As 3D printing emerges as a transformative technology in construction, the structural performance of 3D-printed mortar (3DPM) has become a key research focus. This study conducted shear tests on reinforced specimens combining 3D-printed mortar (3DPM) and normal mortar (NM). Four different shapes of interfacial locking design (I-shaped, K-shaped, C-shaped, S-shaped) were examined, comparing reinforced (CR) and non-reinforced (NR) specimens. The investigation analyzed failure modes, crack propagation patterns, and shear transfer mechanisms at CR series specimens under direct shear loading. CR-S specimens exhibited a shear peak load value 14.0% higher than CR-K specimens, 33.2% higher than CR-C specimens, and 42.9% higher than CR-I specimens. CR-I specimens exhibited pure adhesive failure. CR-K, CR-C, and CR-S specimens showed composite failure patterns combining adhesive and shear failure mechanisms. Strain analysis revealed the maximum horizontal strain εxx across all specimen shapes. CR-C and CR-S specimens recorded strain values exceeding CR-I and CR-K specimens by over 50%. Reinforcement produced pronounced increases in ultimate bearing capacity for I-shaped and C-shaped specimens, achieving gains of 51.9% and 60.4%, respectively. Reinforcement substantially enhanced energy dissipation capacity. Compared with NR series specimens, the performance improvements ranked as follows: CR-C (+164.67%) > CR-S (+70.70%) > CR-I (+52.05%) > CR-K (+9.42%).

Recycled aggregate concrete (RAC) has emerged as a practical route toward low-carbon construction, offering clear environmental advantages by reducing both natural resource consumption and waste generation. The incorporation of recycled aggregates in concrete results in the coexistence of multiple interfacial transition zones (MITZ), which have significant influence on the mechanical performance, durability and functional properties of RAC. However, the formation and performance evolution of MITZ and its influence on performance of RAC remain insufficiently understood. To achieve a better understanding of ITZs in RAC , this Technical Committee (TC) will focus on (1) investigating the formation mechanisms and damage evolution of MITZ, (2) characterizing the anisotropic and heterogeneous characteristics of MITZ across different scales, (3) exploring the key variables on ITZ performance, (4) quantifying the influence of MITZ on the macro properties of RAC, (5) developing innovative technologies to strengthen MITZ, and (6) establishing quantitative models for MITZ evolution.

The challenge of achieving simultaneous pumpability and buildability remains one of the fundamental obstacles in advancing 3D printed concrete (3DPC) technology. This study investigates a novel approach combining CO2 mixing technique with waste glass powder (WGP) incorporation to address this challenge while simultaneously reducing the carbon footprint of construction materials. Through systematic examination of fresh and hardened properties of both cast concrete and 3D printed specimens prepared with varying WGP contents (0-30% cement replacement), it is demonstrated that while WGP initially enhances workability but reduces early-age strength development, the application of CO2 mixing creates a remarkable synergistic effect that reverses these trends. The results reveal that CO2 mixing substantially enhances the yield stress and early-age penetration resistance of concrete which reflects a better buildability of 3DPC, with these improvements being dramatically amplified in the presence of WGP. Specifically, concrete containing 30% WGP showed a 1188% increase in penetration resistance after CO2 mixing, compared to only 304% for WGP-free concrete. Microstructural investigations indicate that this synergistic enhancement occurs through a dual mechanism. WGP releases alkali ions in the carbonation-induced acidic environment, promoting additional calcium carbonate formation, while simultaneously providing silica-rich nucleation sites that accelerate cement hydration. Furthermore, the combination of WGP and CO2 mixing effectively addresses the interlayer bonding challenges typically associated with 3DPC, showing improved interfacial strength compared to either treatment alone. These findings establish a practical strategy for developing sustainable 3DPC with enhanced buildability characteristics.

The global construction industry contributes to nearly half of total anthropogenic CO2 emissions, among which building material production accounts for a dominant share [...]

