Cement and Concrete Research最新文献

The relative slow carbonation efficiency for conventional wet and dry carbonation of recycled concrete fines (RCF) limits its resource industrial utilization. In this study, an innovative mechanochemical carbonation (MC) method was developed. The carbonation kinetics, phase assemblage and microstructure evolution of RCF during the MC process were extensively examined. The results exhibited a substantial enhancement in the carbonation efficiency and CO₂ utilization rate, as evidenced by achieving a notable carbonation degree within 10 min. This accomplishment surpassed what could be achieved even after a prolonged 2 h period of wet carbonation, and the CO₂ uptake capacity and utilization rate achieved via MC reached >0.3 g-CO₂/g-RCF and 80 %, respectively. The superior performance of MC was ascribed to the influence of mechanochemical effects. These effects contributed to the refinement in the geometrical characteristics of RCF, exfoliation of the passivating layers, and facilitation of CO₂ dissolution, which favored the structural disintegration of RCF and carbonation progress. Another distinctive aspect of MC treatment was the production of a greater proportion of metastable CC characterized by reduced crystalline size, which was attributed to modifications in the carbonation environment and the structural alterations induced by mechanochemical effects. Moreover, the precipitation of silica gels commenced at approximately 4 min in the MC process, a notably earlier onset when compared with wet carbonation; additionally, a greater abundance of silica gels was observed in the current MC procedure, resulting from the higher carbonation degree caused by mechanochemical effects. The encouraging conclusions in the present work validated the feasibility of producing carbonated RCF more efficiently and paved the way for future industrial practice.

3D printed concrete (3DPC) creates opportunities, including a reduction in construction waste and time and increased design freedom. However, because of the differences in the construction technique compared to traditional concrete casting, the structures also perform differently; namely, the micro- and meso-structure and durability are shown to be different. For the 3DP technology to find its way to the market, one needs to be aware of these differences and needs to know how to quantify the above-mentioned properties, as differences in the testing methodologies impose themselves when characterizing printed instead of cast concrete. In this paper, we elaborate on the test methods to investigate the micro- and meso-structure and the durability of 3DPC. We start with a discussion on the micro- and meso-structure of the 3D printed concrete and how it is different from conventional mold-cast concrete. An in-depth discussion of the test methods to assess the durability of 3D printed concrete is outlined. Reported findings related to the two aforementioned properties are discussed. In addition, we report on the technologies proposed to improve the durability performance of 3DPC, and we highlight the remaining challenges and opportunities related to 3DPC.

3D Concrete printing requires much more elaborate quality control procedures compared to conventional concrete processing. Due to the various process steps, and the corresponding variation in material behaviour, time-, and length-scales, a single quality indicator and measurement technique (similar to the ‘slump test’ for traditional construction) cannot be selected. Instead, three families of quality indicators have been established: homogeneity during material production and deposition (quality variations), material evolution during printing (transient material behaviour), and macroscopic features and geometric conformity during printing and of the final object (geometry). For each family, quality assessment techniques which have been proven in other fields or for different applications, have been successfully transferred and adapted to the 3DCP process. In some cases, completely new methods have been developed. This paper aims to provide the state-of-the-art in such quality assessment methods, indicating high potential methods and research gaps across all scale levels of 3D concrete printing processes.

The rapid development of 3D concrete printing now offers mechanical efficiency and freedom to push the limits of construction design. The digital manufacturing process holds potential for reducing carbon footprints through design optimization. Printable concrete, which is a mix of cement (based on ordinary Portland cement), aggregates, and admixtures, is attractive due to widespread and cost-effective constituents. However, many common formulations omit gravel, requiring higher cement paste volumes and inducing significant embodied carbon. Assessing the potential of low-carbon cements like Limestone Calcined Clay Cement (LC3), calcium aluminate cement (CAC), or magnesium-based cement for 3D printing is a current challenge that can address this issue. Tailoring these construction materials to printing applications and environmental needs now drives scientific exploration. This paper comprehensively reviews alternative materials and binders such as earthen materials, geopolymers, low carbon binders or gypsum-based materials, addressing fresh and hardened properties, developed digital processes, targeted applications, and discussing advantages and drawbacks of each alternative.

Triboemission of nanoparticle aerosols from construction materials is a growing concern due to its potential impact on air quality and human health. In this study, we investigated the effect of aggregation of polyurethane fibers (PUFs) proceeding from waste on the kinetics of triboemission in cement mortars. A quantitative methodology was employed to assess the deposition rate, particle size distribution, and emissivity for the aerosols within the particle aerodynamic diameter range of 10–400 nm. The triboemission properties were correlated with the pore structure, morphology and tribochemical transformations of the particles and worn surfaces. Our results highlight the intricate influence of PUF aggregation on the kinetics of triboemission in cement mortars through both direct and indirect mechanisms and provide valuable insights into the mechanisms governing triboemission in construction materials.

