Cement and Concrete Research最新文献

Coal fly ashes, long used as SCMs, are quite complex, consisting of a mixture of crystalline and amorphous phases. In this study, we aim to shed light on this complexity by fusing two highly complementary techniques: SEM-EDS and Raman spectroscopy. By closely analyzing hundreds of individual particles from 18 unique fly ashes, we report two major findings. Firstly, there is a distinct correlation between particle shape (roundness/circularity) and degree of crystallinity (FWHM of Raman peaks) where jagged particles happen to be almost always crystalline. Secondly, the mean position of the symmetric stretching Raman band of the silicate tetrahedra (between 600 and 1000 cm⁻¹) is an indicator of the degree of polymerization of the glassy phase in any given particle. These results highlight the importance of understanding these complex systems at the individual particle level, where multiple phases intermixed at the micro-scale ultimately play a dominant role in governing macro-scale ash behavior.

The printability of concrete mixtures is evaluated for varying aggregate content using binder pastes of different compositions. The inter-relations between the binder paste rheology and the aggregate content are explored for printable concrete mixtures made with pastes of alkali-activated binders and cement. Concrete printability depends on the paste content for coating aggregate and filling spaces between the aggregate. The paste coating thickness around the aggregate under pressurized flow conditions is identified with the boundary layer at the aggregate surface, and it depends on the paste viscosity. Pressurized concrete flow in extrusion-based printing requires excess paste content beyond filling spaces in compacted aggregate with coating layer. There is a linear relationship between the excess paste content in printable concrete mixture and the yield stress of the paste. Printable concrete mixtures made with pastes of higher yield stress require larger excess paste content and accommodate smaller aggregate fractions.

Concrete is susceptible to cracking and damage when exposed to severe or repeated temperature gradients, leading to the deterioration of mechanical and durability properties, significantly impacting the normal operation and service life of structures. This study investigates the influence of temperature gradients on the deterioration of mechanical properties and microstructure in concrete. Experiments were designed to establish temperature gradients and detect the microstructure characteristics and mechanical properties post-damage. The microscopic mechanisms governing the evolution of mechanical properties under temperature differences were explored, and a mathematical relationship between them was analyzed. The findings reveal that the severity of thermal damage is directly proportional to the enhancement of temperature differences and proximity to the heat source. The temperature gradient-induced stiffness enhancement and structural damage contribute to a substantial reduction in split tensile strength, while compressive strength initially increases and then weakens. The peak displacement demonstrated a strong exponential relationship with the carbonation parameters. A mathematical model for the strength of concrete influenced by temperature gradients was established, considering the impacts of porosity and carbonation parameters.

Accurate simulation of the hydration process and microstructural evolution of cement paste is essential to predict the diffusion and mechanical properties of cementitious materials. This study developed a continuous hydration model integrated with X-ray computed tomography (XCT) image-based cement particles, for the first time, to simulate the hydration process and the resultant microstructural evolution of cement paste. In this model, real cement particles are characterized analytically by spherical harmonic (SH) functions, followed by their SH coefficients to derive the hydration kinetics and rate controlling equation of hydration product. Then the hydration process is realized by merging and embedding of hydration products between multi-sized particles. After validation, the hydration model is applied to investigate the microstructural development of cement pastes with various water-cement ratios at different curing temperatures, showing the strong simulation capabilities of the developed hydration model.

For alkali-silica reaction (ASR) gels, the relationship between swelling expansion, structure and chemical composition, particularly the effect of aluminum, remains unknown. This study investigates the structure, swelling expansion and associated water uptake of synthetic ASR gels with various Al/Si (0–0.1) and Ca/Si (0.1–0.4) ratios. The results show that aluminum incorporated into the gel structure reduces the overall swelling expansion and the leaching of silicate species during the swelling test. Moreover, they revealed that water in the Al-ASR gels is more tightly bound, reducing the overall water uptake compared to the Al-free ASR gels. Additionally, there is a linear correlation between the maximal swelling results and the ASR gel composition. However, no direct correlation emerged between the amount of water uptake and the free swelling of the ASR gels, which indicates that other factors, like the type of water bonding and pore size of the gels, are decisive for the swelling mechanism.

