Focusing on the recycling of construction waste and intelligent construction, this study investigates the application of steel slag in 3D-printed cementitious composites. The work focuses on resolving the key technical challenges of rheological regulation and the mitigation of printed component anisotropy, with the aim of achieving high-performance fabrication. Through integrated rheological testing, mechanical characterization, and microstructural analysis, the mechanisms governing rheological behaviour and the evolution of mechanical properties are elucidated. The findings reveal that the incorporation of 30% steel slag increases the static yield stress of the paste from 2140.2 Pa to 2714.8 Pa—a rise of 26.9%—whilst the dynamic yield stress increases from 231.23 Pa to 369.61 Pa, corresponding to a 59.85% enhancement. The plastic viscosity, however, exhibits only a marginal increase from 5.69 Pa·s to 5.90 Pa·s. The inclusion of steel slag retards structural build-up, resulting in diminished thixotropy. The low hydraulic reactivity of the slag and the concomitant reduction in cement content led to a decrease in the formation of C–S–H gel and a coarsening of the pore structure, with the total porosity increasing by 21.17%. Consequently, the compressive strength declines from 45.2 MPa to 41.8 MPa, a reduction of approximately 8.7%. Interlayer splitting tensile tests further indicate that the 30% steel slag substitution reduces interlayer bond strength by roughly 14.9%. Moreover, the decrease in ultrasonic pulse velocity and the increase in apparent defect ratio derived from interlayer defect analysis collectively confirm the pronounced anisotropy introduced by the printing process, with the presence of steel slag exacerbating both interlayer defects and the degree of anisotropy. This study provides a theoretical foundation for balancing the rheological benefits of steel slag in 3D-printable materials against the attendant mechanical penalties, and it underscores the necessity for future optimization through material modification and process refinement.

Magnesium silicate cements are increasingly being studied as they could be a low-carbon alternative to conventional cements, particularly for certain applications such as soil stabilization, hazardous waste immobilization or 3D printing. However, several challenges should be addressed, particularly regarding their rheological behavior. These cements have a high water demand and require high dosages of superplasticizers. This study aims to examine the effect of MgO grades on the rheological behavior and reactivity of magnesium silicate cement and to find a method to reduce the required superplasticizer dosage. Several admixtures were used: sodium hexametaphosphate (NaHMP), commercial polycarboxylate-ether based superplasticizer (PCE), and commercial sacrificial polymer (MS) combined with PCE. Three grades of light-burned magnesium were selected. Different rheological procedures allowing the viscoelastic and flow properties to be measured were performed. Reactivity was characterized through in-situ isothermal calorimetry, TGA and XRD analyses. Uniaxial compressive test on cubic specimens were also carried until 180 days of curing. The results show that the grade of MgO affects strongly the rheological behavior and reactivity of MgO-metakaolin blends. This is linked to their specific surface area which is conditioned by the calcination conditions. Concerning the effect of admixtures, the required dosage of PCE can significantly be reduced (up to 50%) by combining it with a sacrificial polymer (MS) which allows maintaining the steric repulsion of PCE.

For the final safe disposal of ¹³⁷Cs-contaminated wastes generated by the Fukushima Daiichi nuclear accident, ¹³⁷Cs-enriched copper ferrocyanide produced during waste decontamination was calcined and subsequently solidified with ordinary Portland cement and natural inorganic cation exchangers. The addition of cation exchangers to a mixture of cement and copper ferrocyanide calcined at 550 °C effectively suppressed the leaching of ¹³⁷Cs from the hardened solid into both deionized water and seawater. The addition of natural mordenite, with an exchange capacity 1.6 times that of the Cs content in the calcined material, prevented 86% of Cs leaching into deionized water and reduced the leaching rate into seawater to 1/160–1/200. Analysis of the suppression effect with a reaction–diffusion equation in conjunction with ion-exchange theory revealed that the addition of a cation exchanger with high exchange capacity and Cs selectivity is very efficient for suppressing the leaching.

Concrete, the most widely used artificial building material, is highly susceptible to damage from freeze-thaw cycles in cold regions. Grounded in practical engineering applications, this study investigates the damage and failure mechanisms of concrete subjected to freeze-thaw cycles. Freeze-thaw cycle tests and numerical simulations were conducted to investigate the mechanical behavior of the material. The experiments focused on changes in surface morphology and mass loss rate, with uniaxial compression tests providing stress-strain curves. A two-dimensional, four-phase mesoscale concrete model incorporating the coarse aggregate, mortar, interfacial transition zone (ITZ), and pores was developed using Python and implemented in ABAQUS for thermomechanical coupling analysis. The results show that as the number of freeze-thaw cycles increases, the freezing and thawing of pore water causes expansion and contraction, which promotes crack propagation. This significantly accelerates the accumulation of internal damage, resulting in higher mass-loss rates and reduced compressive strength. Additionally, an increase in the coarse aggregate content was found to effectively improve the frost resistance of concrete. The simulation results align well with the experimental data, providing a theoretical basis for optimizing concrete performance in freeze-thaw environments.

