γ-dicalcium silicate (γ-C₂S) is a carbonatable binder with excellent carbonation reactivity. However, its extremely low hydration activity prevents the paste from setting and hardening properly, making it difficult to be directly cast and molded. This study introduces β-hemihydrate gypsum as an early-strength regulating agent, which utilizes its rapid hydration to form dihydrate gypsum, thereby imparting early-age strength to the composite system and enabling subsequent carbonation curing after demolding. The experimental investigation examined the effects of β-hemihydrate gypsum content on the fluidity, setting time, and carbonation performance of γ-C₂S paste. The results indicate that when the β-hemihydrate gypsum content is not less than 10%, the paste can obtain sufficient early-age strength to achieve smooth demolding. At a β-hemihydrate gypsum content of 10%, after pre-drying treatment, samples subjected to carbonation curing for 24 h under a CO₂ partial pressure of 0.3 MPa achieved a peak absolute dry compressive strength of 117.29 MPa with a softening coefficient of 0.92. Further increases in β-hemihydrate gypsum content lead to reductions in both strength and water resistance. Microstructural analysis reveals that at a gypsum content of 10%, the crystalline network of dihydrate gypsum interlocks and coexists with calcium carbonate and silica gel generated from γ-C₂S carbonation, forming a compact structure. In contrast, the framework formed by excessive gypsum before carbonation restricts the continuous interlocking and overall development of calcium carbonate products generated from γ-C₂S, resulting in deterioration of structural integrity and mechanical properties. This study provides theoretical foundation and technical support for the engineering application of γ-C₂S-based carbon mineralization materials.

Auxetic cementitious cellular composites (ACCCs) offer high deformability that is attractive for mechanical energy harvesting when integrated with flexible piezoelectric materials. However, the intrinsic brittleness of cement-based materials and the complex coupling between auxetic geometry and damage evolution hinder the efficient design of ACCC energy harvesters. This study proposes a novel learning-driven design framework that, for the first time, integrates a physics-based energy harvesting model with Bayesian Optimization (BO) to directly optimize the recoverable hinge-like strain capacity of ACCCs for enhanced electrical output. The optimization maximizes the voltage generated by piezoelectric materials bonded at hinge regions, while using constraints to prevent splitting failure and non-auxetic behavior under compression. The energy harvesting model combines the concrete damage plasticity (CDP) model for pre-compression damage with a secondary elastic model for cyclic loading, enabling prediction of recoverable strain in generalized ACCC geometries. The learning-driven approach proved far more efficient than random generation in identifying optimal ACCC configurations. Experimental validation of the optimized design achieved a peak-to-peak voltage of nearly 15.0 V per cycle, about 2.7 times higher than a reference design. This study provides a learning-driven approach to designing enhanced compliant auxetic cementitious energy harvesters for smart infrastructure applications.

Woody biomass fly ash (WBFA) is the main by-product of woody biomass energy production. However, its use in cementitious materials remains limited due to its low intrinsic reactivity, largely associated with the scarcity of aluminosilicate phases. At the same time, the high alkalinity and sulphur content in WBFA make it a promising component for formulating cement-free binders without additional chemical activators, when combined with highly reactive precursors. This study investigates the reaction mechanisms and microstructural evolution of binders based on WBFA and ground granulated blast furnace slag (BFS), with the aim of elucidating their synergistic interactions and optimizing performance. Binary pastes with varying WBFA/BFS ratios mixed with water were prepared and characterized by isothermal calorimetry, pore solution analysis, XRD, FTIR, TGA, SEM-EDS, and MIP. The results show that, although increasing WBFA content initially delayed hydration by limiting the dissolution of reactive species, it markedly enhances long-term reactivity and strength through sustained release of alkali and sulphate. The main hydration products are C-(A)-S-H gels, ettringite, Friedel's salt, and hydrotalcite, with their amount and assemblage strongly governed by the WBFA/BFS ratio. Reaction kinetics analysis and thermodynamic modelling confirm the dual role of WBFA as both a reactive precursor and internal alkali/sulphate activator. Among the formulations studied, the mixture with a WBFA/BFS ratio of 50:50 exhibited the best overall performance, achieving the highest compressive strength and lowest porosity. These findings clarify the reaction mechanisms in WBFA-BFS binary pastes, providing practical guidance for designing WBFA-based, cement-free binders for sustainable construction applications.