The emergence of 3D concrete printing technology faces a fundamental challenge to achieve both pumpability and buildability in a single material system. This research investigates the strategic integration of secondary CO2 mixing with water-reducing admixtures (WRAs) as a potential solution to this challenge. Systematic investigation of three WRA types (polycarboxylate ether-based (PCE), naphthalene sulfonate formaldehyde-based (NSF), and sulfonated melamine formaldehyde-based (SM)) indicates that CO2 injection creates a mechanism where carbonation products interact with WRA molecules on cement particle surfaces, transforming flowable concrete into buildable material on demand. This interaction can be tuned to achieve optimal printing performance while potentially contributing to CO2 utilization. Secondary CO2 mixing significantly reduced workability and shortened setting time of mortar with WRAs (e.g., workability of mortar with high dosage of NSF reduced from 264 mm to 128 mm and setting time reduced by 69.5 %), while maintaining comparable 28-day compressive strength across most formulations. Although drying shrinkage increased by 13-26 %, this trade-off appears manageable through optimized mixture design. The mechanism where WRAs promote carbonation that subsequently modifies their own effectiveness becomes an advantage, enabling precise rheological control. This approach provides insights into controlled rheological modification for sustainable construction applications, positioning 3D printed concrete as both a structural solution and a potential carbon utilization technology.

With the growing diversity of concrete materials, developing an interface mechanics theory that can universally describe and regulate multi-system concrete materials has become a key path to advance the field from empirical practice to theoretical formulation. The multiple interfacial transition zones (MITZ) in recycled aggregate concrete (RAC, hereafter recycled concrete) are significantly more complex than the single interfacial transition zone (ITZ) in ordinary concrete in terms of spatial morphology, microstructures, and formation mechanisms. Therefore, MITZ can serve as an ideal research subject for building the interface mechanics theory of RAC. The ultimate goal of this theory is to establish a unified mathematical framework capable of systematically predicting the overall mechanical behavior of RAC. To achieve this goal, three fundamental scientific issues should be systematically addressed: anisotropy, multi-scale characteristics, and time-dependent behavior. Anisotropy arises from the significant differences in the mechanical properties of the ITZ in three axial directions, leading to scattered characterization data and hindering theoretical modeling. The main research challenge lies in the complex spatial morphology of the ITZ and the fact that part of the interface is hidden between the aggregate and the mortar, making accurate testing of the three axial surfaces difficult. For this purpose, this paper proposes the development of ITZ deconstruction technology. This involves designing regular aggregates to standardize the geometry of the ITZ and using aggregate-mortar separation techniques to enable direct characterization of its three axial surfaces, thereby overcoming the experimental bottleneck in anisotropy research. Multi-scale characteristics refer to the fact that the mechanical behavior of the ITZ is jointly influenced by mechanisms across multiple scales, from molecular and micro to meso and macro scales. There is a need to systematically establish quantitative mathematical models that link these scales. In view of the difficulty in directly deriving inter-scale relationships through theoretical deduction, this paper suggests integrating big data and machine learning technologies. This approach involves systematically acquiring multi-scale experimental data and building performance mapping models across scales to achieve accurate prediction of the ITZ's cross-scale mechanical behavior. The key to this path lies in the quality control and integrated analysis of multi-source data, requiring collaborative efforts from materials, informatics, and other disciplines. Time-dependent behavior reflects the evolution of the ITZ's chemical composition and microstructures over time, leading to changes in its mechanical performance. To enhance the universality and forward-looking nature of the interface mechanics theory, it is essential to reveal its time-varying laws. This paper proposes building a multi-period, multi-parameter collaborative analysis framework. This includes developing in-situ monitoring techniques at the experimental level to track the evolution of the ITZ, introducing aging and damage state variables at the simulation level, and establishing performance prediction models that incorporate time factors at the theoretical level. The combination of these three aspects is expected to form a systematic predictive capability for the time-dependent behavior of MITZ. In summary, based on a systematic review of the current research status and shortcomings of MITZ, this paper clearly proposes the concept of a unified "interface mechanics theory of RAC" for the first time, identifies its three fundamental scientific issues, and proposes feasible technical pathways to address the research difficulties of each basic problem. This work not only provides a systematic theoretical framework and methodological support for ITZ research but also lays an important scientific foundation for achieving unified design and performance regulation and optimization of concrete materials. It has foundational significance for promoting the interdisciplinary integration of materials science and civil engineering.