This paper explores the opportunities of digital fabrication with concrete (DFC) to improve structural efficiency and achieve sustainable construction. Efficient structural solutions that drastically reduce material consumption can be achieved by ensuring direct load flow and placing material where needed. More than 50 % of material savings can be achieved by using flanges or hollow sections, providing continuity in beams or slabs, reducing the span of structures or using structural systems such as arches, trusses or deep beams. These concepts are not fully exploited as they often require expensive and complex formwork. DFC tackles the latter point, as it promises to produce complex geometries, minimising extra effort, cost, or waste. The paper discusses the optimisation potential of DFC for several structural elements and presents existing applications that demonstrate this potential. Five case studies of different technological approaches are discussed in detail, highlighting advantages and disadvantages to be addressed for widespread adoption.

Accelerated carbonation of carbonatable clinkers into building products is an effective way of CO₂ utilization. However, due to insufficient understanding on the phase characteristics of carbonatable clinkers, there is still a lack of guidance on the selection and design of carbonatable clinkers. In this study, three γ-C₂S based carbonatable clinkers were designed and synthesized, covering the carbonation active phase, the unavoidable C₂AS and amorphous glass phases when using industrial feedstocks. The differences in the carbonation activity, mechanical properties and microstructure were compared. Results show that the uncarbonated phases have a significant impact on the mechanical properties of carbonated matrix. The presence of unreacted γ-C₂S with self-pulverization induced cleavage planes and the amorphous glass phase with poor binding to the adjacent calcium carbonate crystals leads to reduced compressive strength. The carbonation reactivity of γ-C₂S formed in composite system is significantly higher than that of pure γ-C₂S. Benefiting from the higher degree of carbonation, carbonatable clinkers only need to contain >40 wt% of γ-C₂S to obtain comparable compressive strength as the pure γ-C₂S system.

Digital concrete is advancing due to growing economic incentives for construction automation. Achieving more sustainable concrete construction requires carbon reduction, and digital concrete technologies enable material-saving designs. By decoupling production strength from design strength, two-component (2K) systems utilizing aluminate precipitation offer the most flexibility, allowing more sustainable mixes with higher substitution levels. However, 2K aluminate systems are complex and demand a deeper understanding of their chemistry and strength buildup. This article reviews the basics of 2K aluminate systems, specifically aluminum sulfate-based and calcium aluminate cement/calcium sulfate-based systems, and their use in an inline active mixing reactor. An example reaction engineering analysis predicts the degree of reaction in a given reactor design, relating it to yield stress. The two chemical systems are compared, and future research recommendations are provided.

Layered double hydroxides (LDHs) have emerged as an effective ingredient for enhancing the durability of cement-based materials against chloride-contaminated coastal or marine environments. Yet, the relationship between the surface chemistry and consequential adsorption affinity, which mainly constitutes chloride binding capacity of LDHs, still remain elusive, especially in alkaline cement pore solutions. Herein, we investigate Mg-Al-CO₃-LDHs to demonstrate the cationic-ratio- (M^II/M^III-) regulated surface chloride adsorption in alkaline solutions through a series of progressively in-depth means. The regulatory mechanism of the cationic ratio is microscopically operated through diverse composition and proportion of hydroxylated clusters with varying deprotonation reactivity of bonded hydroxyl on the surface. DFT calculations combined with multiple surface characterization techniques indicate that the varying reactivity is determined by the ionic bonding characteristics of HO bond and the electrostatic attraction ability of different clusters. The LDH with Mg/Al ratio of 2.0 exhibits the optimal surface chloride adsorption among ratios ranging from 1.6 to 3.8 in alkaline solutions due to the strong resistance to nucleophilic attack from OH⁻ of Mg₂Al-OH cluster while maintaining high electrostatic attraction ability. Our findings advance the comprehension of surface interactions of LDHs with alkaline environments while underscoring the role of cationic ratio in surface chloride adsorption.

The influence of NaOH on pore structure, reaction kinetics, volume, and morphology of reaction products in high-volume-fly-ash mixtures was explored by two mixing methods: (i) direct addition of 0.2 M, 0.5 M and 1.0 M NaOH solution into cement-FA powder, and (ii) pre-dissolving FA into NaOH solution before mixing with cement. The pre-dissolution technique improved early-age mechanical performance by enhancing Si release, aiding the expedited precipitation of extra C-A-S-H gel. Both methods improved the degree of FA reaction and alite/belite hydration. However, 1.0 M NaOH negatively affected the strength and microstructure properties due to undesirable silica-gel formation, C-A-S-H gel carbonation, and increased capillary pore volume. NaOH concentration has affected the packing density of C-A-S-H gel, where high-pH systems exhibit loosely packed sheet-like clusters. Ca/Si of C-A-S-H gel in low and high pH systems evolved with increased curing age, with low pH system exhibiting high Ca/Si at 90 days.