Recycled lump-filled concrete is a sustainable concrete material produced by stacking large-sized recycled lumps in moulds and subsequently filling the gaps between the lumps with self-compacting concrete or mortar. However, the homogeneity of the interfacial transition zones (ITZs) and cement paste matrix surrounding the recycled lumps is critical in determining the overall mechanical properties and permeability resistance. In this study, two specimen preparation methods were used including the sequential casting method (directly pouring self-compacting mortar onto the recycled lump stacks) and the reverse casting method (extruding recycled lumps into self-compacting mortar). The results show that both casting methods adequately fill the voids in the recycled lump-filled concrete and form an effective bond between the recycled lumps and self-compacting mortar. However, the sequential casting method relies more strongly on the relationship between the yield stress and shear stress of self-compacting mortar. Particle accumulation occurred mainly at the top and upper sides of the recycled lumps due to sedimentation, while rough surface of recycled lumps and the viscosity of the mortar led to more voids at the bottom of the specimens fabricated by the sequential casting method. In contrast, the ‘wall effect’ was less significant and a more homogeneous interface was achieved in specimens prepared by the reverse casting method, although the bottom interface was slightly denser than the top. Overall, the reverse casting method was more conducive to achieving a homogeneous distribution of micro-pores and phases, thereby stabilising the mechanical properties and permeability resistance of recycled lump-filled concrete.

Despite its critical role in cement and concrete performance and CO2 emissions, the circularity potential of calcium remains insufficiently explored due to the lack of linkage between laboratory-scale recovery and societal-scale material flow dynamics. This study quantifies calcium flows and stocks in Japan in 2020, encompassing reserves, extraction, consumption, accumulation in urban structures, waste, and recycling. Japan holds 4.6 billion tonnes of calcium reserves, primarily in limestone and dolomite, while urban stocks reached 5.5 billion tonnes. Annual calcium inflow was 60.9 million tonnes, of which 77 % was used in construction. By systematically visualizing calcium dynamics, this study identified previously unquantified CO2-related flows at the societal scale and provided a basis for evaluating circularity and decarbonization strategies. Potential recycling strategies include cement paste reuse, carbonated concrete as supplementary cementitious material, and aggregate recycling, with hybrid approaches supporting scenario-based emission reduction and cross-sectoral resource management.

Assessing the risk of alkali-silica reaction (ASR) in large-scale concrete structures remains a critical challenge, particularly due to the scarcity of field-based data and the long timescales involved. This study proposes a methodology to evaluate ASR risk in concrete containing slowly dissolving aggregates. The approach consists of three steps: (i) identifying material properties such as the dissolution rate and chemical composition of the dissolved phases; (ii) determining the critical reaction degree at which amorphous silica forms, using equilibrium calculations based on GEMS thermodynamic simulations; and (iii) estimating the time required to reach this threshold by solving coupled equations for moisture transport and aggregate dissolution under real structural conditions. The methodology is demonstrated using in-situ data from the aged concrete walls of the Hamaoka nuclear power plant. Key factors that mitigate ASR risk are identified, including low dissolution rates, water depletion over time, and the presence of stabilizing species such as Al2O3 and MgO in the dissolved phase. While developed for a specific case, the proposed approach provides an adaptable framework for rational ASR risk evaluation in existing and future concrete structures.

This study investigated the carbonation behavior of ordinary Portland cement (OPC), volcanic glass powder (VGP), and limestone-calcined clay cement (LC3) pastes, focusing on the water content distribution, phase assemblages, and microstructural evolution. Spatiotemporal changes in water and mineral phases were tracked using proton nuclear magnetic resonance relaxometry and micro-X-ray diffraction, respectively. A simplified model reproduced the drying front, defined by a material-specific threshold water content, which aligned closely with the experimentally measured carbonation front. This confirmed that carbonation progression is governed by water diffusion, referred to as the "plugging effect". Furthermore, carbonation advanced until empty space increased to approximately 18 %, resulting from the decalcification-induced agglomeration of calcium (alumino) silicate hydrate (C-(A)-S-H). This limit was consistently observed across pastes with varying Ca/(Si + Al) ratios. These findings provide mechanistic insight into carbonation front development and offer a physically grounded criterion for predicting carbonation depth and assessing CO2 uptake in sustainable cementitious materials.