Amid rising concerns over sustainability, recycled aggregate concrete (RAC) has emerged as a key focus in the construction industry. This study investigates the post-peak behavior of micro steel fiber-reinforced RAC (MSF-RAC) using in-situ 4D computed tomography (CT) and digital volume correlation (DVC) with fiber dosages of 1.0%, 2.0%, and 2.5%. Results demonstrate that micro steel fibers enhance RAC's toughness and ductility, reducing crack volume by 12.3% and crack width by 28.7%. Fiber bridging is shown to control crack propagation and redistribute stress, with 4D CT and DVC revealing concentrated strain at fiber aggregation points that stabilize the matrix and promote ductile failure. These findings provide a theoretical foundation for optimizing MSF-RAC designs and advancing its sustainable applications.

Carbonation plays a crucial role in facilitating carbon cycling within the cement industry. This paper proposes an integrated early-age carbonation approach involving carbonation mixing and carbonation curing for cement-based materials. Self-designed apparatuses were developed for both processes. Cement pastes were prepared using carbonation mixing under varying CO2 pressures (0.1 and 0.2 MPa) and water-to-cement (w/c) ratios (0.35 and 0.4), followed by carbonation curing to enhance CO2 sequestration. The effects on workability, CO2 uptake, phase composition, microstructure, and compressive strength were systematically analyzed. The results show that carbonation mixing reduces workability at a constant w/c ratio, while excessive CO2 pressure or water content can hinder carbonation efficiency. Carbonation mixing also significantly increases subsequent CO2 uptake during curing. The combined carbonation approach achieved a CO2 uptake of 12.17 %, improved microstructural compactness, and increased compressive strength by 10.91 % compared with normally mixed specimens. High-permeability post-curing methods are recommended after carbonation treatment to support long-term performance. This study provides technical and experimental insights to promote the practical application of early-age carbonation technologies in cement-based materials.

The rapid urbanization surge has intensified the dual challenges of natural aggregate depletion and construction waste accumulation, necessitating sustainable solutions like recycled aggregate concrete (RAC). However, the higher water absorption of recycled aggregate (RA) disrupts the effective water-to-cement (w/c) ratio, critically impacting RAC performance. While prior research focused on RA water absorption ratio or empirical w/c ratio correlations of RAC, the interplay between RA moisture dynamics and hydration kinetics remained unexplored. This study bridges this gap by systematically investigating how RA initial moisture degree (Dim) and additional water ratio (Raw) influence hydration behavior through isothermal calorimetry. A modified KrstulovicDabic hydration kinetics model, incorporating Dim and Raw dependent correction coefficients (pi, qi), was developed to elucidate hydration mechanisms. Key findings reveal that RA with Dim <= 0.5 reduced cumulative hydration heat by up to 11.8 % due to water absorption, while Dim >= 0.75 enhanced heat release by up to 14.3 % via internal curing. The duration of the interactions at phase boundaries process gradually shortened and even disappeared with the increase of Raw under low Dim (<= 0.5). A critical Dim threshold of 0.65 balanced water transport equilibrium, aligning effective and nominal w/c ratios. By linking RA moisture states to hydration kinetics, this study provides a framework for optimizing RAC mix designs with controllable effective w/c ratio, advancing sustainable construction practices.

In recent years, with the acceleration of urbanization and the continuous expansion of construction at all scales, the safety, durability, and sustainability of building structures have come to face multiple challenges [...]