Alkali‐activated slag (AAS) materials, composed of blast furnace slag as precursor and alkali solutions as activators, are emerging as sustainable alternatives in construction due to their significantly lower CO₂ emissions compared to traditional Portland cement (PC) ‐based materials. These cement‐free binders often demonstrate comparable or even superior mechanical properties and chemical resistance. However, long‐term durability remains a concern, particularly in humid environments where AAS materials are more susceptible to degradation than PC materials.

Leaching has been identified as the primary deterioration phenomenon in AAS materials under humid environments, yet the underlying mechanisms remain poorly understood. Additionally, efflorescence, characterised by white deposits on the material surface, is believed to be a consequence of leaching. However, the relationship between leaching and efflorescence in AAS materials remains unclear, necessitating further investigation. This thesis aims to bridge these knowledge gaps by investigating the leaching behaviour of AAS materials at multiple scales and developing effective strategies to mitigate efflorescence....

One-part alkali-activated materials (AAM), a low-carbon alternative to cement, can reduce CO₂ emissions while improving the utilization of industrial by-products. In this study, basic oxygen furnace slag (BOFS) was activated by alkali fusion with different contents of sodium hydroxide (NaOH), and the optimum NaOH content was selected by the mineral phase composition and micromorphology of alkali-fused basic oxygen furnace slag (ABOFS). Then, ABOFS and ground granular blast furnace slag (GGBFS) were used to prepare one-part AAM pastes, and the effects of GGBFS content on the reaction products, microstructure, leaching characteristics and mechanical strength of one-part AAM pastes were studied. Finally, the life cycle assessment (LCA) of one-part AAM pastes was conducted. The results showed that alkali fusion activation promoted the formation of reactive mineral phases in BOFS and increased its specific surface area. The optimum NaOH content for alkali fusion activation is 10 wt%. The reaction products of one-part AAM pastes primarily consisted of C-(N-)A-S-H gel and hydrotalcite. As GGBFS content increased from 0 wt% to 80 wt%, the amount of gel products first increased and then decreased, peaking at 60 wt%. The addition of GGBFS reduced the porosity of pastes and increased the proportion of gel pores, resulting in a denser structure. Therefore, the compressive strength of one-part AAM pastes increased with the increase of GGBFS. LCA results indicate that the global warming potential (GWP) of one-part AAM is significantly lower than that of ordinary Portland cement. The findings of this study provide new insights into the application of BOFS in AAM.

Alkali and alkali earth metal ions are normally present in the gels of alkali-activated materials as well as blended PC-based materials. Previous studies have revealed that the leaching of these cations can trigger the change in gel structure and even the gel decomposition. However, the dissolution of cations was rarely known and the underlying mechanisms remained unclear. To address this issue, five calcium-(sodium, potassium-)aluminum-silicate hydrates (C-(N,K-)A-S-H gels) with different Ca/Si ratios (0.8–1.2) and Al/Si ratios (0.1–0.3) were synthesized to investigate the leaching behaviour of Ca, Na and K. For the first time, the dissolution free energies of Ca, Na and K in C-(N,K-)A-S-H gels were calculated using molecular dynamics simulations with the metadynamics method. Experimental results showed that Na showed the highest leaching ratio, followed by K and Ca, attributed to the lowest dissolution free energy of Na. The gel with a higher Ca/Si ratio or a lower Al/Si ratio showed higher charge positivity on the surface, resulting in reduced leaching of the three cations. Additionally, the presence of K was found to promote the dissolution of Na in gels.

Phosphorus slags, abundant industrial by-products, hold significant potential for use as supplementary cementitious materials (SCM). To explore its application, phosphorus slags with varying contents of P₂O₅ were synthesized, and their network structure, dissolution behaviour, and hydration products in cementitious systems were investigated. The results showed that the pozzolanic reactivity initially decreased and then increased with increasing P₂O₅ content from 3.11 to 8.32 wt%. This behaviour was attributed to the competition between silicate network repolymerization and distortion of aluminium polyhedra. Furthermore, the incorporation of phosphorus increased the fractions of Al_V and Al_VI in the glass network, which promoted the incongruent dissolution of Al over Si. Thermodynamic simulation predicted the formation of hydroxyapatite, as the main phosphorus-containing hydration product, which was further confirmed by XRD and NMR results, although with low crystallinity. These findings suggested new perspectives on adopting phosphorus slag as a SCM in cement and concrete production.