Phosphate modified calcium aluminate cement (CAPC) has been demonstrated to be superior to Portland cement under harsh conditions including high temperature and severe corrosiveness. A systematic and quantitative investigation into phase evolution of hardened CAPC pastes cured under 60 °C is conducted to lay a technical basis for cementing CO₂ storage wells. It is found that increasing phosphate dosage results in decreasing contents of C₃AH₆ and AH₃, the typical hydration products of calcium aluminate cement (CAC), and increasing content of C-A-P-H gel in hardened CAPC pastes. The critical phosphate dosage for complete depletion of C₃AH₆ lies in 20%–30% by weight of CAC. Furthermore, C-A-P-H gel, known as the characteristic reaction product of CAPC, is proposed to be constituted of Na-substituted nano-hydroxyapatite and pseudo boehmite clusters. Hardened CAPC pastes exhibit remarkably smaller pore sizes due to C-A-P-H gel, explaining their higher compressive strength compared with hardened CAC pastes.

This study investigates the correlation between the molecular architecture of methacrylate ester-based polycarboxylate (MPEG PCE) superplasticizers and their influence on the rheological characteristics of alkali-activated slag (AAS). Two variants of MPEG PCE superplasticizers were synthesized utilizing distinct acid monomers (methacrylic acid versus acrylic acid), while maintaining consistent structural parameters for both types. These parameters included the uniform length of the MPEG macromonomer side chain, similar anionicity in the trunk chain, and comparable molecular weights (M_w) of the copolymers. The efficacy of these superplasticizers in AAS binders was assessed through spread flow and rheology testing, and their performance was benchmarked against that in ordinary Portland cement (OPC) pastes. To elucidate the interactions between PCE polymers and AAS binders, this study undertook comparative evaluations encompassing adsorption isotherms, anionic charge densities, solubility, and conformational transformations of calcium complexes for both acrylic acid (AA) and methacrylic acid (MAA) variants.

3D-printed concrete is exposed to rapid pore water evaporation immediately after extrusion, leading to plastic shrinkage and cracking. Plastic shrinkage and related cracking can severely impair the durability, serviceability, aesthetics and structural stability of 3D-printed concrete elements. This article addresses the specific evaporative and deformation behaviours of 3D-printed concrete elements. The absence of the formwork was found to more than double the extent of plastic shrinkage. Moreover, the study showed that the extent of plastic shrinkage is proportional to the exposed surface-to-volume ratio. In addition, the study confirms that concrete elements printed with thin filaments are more susceptible to plastic shrinkage than those printed with thick filaments.

Water confined within cement paste significantly influences the material's physical and chemical behaviors. Using quenched solid density functional theory (QSDFT), we investigate the complex behaviors of water confined within C-S-H interlayers. Confined water exists in forms of double layers (0–0.29 nm pores), three layers (0.32–0.59 nm pores), and multiple layers (≥0.65 nm pores). The adsorption isotherms reveal distinct adsorption behaviors depending on the pore size. For pores smaller than 0.59 nm, adsorption occurs as monolayer water adsorption and phase transition. In contrast, larger pores exhibit three stages: monolayer adsorption, multilayer adsorption, and phase transition. The pore pressure is positive for smaller pores (≤0.03 nm) and negative for larger pores; however, after the phase transition, the negative pressure is released with increasing relative humidity (RH). Additionally, temperature increase reduces the adsorption capacity, disrupts the water ordering, shortens the phase transition period, and affects the saturated pore pressure.

The hygrothermal stability of ettringite in cementitious systems composed of calcium aluminate cement (CAC) blended with ordinary portland cement (OPC) and calcium sulfate (CŠ) has been investigated for five simulated environmental conditions- i) 23 °C-100% RH (Relative Humidity), ii) 23 °C-50% RH, iii) 38 °C-50% RH, iv) 50 °C-50% RH, and v) 60 °C-50% RH. Two types of ternary systems were used, one rich in CAC (with a higher mass ratio of CAC compared to OPC) and the other rich in OPC (with a higher mass ratio of OPC compared to CAC), both with a water-to-cement (w/c) ratio of 0.35. The volume change and mechanical properties were evaluated by dimensional change, mass change, compressive strength, and flexural strength measurements. Hydration and microstructural properties were evaluated by isothermal calorimetry, thermogravimetric analysis (TGA), and Quantitative X-Ray Diffraction (QXRD) via the G-factor method. QXRD results showed that the studied CAC-rich binder formed 26.0 ± 1 wt% and the OPC-rich binder 28.9 ± 1 wt% ettringite after 24 h of hydration. However, with the increase in temperature, decrease in relative humidity, and time of exposure, the OPC-rich binder showed higher shrinkage, higher mass loss, and a significant decrease in ettringite content compared to the CAC-rich binder.