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.

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.

Wood biomass fly ash (WBFA) has emerged as one of the most dominant by-products in the biomass energy sector. Circulating WBFA for construction practice can mitigate the secondary pollution caused by improper ash management, and provide a new material source to compensate for the scarcity of raw materials in the construction industry. This paper reviewed the current research progress on recycling WBFA in cementitious materials. The physicochemical properties of WBFA were summarized based on the literature. Further, the implementations of WBFA for the development of cementitious materials were categorized into three binder systems: clinker, blended cement, and alkali-activated materials (AAMs). Owing to the large variation in chemical compositions of WBFA and strict requirements in clinkering parameters, employing WBFA in blended cement and AAMs seems to be more promising. A new classification approach for WBFA was proposed to divide WBFA into two categories. This helps to provide simple guidance for ash recycling in construction practice. Finally, the current research gaps in WBFA valorization in cementitious materials were summarized, outlining the research for further exploration.

Ferric-rich sulfoaluminate cement (FR-SAC) prepared from solid waste has attracted more and more attention. However, the mineral compositions of solid waste-based FR-SAC clinkers are changeful due to the complex composition of raw materials. In this study, the influences of mineral composition and impurities in solid wastes on the mineral formation of FR-SAC clinker were investigated. The results showed that mullite in fly ash and sodium aluminosilicate in red mud did not decompose but easily reacted with other raw materials to form FR-SAC clinker. Al₂(SO₄)₃ in galvanic sludge decomposed before the formation reaction of FR-SAC clinker. The insufficient SO₂ involved in the reaction should be noted. Besides, Na₂O and K₂O promoted the decomposition of CaSO₄ and the formation of C₄AF. Most of the TiO₂ formed CaTiO₃, resulting in the increase of gehlenite and the decrease of C₂S and CaSO₄. These results can reduce the error when these solid wastes were used to prepare FR-SAC.

Due to low activity or long mineralization time, traditional mineral agents for self-healing concrete generally need a long time to achieve a desired repair efficiency. Inspired by epoxy resin AB glue which can consolidate in a short time when mixing the two components together, a novel type of fast-responsive capsules based on two soluble components was designed for self-healing concrete. Component A (sodium carbonate) and component B (calcium acetate) were encapsulated in two different groups of capsules, respectively, coated with three layers consisting of epoxy resin and fine sands to achieve superior waterproof and alkali resistance properties. After rupture of the capsules, the rapid response with respect to core material dissolution and precipitation can be realized in presence of water, by which the cracks below 200 μm can be healed in 3 days. Super absorbent resin (SAP) embedded in the capsules could expand in contact with water, and further improve the self-healing efficiency of the capsules by blocking the crack.

Solid waste-based calcium sulfoaluminate (SW-CSA) cement is a type of low carbon cement that uses solid waste as raw material. It is usually used to prepare lightweight porous concrete (LPC) due to its short setting time. However, high water absorption of LPC based on SW-CSA cement limits its extensive application. Water repellent can be mixed into binder material to reduce the water absorption of LPC, but it may affect the hydration properties of SW-CSA cement paste, which influences the performance of LPC correspondingly. Calcium stearate (CS), sodium oleate (SO) and sodium methyl siliconate (SMS) are three familiar commercial water repellents. To find the suitable internal mixing water repellent for LPC based on SW-CSA cement, the effects of three CS, SO and SMS on the water absorption, hydration, compressive strength, fluidity, and setting time of SW-CSA cement paste were explored. Besides, the properties of LPC with CS and SO added were also studied. The results indicated that using CS as the water repellent could reduce the 1 day water absorption of SW-CSA cement paste by 45.9% and the water absorption of LPC by 33.0%. It also reduced the setting time of SW-CSA cement paste and increased the final compressive strength of LPC, which was conducive to the preferred rapid setting and high compressive strength of LPC. The hydrophobicity of SW-CSA cement paste with SO was better than that of SW-CSA cement paste with CS. But using SO and SMS as the water repellent retarded the early hydration of SW-CSA cement and prolonged the setting time of SW-CSA cement and reduced the final compressive strength of SW-CSA cement paste. Therefore, SO and SMS can't be used as the internal mixing water repellent of LPC based on SW-CSA cement, while CS is a promising internal mixing water repellent of SW-CSA cement to prepare LPC.

Carbonation of hydrated cement paste (HCP) causes numerous chemo-mechanical changes in the microstructure, e.g., porosity, strength, elastic modulus, and permeability, which have a significant influence on the durability of concrete structures. Due to its complexity, much is still not understood about the process of carbonation of HCP. The current study aims to reveal the changes in porosity and micromechanical properties caused by carbonation using micro-beam specimens with a cross-section of 500 μm x 500 μm. X-ray computed tomography and micro-beam bending tests were performed on both noncarbonated and carbonated HCP micro-beams for porosity characterization and micromechanical property measurements, respectively. The experimental results show that the carbonation decreases the total porosity and increases micromechanical properties of the HCP micro-beams under the accelerated carbonation. The correlation study revealed that both the flexural strength and elastic modulus increase linearly with decreasing porosity.