Cementitious materials can achieve desirable strength development and reduced cracking potential under moist or immersed conditions. However, in this work, we found that alkali-activated slag (AAS) pastes can crack underwater, with a higher silicate modulus showing more pronounced cracking. Chemically, the C-(N-)A-S-H gel in the paste with a higher silicate modulus showed a higher Na/Si ratio and a higher leaching loss of Na, which led to more significant structural changes and gel deterioration underwater. This triggered the propagation of cracks initially present in the material. Physically, the paste with a higher silicate modulus featured a denser microstructure, lower water permeability and higher pore pressure, which resulted in a steeper gradient of pore pressure in the matrix. Consequently, the concentration of tensile stress was simulated at the centre and the corner of the cross-section of the sample. As this simulated concentrated stress exceeded the flexural strength of AAS pastes, significant fractures at the centre and spalling at the corner occurred, consistent with the experimental observation. This work not only elucidated the cracking mechanisms of AAS materials underwater but also provided new insights into mixture designs for these materials under high-humidity conditions.

This research investigated the use of wood biomass fly ash (WBFA) as a key component in developing low-carbon cementitious materials. WBFA was first subjected to water pretreatment and grinding to remove metallic aluminum and free lime, reducing expansion and cracking risks. Characterization of WBFA showed its high calcium and alkali-bearing phases but limited aluminosilicates. Dissolution test showed WBFA had strong alkalinity, suggesting its role as an activator for aluminosilicate-bearing minerals. A novel cement- and chemical-free binary binder was developed using 50 % treated WBFA and 50 % blast furnace slag (BFS). Paste with a water-to-binder ratio of 0.4 achieved 40 MPa compressive strength at 60 days. The use of superplasticizer significantly improved flowability, allowing the water-to-binder ratio to be reduced to 0.25, which resulted in compressive strength up to 58 MPa at 60 days. Calcium aluminate silicate hydrates (C-A-S-H) gels and ettringite were identified as the main reaction products in the pastes.

The present research investigated the hydration characteristics of model (C₃S-C₃A-gypsum-blast furnace slag) and real cement slag pastes at an early age, and emphasis was laid on the interaction of gypsum, aluminate phases dissolved from both C₃A and slag. Two additional peaks arose during the decelerating stage of calorimetric measurement. The first one was originated from the renewed aluminate reaction between C₃A and gypsum, while the second one was attributed to the reaction between aluminate phase dissolved from slag and gypsum. High alumina content in slag promoted the aluminate reactions between gypsum and C₃A. On the other hand, the reaction between slag and gypsum was suppressed because of the addition of C₃A. Besides, factors affecting the optimal sulfate requirement of cement slag paste, e.g., C₃A content in cement clinker, slag substitution level, and slag chemistry, etc. were analyzed. The authors believed that the results found in this paper provided essential insights to quantitatively understand the early-age aluminate interactions of gypsum, C₃A as well as blast furnace slag in cement slag paste.

Fiber-reinforced cemented paste backfill (FR-CPB) has attracted considerable attention in modern mining applications due to its superior mechanical properties and adaptability. Despite its potential, understanding its rheological behavior remains limited, largely because of the absence of quantitative methods for assessing fiber packing behavior within CPB. This study develops a rheology-based approach to determine the maximum packing fraction of polypropylene fibers in fresh CPB, revealing that shorter fibers (3 mm) achieve a maximum packing fraction of 0.661, significantly higher than longer fibers (12 mm) with 0.534. Building on these findings, a quantitative model for the static yield stress of FR-CPB was developed, showing that under a high fiber content (0.9%) and with longer fibers (12 mm), the yield stress reached 274.34 kPa, a 40% increase compared to shorter fibers. Additionally, the study modeled the time-dependent evolution of yield stress, achieving a prediction accuracy with a correlation coefficient of 0.92. These advancements enable the optimization of FR-CPB composition, which can reduce material usage, enhance pipeline transport efficiency, and improve backfill stability in underground voids. By minimizing the risk of structural failure and optimizing resource allocation, this research provides a theoretical foundation for safer and more cost-effective mining operations.

Cementitious capillary crystalline waterproofing materials (CCCW) can be used as a self-healing additive. Compared with other self-healing materials and methods, CCCW directly added to the concrete matrix is easier to apply in specific projects. In this study, long-life self-healing concrete was prepared for the Palace Museum North Campus project by adding expansion agents, polypropylene fibers, and CCCW. Its mechanical properties and durability were tested. At the same time, non-destructive testing methods were used to test the crack resistance and self-healing performance. The compressive strength was around 41 MPa (28 d) for C40 group and 47 MPa (28 d) for C45 group. The compressive strengths exhibited no loss after 300 freeze-thaw cycles. The results showed that CCCW reduces early cracking and exhibits a good repairing effect on cracks below 0.2 mm. The method provided a new technical route for evaluating the healing effect.