The long-term microstructural evolution of hardened cement paste under nearly two years of enforced carbonation was examined using water vapor sorption and H-1 NMR relaxometry. The results show that nanopore evolution, including interlayer spaces and gel pores, in carbonation at different relative humidity (SDC) was reduced significantly by as much as approximately 50 % degree of carbonation (DoC). In contrast, wet carbonation (WC) showed distinct pore changes and rapid matrix degradation from structural breakdown, with pore evolution following a trend similar to pH. The specific surface area (SSA) in WC is higher than in SDC owing to the formation of highly polymerized Al-Si gel, which also increases the SSA measured by H-1 NMR relaxometry. Lower relative humidity (RH) leads to a much greater reduction in nanopores. The reduction in interlayer spaces and gel pores and metastable CaCO3 polymorph transformation retards further carbonation in SDC. In SDC, vaterite tends to form at DoC <50 %, whereas stable forms, such as aragonite and calcite, dominate at later stages, depending on the RH. At 60 % RH, aragonite and vaterite were depleted at a DoC of approximately 50 %, owing to the disappearance of the interlayer spaces and gel pores. By contrast, at 90 % RH, the interlayer spaces decreased to approximately 50 % DoC, whereas the gel pores decreased significantly beyond 70 % DoC. Vaterite is depleted around 50 % DoC as it transforms into aragonite alongside nanopore reduction, and aragonite seeds promote further aragonite growth. This behavior is attributed to aragonite crystal growth through Ostwald ripening and polymorph transformation.

The thickness of water film on cementitious minerals is said to influence their carbonation reactivity. To understand it, this study simulated water adsorption of portlandite (CH) using Grand Canonical Monte Carlo simulations combined with Molecular Dynamics to sample the grand canonical ensemble. The effects of (i) different cleavage planes (0 0 1 and 1 0 0), (ii) presence of surface defect, (iii) temperature (from 20 to 80 ℃), and (iv) different water models (SPC/E and TIP4P) were investigated and compared with experiments. The results revealed that the adsorbed amount was significantly lower for 0 0 1 surface compared to that of 1 0 0 surface at low relative humidity (RH). Creating surface edges on the former significantly increased the adsorption. This behavior stemmed from the presence of insufficiently coordinated calcium on the surface. In addition, a decreasing trend of adsorption amount at higher temperature, following Clausius-Clapeyron theory, was observed. Finally, the choice of different water models, three sites (SPC/E) or four-point TIP4P did not greatly affect the results.

Carbonation of cementitious materials has attracted increasing attention as a potential approach for CO2 fixation. However, quantitative evaluation of the amount of CO2 fixed from a designated source under accelerated carbonation conditions remains challenging, because atmospheric CO2 mixing may occur. In this study, carbonation experiments were conducted on well-hydrated cement paste powders under controlled relative humidity conditions using fossil-derived CO2, and carbon uptake and isotopic compositions were investigated using combined 13C and 14C analyses. The results show that atmospheric CO2 can contribute significantly to the isotopic composition of the fixed carbon in the samples. Two approaches were proposed to quantify this atmospheric contribution, which yield contamination estimates of comparable magnitude. Based on the estimated atmospheric mixing, a practical framework is established to quantify the amount of CO2 fixed from the 14C-free fossil-derived source gas. The proposed approach enables source-specific evaluation of CO2 fixation in cementitious materials and supports quantitative assessment and labeling of carbon-captured materials.

