Cement and Concrete Research最新文献

The colloidal dispersion of biopolymer-treated HNTs in the cementitious environment was studied. Three different biopolymer treatments, namely casein (C), guar gum (GG), and xanthan gum (XG), were utilized to prepare the stable colloidal dispersions of HNTs in such environments. The examination of the dispersion and time-dependent stability of untreated HNTs in a synthetic pore solution (SPS) revealed rapid agglomeration and settling of the particles. Specifically, the stabilization of XG-treated HNTs was superior compared to those treated with GG and C biopolymers. XG-treated HNT suspension in the pH 13 environment existed as monodisperse nanoparticles centered around 20 nm, while bare HNTs existed as polydisperse across 200 nm to 7000 nm. Moreover, XG-treated HNTs boosted cement mortar samples, increasing compressive strength by 29.4 % and reducing initial and final water absorption by 15.8 % and 19.9 %, respectively, after 90 days. Furthermore, the microstructural morphology of the HNT-XG-modified mortar exhibited improved compactness and homogeneity.

Most carbonation reactions are characterized by products encasing unreacted particles, leading to incomplete reactions and consequently lower material utilization efficiency. This study introduces a novel approach for preparing gradient carbonated materials by partially calcined limestone (PCL), which is calcined below full calcination temperatures to maintain some original limestone while partially transforming into lime. The mechanical strength, phases evolution and microstructure were investigated. The results indicated that the mechanical properties of the materials improve continuously and porosity markedly decreases as the calcination degree of PCL increases, peaking at around 35 %. However, excessive calcination degree impedes the formation of a compact structure. Simultaneously, the carbonation process yields CaCO₃ with a lower decomposition temperature, exhibiting two distinctive microstructural features: an encapsulating layer on the surface of unreacted limestone and the tiny particles (<5 μm) scattered between the layers. This study presents a promising approach to carbonated material design, demonstrating that through controlled partial calcination of limestone, opening avenues for more efficient material utilization.

The present study advances the understanding of deterioration progress in concrete foundations containing iron-sulfide bearing aggregates through electrochemical accelerated testing, coupled with non-destructive damage evaluation. The method addresses the challenge of time constraints in conventional laboratory studies by expediting the iron-sulfide oxidation process and subsequent concrete deterioration mechanisms in a controlled environment. Concrete cylinders cast in the laboratory and cored from an affected foundation in the field were subjected to the acceleration method under varied applied voltage and electrolyte exposure. Samples with iron-sulfide aggregates showed field-typical signs of deterioration within days to weeks, while those without these aggregates did not exhibit any measurable damage. The deterioration progressed with increased exposure time and applied voltage, evidenced by visual observation, quantified by decreased resonance frequency and increased total crack length. Complemented by microstructural and phase analyses, the results showed the successful accelerated replication of pyrrhotite oxidation and subsequent concrete deterioration.

Current packing models for aggregates are robust but they largely prioritize Particle Size Distributions (PSDs). There may be room for further improvement by incorporating aggregate shape. Here, we image up to 250 unique sand particles from 4 different sands and obtain key shape parameters such as Circularity (C), Roundness (R), and Aspect Ratio (AR). Combining these independent parameters, we introduce a new, fundamental parameter – the Particle Shape Metric (P_SM = C.R/AR). Interestingly, this P_SM has a strong correlation with the Packing Coefficient (R² = 0.98), obtained independently from helium pycnometry. Most importantly, P_SM of the sands seems to have a direct influence on the yield stress, compressive strength, and open porosity of a variety of mortar mixtures. These findings highlight the potential role of shape metrics in influencing the granular skeleton of cementitious composites, marking a paradigm shift towards optimizing their workability, strength, and durability.

In recent years, the technical capabilities of high-resolution Scanning Electron Microscopes (SEM) improved significantly. To fully utilise their potential, it is crucial that also the preparation techniques are advanced to obtain a high-quality surface which preserves even nanometre sized microstructural features. In this study, state-of-the-art resin-embedded polishing, and novel Broad Ion Beam (BIB) milling of hydrated cement paste are compared. SEM microstructural investigations are aided by image processing to determine porosity and pore geometry factors. Additionally, a comprehensive quantitative surface roughness analysis based on Atomic Force Microscopy scans is carried out. BIB-milling enabled the study of nanoscale features such as gel porosity or the acicular morphology of calcium-silicate-hydrates. Pores exhibit increased aspect ratios whereas resin-embedded polishing results in higher circularity. However, a superior vertical surface roughness was achieved by the resin-embedded polishing approach. The research highlights the advantages and drawbacks of BIB-milling as a polishing method for hydrated cement pastes.