The recycling of municipal solid waste incineration (MSWI) bottom ash as a supplementary cementitious material (SCM) has attracted global attention, driven by the increasing availability of this by-product and the demand for sustainable SCMs to lower CO₂ emissions from cement production. Currently, the widespread use of MSWI bottom ash in the cement industry is hindered by the lack of guidelines to regulate material composition, optimize pretreatment processes, and specify mix design requirements. This review compiles and analyzes literature data on mix design, microstructural evolution, fresh properties, mechanical properties, durability, leaching risks, and environmental impacts of MSWI bottom ash-blended cement pastes, mortars, and concretes. The analysis aims to assess the influence of the pretreatment and physicochemical properties of bottom ash¹ on the microstructure and performance of blended cementitious materials.² The Ash Impact Strength Index (AISI) is introduced to quantify the effects of various factors on compressive strength, enabling direct comparison across different studies. Based on the statistical analysis of the 28-day AISI, the key quality requirements for MSWI bottom ash as an SCM are proposed, along with the optimal mix design. This work provides valuable insights and practical guidance to support the integration of bottom ash into the cement industry.

Efflorescence is a significant aesthetic and structural issue for alkali-activated materials (AAMs). This study addressed this issue at the aggregate level for the first time. The results indicated that substituting sand with aluminosilicate-based lightweight fine aggregate (LWFA) by 20 % or 50 % in volume reduced the efflorescence of alkali-activated slag (AAS) mortars by 14.6 % or 43 %, respectively. The mitigation mechanisms of LWFA were proposed in terms of gels, pore solution and microstructure. Specifically, the pozzolanic reaction of LWFA provided gels with additional Si and Al, which contributed to binding Na⁺ in the pore solution, thereby reducing the available Na for efflorescence formation. Combined with its internal curing effect, LWFA densified the surrounding pastes, which hindered the transport of ions and water, thus limiting the formation of efflorescence products. Furthermore, the incorporation of LWFA enhanced the flexural strength of mortars without significantly compromising compressive strength.

Recycling aluminosilicate-based solid wastes is imperative to realize the sustainable development of constructions. By using alkali activation technology, aluminosilicate-based solid wastes, such as furnace slag, fly ash, red mud, and most of the bio-ashes, can be turned into alternative binder materials to Portland cement to reduce the carbon footprint of the construction and maintenance activities of concrete structures. In this paper, the chemistry involved in the formation of alkali-activated materials (AAMs) and the influential factors of their properties are briefly reviewed. The commonly used methods, including X-ray diffraction (XRD), scanning electron microscopy (SEM), thermogravimetric analysis (TG), nuclear magnetic resonance spectroscopy (NMR), and X-ray pair distribution function technology, to characterize the microstructure of AAMs are introduced. Typical characterization results of AAMs are shown and the limitations of each method are discussed. The main challenges, such as shrinkage, creep, efflorescence, carbonation, alkali–silica reaction, and chloride ingress, to conquer for a wider application of AAMs are reviewed. It is shown that several performances of AAMs under certain circumstances seem to be less satisfactory than traditional portland cement systems. Existing strategies to improve these performances are reviewed, and recommendations for future studies are given.

Alkali-activated materials (AAMs) are a class of potentially eco-friendly construction materials that can contribute to reduce the environmental impact of the construction sector by offering an alternative to Portland cement (PC). With the rapid development of both computational capabilities and theoretical insights into alkali-activation reaction processes, there has been a surge in research activities worldwide, leading to a growing demand for computational methods that can describe different characteristics of AAMs. This review summarizes the collective efforts made in the past two decades on this topic, and highlights the most relevant results and advances in the aspects of atomistic simulation, thermodynamic modeling, microstructure/−based simulation, and multi-scale modeling. The gaps and challenges in current numerical research on AAMs are pointed out and discussed in comparison with PC-based materials. This review aims to provide a critical overview of the state-of-the-art in modeling and simulating AAMs, while also outlining potential avenues for future development.