This study aims to provide a better understanding of the autogenous shrinkage of slag and fly ash-based alkali-activated materials (AAMs) cured at ambient temperature. The main reaction products in AAMs pastes are C-A-S-H type gel and the reaction rate decreases when slag is partially replaced by fly ash. Due to the chemical shrinkage and the fine pore structure of AAMs pastes, drastic drop of internal relative humidity is observed and large pore pressure is generated. The pore pressure induces not only elastic deformation but also a large creep of the paste. Besides the pore pressure, other driving forces, like the reduction of steric-hydration force due to the consumption of ions, also cause a certain amount of shrinkage, especially in the acceleration period. Based on the mechanisms revealed, a computational model is proposed to estimate the autogenous shrinkage of AAMs. The calculated autogenous shrinkage matches well with the measured results.

In this study, internal curing by superabsorbent polymers (SAP) is utilized to mitigate self-desiccation and autogenous shrinkage of alkali-activated slag (AAS) pastes. Absorption and desorption kinetics of SAP incorporated in AAS pastes were studied with X-ray tomography. Internal curing delayed the peak of the rate of heat liberation but increased the total reaction degree of AAS pastes. Internal curing by SAP mitigated effectively the drop of internal relative humidity and the self-desiccation-induced autogenous shrinkage of AAS pastes. The cracking tendency of AAS pastes undergoing shrinkage in restrained conditions was also significantly reduced with SAP. Nonetheless, adding SAP, regardless of their content, cannot eliminate the autogenous shrinkage of AAS pastes, suggesting the existence of other autogenous shrinkage mechanism(s) besides self-desiccation.

Ferric-rich calcium sulfoaluminate (FR-CSA) cement is an eco-friendly cement. Fe₂O₃ exists in different minerals of FR-CSA clinker, e.g., Ca₄Al₂Fe₂O₁₀ (C₄AF), Ca₂Fe₂O₅ (C₂F), and Ca₄Al_6-2xFe_2xSO₁₆ (C₄A_3-xF_xS-). The mineral composition depends on the chemical composition of the raw materials and significantly determines the reactivity of FR-CSA cement. To optimize the phase composition of the FR-CSA clinker, chemical reagent raw mixtures with different amounts of CaO were used to prepare the FR-CSA clinker. X-ray diffraction (XRD) analysis, Rietveld quantitative phase analysis (RQPA), Fourier Transform Infrared spectroscopy (FT-IR), and scanning electron microscopy/energy-dispersive spectroscopy (SEM/EDS) were used to identify the mineralogical conditions of the FR-CSA clinker. The results indicated that the amounts of CaO in raw materials greatly affected the iron-bearing phase formation in the FR-CSA clinker. With decreasing CaO content involved in calcination reaction, the amounts of Fe₂O₃ incorporated in C₄A_3-xF_xS- increased up to 17.72 wt% (where x = 0.36). The findings make it possible to optimize the mineral composition of the FR-CSA clinker by changing the CaO content in raw materials. Furthermore, low CaO content in the raw material is beneficial to the formation of C₄A_3-xF_xS-, which enables the use of solid wastes containing low calcium for producing FR-CSA cement.

Mercury intrusion porosimetry (MIP) measurements are widely used to determine pore throat size distribution (PSD) curves of porous materials. The pore throat size of porous materials has been used to estimate their compressive strength and air permeability. However, the effect of sample size on the determined PSD curves is often overlooked. In pursuit of a better understanding of the effect of sample size on mercury intrusion into porous materials, a combined experimental and numerical approach was applied. Quartz sand and epoxy resin were mixed to form artificial sandstone. Digital microstructures of the sandstone were obtained by using X-ray computed tomography (CT scan) technique. PSD curves of the artificial sandstone with different sample sizes were determined both by MIP measurement and by simulation of mercury intrusion (i.e., MIP simulation). Percolation analysis was performed on mercury-intruded pores in the digital microstructures. The PSD curves determined both by MIP measurements and by MIP simulations show that there was a significant effect of sample size on mercury intrusion before percolation of mercury-intruded pores. The effect of sample size decreased with the increasing pressure. After the mercury-intruded pores percolated through the samples, the effect of sample size on mercury intrusion became minor. The pore throat size of the artificial sandstone was used to estimate the air permeability using the relation proposed in the literature. The calculated air permeability of the smaller sandstone sample was higher. However, in principle, the air permeability of sandstone samples should be independent of the sample size. Two main conclusions can be drawn: (1) a fixed sample size should be used in MIP measurements or MIP simulation so that the PSD curves of different samples can be properly compared, (2) sample size needs to be considered when the pore throat size determined by MIP measurement is used for estimating air permeability.

This article presents the results of an interlaboratory experimental study performed by 13 international research groups within the framework of the activities of the RILEM Technical Committee 225-SAP “Applications of Superabsorbent Polymers in Concrete Construction”. Two commercially available superabsorbent polymers (SAP) were tested in terms of their influence on the freeze–thaw resistance of ordinary concrete. To test the robustness of the method, all participating laboratories used locally produced materials. Furthermore, following this aim, various accelerated methods were used to estimate the resistance of the concrete to freeze–thaw cycles. The effect of adding SAP was from insignificant to considerably positive in terms of improvement in material performance as determined by reduced mass loss after freeze–thaw cycles; only one participant observed worsening of the material behaviour. At the same time, due to the addition of SAP, a much less pronounced decrease in the dynamic Young’s modulus was observed as a result of freeze–thaw testing without deicing salt.