Nanomaterials provide a new way to control the properties of cement-based composites due to their unique microstructural effects and surface properties. Nano-Al2O3 (NA), Nano-MgO (NM) and Nano-Fe2O3 (NF) are commonly used to modify concrete composites. In this study, these three functional nanomaterials are selected to systematically study their effects on the fresh properties, printing properties and hardening properties of cement-based materials. The research shows that adding an appropriate amount of NA, NM, and NF nanomaterials can reduce the flowability, improve the printability and enhance the physical and mechanical properties of the printing material. Among them, the physical and mechanical properties of the printing mortar with 3 similar to 4 wt% NA and 4 wt% NF are the best, but the compressive strength of the printed specimens with 2 wt% NM reach the highest values. The microscopic mechanism indicates that the 4 wt% NA sample has the fewest pores due to NA accelerating cement hydration. There are more harmful pores in 4 wt % NM specimens because NM exhibits expansive properties. NF can make the cement matrix relatively dense by refining the crystal size of Ca(OH)(2) crystal. The research results can provide new ideas and methods for the nanomaterials modification of cement-based 3D printing materials.

CO2 mixing is a promising technology to solve the inherent contradiction between workability and buildability in 3D printing concrete (3DPC), which also has the potential to sequester CO2. This study proposed a novel CO2 mixing technique called pressurized CO2 mixing. The properties of fresh and hardened mortar under pressurized CO2 mixing were investigated. The evolution of the microstructure and chemical composition induced by pressurized CO2 mixing were also studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), and thermogravimetric analysis (TGA). The findings demonstrated that pressurized CO2 mixing enhanced the efficiency of CO2 absorption compared to normal CO2 mixing, and it also increased the buildability of mortar while slightly improving the compressive strength. This was because nanosized CaCO3 was produced during pressurized CO2 mixing, which accelerated the hydration of cement clinkers. However, the accelerated hydration reaction resulted in an expansion of gel pore volume, which led to an increase in mortar shrinkage. Compared with normal CO2 mixing, pressurized CO2 mixing reduced setting time and workability more quickly and effectively because more nano-sized CaCO3 was produced during CO2 mixing, which indicated that pressurized CO2 mixing might be a better way to deal with the dilemma encountered by 3DPC.

Polycarboxylate superplasticizer (PCE) and hydroxypropyl methylcellulose (HPMC) are commonly used chemical admixtures to control the rheological properties of concrete, such as 3D printing concrete (3DPC). CO2 mixing is a promising technology that improves the rheological and mechanical properties of 3DPC and has the potential to sequestrate CO2. But at present, the coupling effects of CO2 mixing with PCE and HPMC on the properties of cementitious materials are unknown. Therefore, this study investigated the influence of CO2 mixing on the fresh and hardened properties of cement mortar with varying amounts of PCE and HPMC. The results showed that after using CO2 mixing, the effects of HPMC and PCE on the properties of cement mortar changed obviously. The PCE in cement mortar almost lost its effect after using CO2 mixing. The addition of 0.2 %PCE increased the workability of mortar by 50.3 % and increased setting time by 84.6 %, but after using CO2 mixing, the workability and setting time of mortar with 0.2 %PCE exhibited only limited increase compared to mortar without PCE. The effect of HPMC was also weakened after using CO2 mixing. Furthermore, the heat evolution and microstructural properties of fresh and hardened mortar were analyzed to reveal the mechanism of the coupling effects of CO2 mixing with PCE and HPMC. In addition, based on the coupling effect of CO2 mixing and PCE, a strategy for improving the buildability of 3DPC using CO2 mixing and PCE was proposed.