Calcium sodium aluminosilicate hydrate C-(N-)A-S-H gels, formed through the alkali-activation of calcium silicate-based materials, may exhibit greater susceptibility to aqueous environments when compared to traditional C-(A-)S-H phases formed by hydration of blended Portland cements. This study investigates structural changes in synthesized C-(N-)A-S-H gels triggered by water immersion. Three gels have been examined, each with stoichiometrically controlled ratios of Ca/Si (0.8 and 1.2), Al/Si (0.1 and 0.3), and Na/Si (0.1, 0.2, and 0.3). The gel with a higher Ca/Si ratio demonstrated enhanced resistance to water leaching and only experienced marginal decalcification whereas the gels with lower Ca/Si ratios exhibited more pronounced effects including leaching losses of Si. Notably, all gels displayed rapid and substantial sodium leaching, contributing to an increased degree of polymerization for the aluminosilicate tetrahedra in the gels. A plausible mechanism for this change is that Na leaches out from the interlayer and Ca ions progressively take over the role of charge compensators in the interlayer of the C-(N-)A-S-H structure.

The maximum packing fraction (φ_fm) of flexible fibers is an essential parameter for understanding the rheological behavior of flexible fiber-reinforced cement paste (FFRCP). However, direct measurement of φ_fm of flexible fibers is still lacking. In this study, a shear rheology-based method for direct measurement of φ_fm was proposed and the assumption of fiber conformation under shear was verified by micro-CT. Based on this, a yield stress model for FFRCP was constructed to explain the entanglement and friction effects in the fiber network. Finally, static yield stress tests and small amplitude oscillatory shear (SAOS) tests were carried out to explore the structural build-up of FFRCP. It was found that the proposed method enables direct determination of φ_fm through only a few viscosity-fiber content data for a given FFRCP. Furthermore, the proposed model can describe the static yield stress of FFRCP well. Finally, the relative structural build-up rate of FFRCP follows a similar trend as the relative yield stress, with a critical relative fiber volume fraction (0.299) as the boundary. Subsequently, the relative structural build-up gradually deviates from the relative yield stress due to the limiting effect of the fibers.

Efflorescence presents not only as a cosmetic concern but also as a structural issue, which impacts the performance of alkali-activated materials (AAMs). In this study, the mechanisms of efflorescence of alkali-activated slag (AAS) pastes are investigated. First, the efflorescence of AAS pastes with different alkali dosages (3 %, 5 % and 7 %), activator types (sodium hydroxide (NH) and sodium silicate (NS)), exposure atmospheres (ambient, N₂ and 0.2 vol% CO₂), and relative humidities (40 %, 60 % and 80 %) was observed. Subsequently, leaching tests were performed and the impacts of efflorescence on AAS pastes at different heights were studied. It was found that a lower relative humidity facilitated more rapid and severe efflorescence. The positioning of efflorescence products was dependent on the porosity of the matrix. Compared to NH pastes, NS pastes subjected to semi-contact water conditions were more vulnerable to cracking problems, which turned out to be exacerbated by the formation of efflorescence products. A new method to quantify efflorescence was developed and it corresponded well with both efflorescence observations and leaching experiments. Furthermore, a competitive reaction between Ca and Na in the presence of carbonate ions was identified. CaCO₃, a representative product of natural carbonation, was rarely found in the regions where efflorescence products (sodium carbonate) formed. Regarding compressive strength, NS pastes were more adversely affected by efflorescence than NH pastes.

Carbonation of alkali-activated slag (AAS) materials has been primarily concerned in atmospheres with gaseous CO₂. This study, by contrast, highlights that AAS pastes would also be carbonated under tap water immersion. Calcite is the main CO₂-bear phase in both sodium hydroxide- and sodium silicate-activated AAS pastes, and the paste pre-cured for a longer curing period shows more severe carbonation. Additionally, calcium carbonate can densify the deteriorated microstructure of sodium hydroxide-activated paste caused by long-term leaching. The indentation modulus of pastes subjected to tap water immersion is higher than those under deionized water immersion. The uptake of CO₃^2- by hydrotalcite (Ht) and gels is also detected, resulting in the formation of Ht-CO₃ and decalcification of gels. Due to the synergistic effect of leaching and carbonation, a characteristic layered distribution of pastes close to the exposure front is observed, comprising the carbonated layer, transitional (carbonated + leached) layer, and leached layer, progressing from the outermost to the inner regions. Eventually, the kinetics of underwater carbonation, as well as the discrepancy between dry and underwater carbonation, is revealed.