantly reduced resistivity of RCFC, with longer fibers achieving the percolation threshold at lower dosages. At equivalent RCF dosages, the resistivity of RCFC increased with the curing age. Additionally, the resistivity of RCFC decreased with rising moisture content and temperature. Upon reaching the percolation threshold, the sensitivity of concrete conductivity to moisture content and temperature was markedly reduced. SEM observations confirmed that the formation of a conductive network via overlapping RCFs, contributing to the enhancement of concrete electrical properties.

To more accurately simulate the alkaline environment of basalt fiber in cement-based materials, this study innovatively employs the benchmark cement supernatant as the alkaline solution for the long-term corrosion testing of basalt fiber (BF). Additionally, through mechanical performance testing of mortar containing basalt fiber at different ages, the study reveals the corrosion behavior of basalt fiber in an alkaline environment. The results indicate that basalt fiber undergoes corrosion in the benchmark cement supernatant, where the OH- ions in the solution react with SiO2 in the basalt fibers, thereby disrupting the silicate framework of the fibers and causing pitting corrosion on the fiber surface. The black and white binarization analysis of the SEM images of BF7 after 360d erosion revealed that the corrosion pits covered 31.50 % of the fiber surface area, indicating severe corrosion. Furthermore, the study on the long-term mechanical properties of basalt fiber mortar demonstrated an initial increase in performance, followed by a gradual decline as the curing age progressed. Notably, after 360d of curing, the flexural strength of basalt fiber mortar was lower than that at 30d. For instance, the 360d flexural strength of BF10.1 and BF7-0.1 decreased by 18.85 % and 14.54 %, respectively, compared to 30d. This decline is primarily attributed to the corrosion of basalt fibers in alkaline environments, leading to a decrease in their mechanical properties. Therefore, it is essential to enhance the alkali resistance of basalt fibers when used in cement-based materials.