Limited research has been conducted to elucidate the micro-scale strength degradation of seawater-mixed alkali-activated slag (AAS). In this study, the effects of the primary salts (e.g., NaCl, MgCl2, CaCl2 and Na2SO4) in seawater on the micro-mechanical properties of the AAS were evaluated by a combination of nanoindentation and quantitative BSE-EDS image analysis. The results showed that the sea salts could react with silicates from the activator, decreasing the content of silicates, which is necessary for strength development. In a low alkali system, the various salts in seawater caused severe strength degradation on the AAS, and the Mg ion was identified as a primary factor contributing to the deterioration of the modulus and hardness properties of the C-(A)-S-H gels. When a high alkali content was used, the micro-mechanical properties of the AAS were less vulnerable. Additionally, chloride in seawater preferentially reacted with dissolved Al to form Friedel's salt rather than hydrotalcite, as observed in the AAS prepared with deionized water. The formation of additional hydrotalcite and Friedel's salt in seawater-mixed AAS resulted in the improvement of the micro-mechanics of the gels around a dark rim of reaction products surrounding the slag particles observed in BSE-SEM images. This investigation provided guidance for using seawater as mixing water in AAS.

The source complexity of recycled aggregate (RA) generally leads to significant fluctuations in the aggregate quality, which brings difficulties in the mix design of the recycled aggregate concrete (RAC). In this study, machine-learning models were developed to predict the cubic compressive strength by collecting mix proportions data including aggregate properties. Then a carbon emission model considering carbon sequestration was established. Based on these models, optimization algorithms were employed to obtain the optimal mix proportions. The results indicate that the backpropagation neural network model optimized by genetic algorithms (GABP) demonstrates the best predictive capability. Furthermore, the GABP model exhibits the highest sensitivity towards cement strength and water absorption of recycled coarse aggregate (RCA). In addition, single-objective optimization achieves a carbon reduction of 61.8-68.4 %. In dual- objective optimization, when the optimized mix proportions exhibit a low level of carbon intensity, the cubic compressive strength of RAC exceeds 50 MPa. This suggests that more efficient and valuable utilization of RA in concrete can be achieved after mix proportion optimization.

The corrosion of alkaline concrete materials exposed to a sulfuric acid environment is becoming more and more prevalent, and its damage assessment is becoming more and more imperative. This study aims to describe the corroded surfaces of concrete with different strength grades (C30, C50, C80) in sulfuric acid environments in terms of their three-dimensional fractal dimension. Three kinds of concrete with varying strength grades, namely C30, C50, and C80, were immersed in a sulfuric acid solution with pH ≈ 0.85 for four distinct corrosion durations, specifically 0, 28, 56, and 165 days, in accelerated corrosion tests. The 3D laser scanning technique was utilized to capture the 3D coordinates of the surface points of the concrete cylinder before and after corrosion. The fractal dimension of concrete’s uneven surface before and after corrosion was computed via the cube covering method, and the mass loss of the concrete specimen was also obtained. The outcomes demonstrate that the three-dimensional fractal dimension provides a new method for characterizing the degree of corrosion deterioration of concrete samples affected by sulfuric acid via laser scanning technology. From the perspective of the appearance, mass loss, and fractal dimension of a rough surface in the sulfuric acid environment at a pH level of approximately 0.85, the degree of the corrosion deterioration of concrete is ranked from high to low as C80 > C50 > C30. These fractal dimensions of the concrete’s corroded surfaces with various strength grades increase rapidly in the initial period. However, as the corrosion time progresses, the growth rate of the corroded surface fractal dimension gradually decelerates and tends towards stability, which accords with the law of exponential function. The widespread belief is that the higher the strength grade of concrete, the better its durability; however, this pattern varies in sulfuric acid corrosive environments. Therefore, based on this research, it is recommended that in extremely acidic environments (i.e., very low pH), more attention should be paid to high-strength grades of concrete.