Neutron irradiation of alpha-quartz induces amorphization accompanied by a reduction in density. Experimental studies have shown that this density reduction slows at elevated temperatures, a phenomenon interpreted as the "thermal healing effect" of neutron-irradiated alpha-quartz, i.e., a crystallization transition. However, the atomic rearrangement mechanisms underlying the recovery of long-range order remain poorly understood. Here, we reproduce amorphized alpha-quartz using neutron irradiation simulations and capture the crystallization transition at the atomic scale. alpha-quartz structures with crystallinities of 80%, 60%, and 40% are generated through irradiation simulations, and their thermal healing behavior is investigated using annealing-based and metadynamics (MetaD) simulations to analyze the structural changes accompanying the phase transition. In the annealing-based simulations, local structural defects in SiO4 units are partially repaired, but restoration of long-range order is not achieved. In contrast, MetaD simulations successfully reproduce structural transitions up to nearly 100% crystallinity. Analysis of atomic rearrangements during these transitions reveals that Si atoms diffuse through void channels aligned with the 3-fold helical axis of irradiated alpha-quartz. This diffusion pathway allows Si atoms to occupy crystalline sites, thereby driving the thermal healing effect.

This study examined the effect of water on carbonation of hardened cement paste with three water-to-binder ratios, considering four types of relative humidity (RH) during carbonation. The role of the water content in calcium aluminosilicate hydrate (C-A-S-H) on decalcification and decomposition under RHs was quantitatively assessed using total carbon analysis, XRD/Rietveld-PONKCS, 1H NMR relaxometry, SEM-EDS, and carbonation shrinkage measurements. At different RHs, carbonation shrinkage increased with reduced Ca/(Si + Al) ratio and increased aluminosilicate gel content. Regardless of RHs, the H2O and Ca released from C-A-S-H during decalcification were equimolar. Minor variations in the released H2O/Ca ratio during decomposition were observed, which could be attributed to the effect of RH on Ca-modified silica gel formation. Carbonation shrinkage was assessed based on the Ca and Si released from C-A-S-H, demonstrating that decomposition from aluminosilicate gel formation significantly affects carbonation shrinkage compared with decalcification and water loss.

At present, Japan—similar to many other countries—is undertaking government-supported research and development aimed at transitioning the cement and concrete sectors toward carbon neutrality and a circular economy. This study introduces the approaches adopted by Japan’s cement industry, which operates under unique geological conditions with limited domestic resources, as well as the initiatives pursued within the concrete sector. Because Japan has emphasized resource and energy efficiency since the oil crisis of the 1970s, background information on research and development originally conducted to reduce resource consumption during that period is also incorporated. Although carbon neutrality is a global challenge, each country faces distinct historical and contextual constraints, including geological and resource-related boundary conditions. Research and development efforts must therefore be tailored to national circumstances. In this respect, Japan’s experience is presented here as an illustrative case.

A cement-free carbonated concrete waste composite (CWC) was developed through cold compaction of carbonated recycled concrete waste particles. This study explores the influence of key processing parameters—loading pressure, loading rate, and holding time—on the compressive strength of CWC. Results show that these parameters significantly affect strength development, and an optimal balance must be struck between energy consumption, target strength, and the intrinsic properties of the raw materials. Among the compaction aids tested, calcium bicarbonate [Ca(HCO3)2] solution at 20

Determining the elemental composition of ordinary concrete for radiation shielding calculations is challenging, as existing guidelines often present differing compositions and typically lack essential information such as constituent materials, mix proportions, and environmental conditions (e.g., heating or drying histories). Consequently, it is difficult to verify whether a given elemental composition accurately reflects the actual concrete used in shielding structures. This study emphasizes the importance of establishing a clear relationship between the elemental composition used in shielding calculations and ordinary concrete properties. Ensuring this correspondence is essential for the validity and appropriateness of shielding design.In nuclear power plant design, shielding calculations are sometimes required prior to finalizing the concrete mix design. For such cases, we propose elemental compositions in which all aggregate oxides are substituted with silicon dioxide to model silica aggregate-based concrete. This approach ensures conservative results when precise mixture data are unavailable. We also propose a methodology that allows for flexible input of the elemental composition of concrete, accounting for water mass and concrete density. This makes it possible to incorporate known parameters and account for the designer's intended level of conservatism. The proposed approach contributes to the development of more reliable and adaptable shielding design practices.