To explore the influence of freezing temperature on the freeze-thaw (F-T) damage of cement matrix, two F-T cycling regimes were applied: −15 to 5 °C and −20 to 5 °C. The mass loss, surface damage, and mechanical performance evolution of cement specimens following F-T cycles were evaluated at the macro-scale. Additionally, scanning electron microscopy (SEM) and micro-indentation experiments were used to examine the microcrack evolution and micro-mechanical responses at the micro-scale. The results demonstrate that freezing temperature has a crucial regulatory effect on the F-T damage of cement matrix. Low-temperature environment accelerates the initiation and propagation of microcracks, promoting the rapid formation of microcrack networks. This subsequently causes mass loss, surface spalling, and a marked degradation in the mechanical performance of cement-based materials. Moreover, F-T damage exhibits a two-stage characteristic. The initiation and propagation of microcracks in the early stages have limited influence on the macro- and micro-mechanical properties of the material. In the later stages, microcracks rapidly coalesce into a large-scale crack network, intensifying surface spalling and causing a sharp decline in mechanical performance. This study elucidates the mechanisms of F-T degradation of cement matrix under freezing temperatures from both micro- and macro-scales. It provides experimental support for the freeze-resistance design of cement-based materials in cold regions.

The shrinkage of shotcrete under different drying directions were tested. The overall pore distribution and aggregates microcracks in shotcrete were tested using low field NMR. The pores, microcracks, hydration products and unhydrated cement particles in the interfacial transition zone (ITZ) of shotcrete were analyzed using image recognition technology based on BSE morphology test. The formation reasons of pores or microcracks in shotcrete were also analyzed. The results show that the drying shrinkage of shotcrete exhibits anisotropy under different drying directions. The drying shrinkage under the vertical spraying direction drying is about 2.16 times that along spraying direction. The spraying process will result in an increase in the average pore size, pore content and aggregates microcracks in shotcrete. There are pores and microcracks in the ITZ of shotcrete. The pore contents along the four directions of aggregate are significantly different. It is higher along the spraying direction and lower in the vertical spraying direction. There are microcracks in the coarse aggregate of shotcrete along the spraying direction which was directly contacted with the previous slurry layer. The pore and microcrack distribution characteristics in ITZ and aggregate are the two reasons for the anisotropy of drying shrinkage in shotcrete.

Magnesium potassium phosphate (MKP) cements are acid-base binders that are attractive for a range of applications. Their interactions with the surrounding environment are largely governed by the chemistry of their pore solution, which remains poorly documented, particularly for near stoichiometric cement pastes (molar ratios Mg/P ≈ 1 and H₂O/P ≈ 5), due to the limited volume of residual free water in the pore network.

This study investigates the pore solution composition of MKP cement pastes through a combined experimental and thermodynamic approach. Several extraction protocols are compared and allow, through the use of correction factors derived from a statistical analysis, a more reliable determination of the pore solution composition at low water content. Experimental results show that the pH increases rapidly during early hydration, and that the solution is highly concentrated in potassium and phosphorus, while the magnesium concentration remains much lower. Thermodynamic simulations reproduce the general trends, with an increase in pH and a decrease in ion concentrations as the water content increases in the paste. However, the predicted concentrations are approximately one order of magnitude lower than the measured values. This discrepancy indicates that the pore solution remains oversaturated with respect to the identified phase assemblage, even after prolonged curing (up to 6 months). Finally, protocols are proposed to prepare synthetic solutions mimicking the pore solution of stoichiometric MKP cement pastes after 28 days of hydration, or at thermodynamic equilibrium.

Shear loading tests were conducted on steel fiber-reinforced concrete (FRC) beams to clarify the effects of fiber volume fraction (v_f), shear reinforcement ratio (r_w), tensile reinforcement ratio (p_w), and shear-span ratio (a/d) on the shear resistance mechanism of steel FRC beams. The shear resistance carried by fibers was calculated through image analysis to obtain a detailed understanding of the opening and propagation of diagonal cracks, and its change with the fracture progression was quantified. The results showed that when shear reinforcement bars and steel fibers were used in combination, the shear resistance carried by fibers was maintained from the initiation of diagonal cracks to failure. In addition, the change in the shear resistance carried by fibers as the fracture progressed was similar to that of the beam action, regardless of the v_f, r_w, p_w, and a/d. This paper is an English translation of the authors’ previous work [Abe, K., Yanagida, R., Nakaya, S. & Igarashi S., (2023). Evaluation of shear mechanism of steel fiber reinforced RC beams using image correlations method. Proceedings of the Japan Concrete Institute, 45(2), 781-786. (in Japanese)].

Curing plays a critical role in determining the mechanical performance and durability of cement-based materials. However, existing studies remain fragmented, with most reviews focusing on individual techniques rather than providing a unified understanding of the underlying mechanisms. To address this limitation, this study presents a combined bibliometric and mechanism-oriented review of curing technologies based on 2350 publications (between 2000 and 2026). Bibliometric analysis reveals a clear evolution from conventional empirical curing methods toward advanced, sustainability-driven, and intelligent control approaches. More importantly, a unified thermo-hygro-mechanical (THM) framework is established to interpret curing processes as coupled regulation of temperature and moisture fields, which govern hydration kinetics, microstructure evolution, and stress development. Based on this framework, the reviewed approaches are classified into thermal regulation-dominated curing, moisture regulation-dominated curing, reaction-assisted curing or controlled environmental conditioning, and data-driven intelligent control. The framework explicitly distinguishes curing measures for freshly cast concrete from environmental conditioning used to reproduce or accelerate prescribed conditions, while providing a common THM basis for comparing their regulation pathways and material responses. A mechanism-based analytical scheme is further proposed to unify different curing approaches through their regulation pathways and performance outcomes. The results highlight a fundamental transition from single-field control to multi-field coupled regulation, and ultimately toward closed-loop intelligent curing systems. Future research should focus on multi-field coupling mechanisms, predictive models, and low-energy intelligent curing strategies. This work provides a unified theoretical perspective and practical guidance for advancing curing technologies in modern engineering applications.