To fulfil the requirements of pumpability and buildability, the rheological properties of mortar/concrete for 3D printing should follow much stricter requirements than traditional-cast mortar/concrete. This study proposed to use a secondary CO2 mixing technique coupled with the addition of silica fume (SF) to achieve in-situ rheology control of 3D printed mortar. The influences of CO2 mixing on the properties of the cast mortar and the 3D printed mortar prepared with SF were systematically studied. The pH, conductivity and ion concentration were tested to revel the essential chemical reactions and phase evolutions during CO2 mixing. It was found that the CO2 mixing effectively enhanced the early-age penetration resistance and yield stress of mortar incorporated with SF, and thus significantly improved the buildability of 3D printed mortar, while the maximum printing layer increased higher than 33 layers. Meanwhile, the mechanical properties of the SF-incorporated mortar were improved by this process. It was revealed that during the CO2 mixing process, the injected CO2 rapidly reacted with calcium ions to form CaCO3, which further reacted with C3A to form monocarboaluminate (Mc). The fast precipitation of calcium ions during CO2 mixing accelerated the hydration of C3A and C3S, contributing to the rapid development of strength at early ages and the improved performance of hardened mortar. The addition of SF in mortar greatly amplified the beneficial effect of CO2 mixing, which due to SF as nucleation sites promoted the formation of CaCO3 and Mc during CO2 mixing.

Accurate prediction of corrosion depth degradation law can provide a basis for the determination of the sacrificial layer thickness and the cover thickness of the concrete structural members in a corrosive environment. This study aims to explore a more accurate and safe method to predict corrosion depth. Six sets of concrete cylinder concrete specimens were adopted to perform an accelerated corrosion test in sulfuric acid solution with pH approximate to 0.95 for six different scheduled durations. 3D coordinates of a large number of points on the corroded cylindrical surface were obtained with the help of the 3D laser scanning technology. Then, a comparative study was carried out for the corrosion depth measured with the laser scanning method and vernier caliper method. The results prove that concrete corrosion depth predicted by the Gaussian random process model established by the laser scanning method was considered to possess higher credibility and better security.

A highly efficient, low-carbon crushing technology is urgently needed for recycling demolished reinforced concrete (RC) components. Static pressure crushing (SPC), based on fragmentation theory, is a proposed solution. This paper presents a numerical model and a corresponding quantitative analysis method for the SPC of RC components. The model was developed based on a full-scale crushing application of an RC beam, wherein the failure of concrete and steel was characterized by the traditional finite element method (FEM) and the "erosion algorithm". A volume reduction-based crushing ratio was suggested and proven feasible for the quantitative evaluation of SPC. Subsequently, the effects of component length, boundary conditions, and top die shape on crushing performance were numerically investigated. The results show that the established model effectively simulates the crushing behavior of RC beams, influenced by boundary conditions and top die shape. Furthermore, modifying the top die shape can significantly enhance crushing performance. A crushing ratio of 49.8 % was achieved using the cross-shaped top die. This study provides a practical quantitative analysis paradigm for SPC process optimization.

The use of recycled coarse aggregate (RCA) produced from waste concrete in new concrete is an effective way to realize the recycling of construction and demolition waste. The utilization of accelerated carbonation to enhance RCA has attracted a lot attention because it can enhance performance of recycled aggregate concrete (RAC) and has potential for CO2 sequestration. This study systematically investigated the influences of the duration of pressurized carbonation on the properties of RCA, RCA-new mortar interface, and RAC. The results showed that water absorption of carbonated RCA (CRCA) decreased with the increase of carbonation duration. But the beneficial effect of carbonation was less obvious after 1-day carbonation because most of calcium hydroxide and Ca-rich calcium silicate hydrates in RCA had reacted with CO2 within 1 day. After using CRCA, the improvement in different performance indicators of RAC was dependent on different factors. The enhancement in compressive strength and chloride penetration resistance of RAC prepared with CRCA were dependent on the reduced porosity of CRCA. That was why they correlated with the water absorption of CRCA as carbonation duration increased. However, the decrease in initial absorption of water of RAC was mainly dependent on the denser surface layer of CRCA, rather than the reduced porosity of CRCA. This is because the denser surface layer can change water path from “penetrating through RCA” to “passing around RCA”. It is recommended to use 1-day carbonation to enhance RCA, which might have higher cost-effectiveness.