The drying of concrete has been recognized as a key phenomenon in the deterioration of concrete structures. Nevertheless, the complex conditions of real RC structures may lead to unforeseen responses observed from existing laboratory experiments on RC members. To clarify the influence of drying, a quasi-static cyclic loading experiment was conducted for one-third scale, three-story RC buildings under wet (saturated) and two-year-dried conditions. The significant decrease in the initial stiffness emphasized the influence of drying on the structural performance regarding residual stress and drying shrinkage cracks affecting the stress-transferring process. In addition, the different deformations of the frame structure indicated the influence of drying on the failure mode. The localized damage occurred early in the wet specimen due to the stress concentration. By contrast, the dried specimen showed only distributed damage during the same cycle. These influences emphasize the impact of drying, which should not be neglected in structural designs.

In this study, nanocarbon black (NCB)-filled cement-based sensors (CuNCBS) were developed using 20% mechanically activated copper tailings (CuT) by weight of cement. CuNCBS exhibited acceptable mechanical properties and enhanced conductive networks. Superior resistance-and capacitance-based sensing performances were demonstrated across thirteen application configurations with multiple loading conditions and electrodes design. Under low dynamic loads, capacitance-based sensing not only showed greater sensitivity but also enabled clearer differentiation of load magnitudes compared with resistance-based sensing. Microstructural characterisation revealed that the improved sensing performance is primarily governed by a hierarchical conductivity-strengthening mechanism, originating from multiple NCB/C-A-S-H layers formed on CuT surfaces with different NCB loading capacities. The formation of these layers can be explained by two synergistic mechanisms, including early hydration inhibition induced by aqueous Al, followed by continuous Si and Al release that modifies C-A-S-H composition, and the prolonged lifetime and enhanced stability of amorphous globular intermediates during early-stage C-A-S-H crystallization compared with C-S-H crystallization. The NCB loading capacity is closely correlated with the compositional and structural evolution of C-A-S-H, where increasing NCB loading capacity corresponds to a transition from higher Si/Ca and Al/Si ratios to lower ones. These insights are expected to advance the development of low-carbon, tailings-incorporated cement-based sensor for efficient structural health monitoring and practical applications in smart and sustainable mining infrastructure.

This study systematically investigates how sustained shear history, with a water-to-cement ratio of 0.50 under shear rates ranging from 2 to 15 s⁻¹ for up to 2 h, regulates the rheological properties and hydration behavior of cement paste. By integrating macroscopic rheological testing with microscopic structural analysis, the study reveals that shear action is far from simple physical mixing and is instead a key factor that appears to alter hydration pathways and structural evolution. Results indicate the existence of an optimal 'process window' of the tested system (8 s⁻¹ shear rate sustained for 1 h), under which the paste exhibits synergistically optimized properties: significantly enhanced dynamic yield stress (89 Pa) and plastic viscosity (1.96 Pa·s), coupled with a minimal thixotropic ring area (5416.5 Pa·s). This suggests the formation of a stable, high-strength, low-reversibility three-dimensional structure within the paste. The underlying mechanism shows that moderate shear modulates the pore solution chemistry (by increasing pH and Ca²⁺ concentration while decreasing zeta potential), promoting silicate mineral dissolution and uniform nucleation of low Ca/Si ratio (Ca/Si ≈ 1.8) C-S-H gel. This process constructs a robust gel skeleton. Furthermore, the study identifies a critical shear ageing threshold, where prolonged shearing beyond 2 h compromises long-term mechanical properties. The proposed concept of shear-assisted hydration provides a novel theoretical framework for actively designing and controlling the performance of cementitious materials under complex construction conditions.