This study investigates the effect of thixotropy and stiffness evolution on the extrudability and buildability of 3D printing concrete. Different types of supplementary cementitious materials and limestone filler were used to prepare mortars with varying levels of thixotropy and early-age stiffness. Mixtures with yield stress and flocculation (τ_floc) of 180–400 and 420–950 Pa, respectively, exhibited adequate extrudability without plastic collapse. The printing was completed 8–15 min after mixing at a vertical build-up rate of 138 mm/min. Several process parameters, including rest time to secure a penetration resistance of 150 kPa (T₁₅₀) and penetration resistance at various rest times were developed to assess early-age stiffness. Results indicate that the penetration resistance at rest time of 10–30 min and T₁₅₀ can be correlated with the buildable height at elastic buckling. A prediction model of buildable height based on penetration resistance was proposed. The early-age hydration was analyzed to evaluate the early-age stiffness.

Ettringite (AFt) is critical to the durability and strength development of cement, and its structure and performance can be affected by the presence of chemical substitutions as well. Fe is one of the major constituents of cement, and its effect on the properties of AFt needs to be further understood. Combined density functional theory (DFT) with thermodynamics modeling, the crystal properties of Fe-bearing AFt, thermodynamic stability, and mechanical properties were illustrated in this study. The results showed that Fe is preferred to incorporate into AFt for the Fe/(Al + Fe) molar ratio of AFt <20 %, and the incorporated Fe is sensitive to the pH values. Thermodynamics modeling results indicated that low Fe-bearing AFt is preferentially formed and equilibrated during C₄AF hydration. Fe-doped AFt improves the elastic isotropy, where the elastic properties in [001] direction are lowered and enhanced in the xy plane. This study illustrated the physicochemical properties of Fe-bearing AFt, providing theoretical support for the utilization of sustainable ferrite-rich cements.

Accurate model predictions of corrosion-driven damage in reinforced concrete structures necessitate a comprehensive understanding of the rate of corrosion product formation. Here, we investigate the influence of dissolved Si characteristic of cementitious systems on the rate of corrosion product transformation at alkaline pH. Compared to systems aged in the absence of Si, small amounts of Si decrease the formation rate of the thermodynamically stable corrosion product goethite by a factor of 10. The estimated first order rate constant of transformation $k$ decreases exponentially as a function of the dissolved Si concentration and follows the progression ${\text{log}}_{10}k={\text{log}}_{10}{k}_0-$ 14.65×[Si]${}^{0.28}$. Findings further suggest that the observed retardation is primarily due to the formation of a mobile aqueous Fe-Si complex. The concentration of Si in cementitious systems has a crucial influence, and additional research is required to fully incorporate this factor into reactive transport models, ultimately essential for accurate service life predictions.

This study investigates the potential of xanthan gum (XG) to serve as a biopolymer binder for improving the rheological, mechanical, and 3D printing properties of earth-based concrete, aligning with the pressing need for sustainable, low-carbon construction materials. Experimental results indicate that XG could disperse kaolinite clay particles, which likely arises from the highly negative charges of both kaolinite and XG. Rheological parameters display two trends with increasing XG concentration: initially decreasing yield stress, viscosity, and storage modulus owing to XG's dispersing effect, followed by an increase due to polymer overlapping. The same trend is observed in 3D printing experiments, where the kaolinite clay suspensions exhibited enhanced buildability with increasing XG concentration and eventually achieved a “Printable” state at 5 % XG. Additionally, compressive strength was observed to steadily increase with increasing XG content, for instance, nearly tenfold with 2.4 % XG compared to 0 % XG (0.34 MPa to 3.58 MPa). This exploration highlights the pivotal role of XG as a dual-functionality agent, acting as a robust binder and a promising rheology modifier.

The thixotropic behaviour of cementitious materials received widespread attention in recent years. However, structural break-down is often ignored in most research. The role of structural break-down in thixotropy of magneto-responsive cementitious materials should be discussed in order to achieve comprehensive active rheology control of cementitious materials, not only from more liquid-like to more solid-like (set-on-demand), but also from more solid-like to more liquid-like. This research investigates the thixotropy of magneto-responsive cementitious materials in combination with the structural break-down. The magnetorheological response of cement pastes subjected to unipolar and bipolar magnetic fields is experimentally evaluated. The effects of magneto-responsive particle size, magnetic field direction, intervention duration, magnetic flux density and magnetic field frequency on thixotropy of cementitious materials containing carbonyl iron particles (CIPs) are studied. The shear stress of magneto-responsive cementitious materials can be changed in time and on demand by the structural build-up and structural break-down of magneto-responsive particles' alignment and/or movement. With the increase of magnetic flux density, the shear stress of magneto-responsive cement paste reaches a minimum first and afterwards increases dramatically. When the magnetic field frequency increases from 0 Hz to 0.2 Hz, the yield stress of magneto-responsive cement paste decreases dramatically, followed by a slight increase. The origins of thixotropy for fresh magneto-responsive cement paste considering the magneto-responsive particle agitation (e.g. vibration and movement, etc.) are presented.