Cement and Concrete Research最新文献

Quantitative phase analysis of cementitious materials is crucial for understanding and predicting their performance. For anhydrous Portland cements, a variety of techniques based on microscopy, spectroscopy, and diffraction exist. Here, we employ two highly diverse techniques (X-ray Diffraction and Raman Imaging) to a common sample (ASTM Type I/II OPC). We find that, firstly, the two methods are highly complementary in the initial phase identification, given that certain clinker phases are difficult to detect in one as opposed to the other. Secondly, quantitative analysis can be influenced by parameters such as preferred orientation, order of phase addition, and the nature of the binarization algorithm. Specific signal-to-noise (SNT = 5) and epoxy-to-signal (EST = 10) thresholds resulted in the highest alignment between the two methods (R² > 0.99, Δwt% < 2.5 %). Thirdly, Raman imaging area can be reliably reduced (25 mm² to 9 mm²) to shorten scan time by 2.5× (8 h to 3 h).

Fe-rich cementitious phases, such as tetracalcium aluminate (C₄AF), are likely the least characterized and understood components of ordinary cements, in terms of their reactivity and early-age reactions. In this study, we provided the first in-situ measurements of C₄AF's absolute dissolution rates in water and aqueous sulfate solutions (Na₂SO₄, MgSO₄ and CaSO₄) along with its early-age hydration characterization. Upon contact with water, C₄AF dissolved at a rate of 1.74 ± 0.23 μmol·m⁻²·s⁻¹, and sulfates were found to inhibit its reaction. Among the three types of sulfates tested, CaSO₄ exhibited the strongest inhibition effect, while Na₂SO₄ showed the weakest inhibition. Soon after the initial dissolution, the precipitation of hydrates occurred, and the presence of sulfates affected the overall hydration process of C₄AF. These results enhance our understanding of C₄AF's dissolution and early-age hydration behaviours, thereby advancing our knowledge of its hydration mechanisms.

The mechanisms responsible for the compromised or lost dispersing capability of polycarboxylate ether (PCE) superplasticizers in two-part alkali-activated slag (AAS) have been extensively studied, but limited scientific understanding is available on how PCEs function or lose their dispersing efficiency in one-part AAS systems, particularly those prepared using greener carbonate -based solid activators. This study investigates the dispersing and adsorption behaviors of phosphate-based PCE in one-part AAS activated by K₂CO₃ or a combination of K₂CO₃ and CaO. Our findings indicate that in K₂CO₃-activated slag systems, the loss of phosphate-based PCE efficiency primarily results from the conformational contraction of PCE molecules, which deteriorates steric repulsion in the alkaline activating solution, rather than the insolubility of PCEs in alkaline solution or competitive adsorption between PCEs and carbonate anions on dissolving slag grains. In K₂CO₃-CaO-activated slag systems, when PCE dosage is below 5 mg/g, the consumption of PCE by early precipitates (e.g., C-A-S-H, gaylussite, calcite) contributes substantially to its lost efficiency; however, once the PCE dosage surpasses this threshold, the dominant factor influencing PCE efficiency shifts to the conformational change of PCE.

This study compares, for the first time, MgO-based (MB) cement and Portland-based (PB) cement for stabilizing earth mortars. While MB and PB earth mortars reach similar strength, MB cement stabilization demonstrates superior early-age performance. Thermogravimetric analysis, X-ray diffraction, ²⁹Si and ³¹P NMR spectroscopies show that the cement reacts in both systems and allow to establish the phase assemblages. The stabilized earth pastes contain less hydroxide phases, indicating a pozzolanic reaction in both cases.

MB-stabilized clay mortars retain about 1/3 of the compressive strength of pure MB mortar, while with PB this proportion is only 1/5. This difference demonstrates that MB is more compatible with clay minerals and more suitable for stabilizing earth mortars. If MB cement could be produced with renewable energy from CO₂-free sources (instead of from magnesite), stabilization of earth mortars with MB would be substantially more CO₂ efficient than with PB.

Hypothesis

Accurately predicting the maximum particle loading fraction (ϕ_max) of a suspension remains a significant challenge in both theoretical modeling and industrial applications.

Experiments

We present a novel method that surpasses existing approaches by precisely determining ϕ_max through predicting the general suspension viscosity at a constant shear rate as a function of particle volume fraction. Our approach leverages boundary and initial conditions to pinpoint ϕ_max with precision.

Findings

The proposed model flawlessly captures viscosity behavior across the entire range of volume fractions without pre-assumptions or limitations, showcasing its remarkable versatility. We validate the efficacy of our method by comparing its predictions with established theoretical models and diverse experimental data for various suspensions, including nanofluids and yield stress fluids, as reported in the literature. This extends to the evaluation of crucial parameters related to ϕ_max within existing suspension viscosity models. Beyond its immediate applications, this approach opens avenues for exploring relationships between ϕ_max and other suspension properties, potentially leading to broader advancements in understanding or manipulating suspension rheological behavior in material science.

Additive Construction has increased dramatically within the United States in the last few years. Efforts to develop acceptance criteria have increased since 2020 and are being developed through integration of research efforts and early engagement with partners across academia, industry, and government. This review paper builds on the work by Bos et al. (2022) through addressing gaps identified and outlines developments within the United States through engagements with the international community to align experts within the field towards common goals, acceptance criteria, and the early integration of acceptance criteria with lessons learned for Additive Construction in practice.

Digital fabrication technologies, such as 3D concrete printing, are currently making their way into the construction industry. The primary focus in this field is often on the depositing processes, such as extrusion 3D concrete printing, where material is typically applied in horizontal planar layers. This area has seen substantial progress in recent years. However, numerous research and development projects are specifically targeting the additive manufacturing of unreinforced raw concrete components. When implementing these technologies in practice, it has become clear that additional processes, such as fully automated process-parallel reinforcement integration, application of cover layers and formative and subtractive post-processing of the components, are essential for successful application. In addition, by varying the orientation, characteristics and arrangement of the layers, new shapes and functions can be realised. Examples include angled layer orientation for producing vaulted geometries without support structures, as well as non-planar layer formation for complex component geometries or assembly joints. Moreover, alternative innovative manufacturing processes, such as KnitCrete, Smart Dynamic Casting or Injection 3D Printing, reveal new potential for the application of digital manufacturing technologies in the construction industry. This article aims to demonstrate the possibilities offered by digital fabrication with concrete beyond the stacking of horizontal planar layers, and how these technologies can complement and expand a future digital fabrication strategy in the construction industry.

Accelerated impressed current testing is the most common experimental method for assessing the susceptibility to corrosion-induced cracking, the most prominent challenge to the durability of reinforced concrete structures. Although it is well known that accelerated impressed current tests lead to slower propagation of cracks (with respect to corrosion penetration) than in natural conditions, which results in overestimations of the delamination/spalling time, the origins of this phenomenon have puzzled researchers for more than a quarter of a century. In view of recent experimental findings, it is postulated that the phenomenon can be attributed to the variability of rust composition and density, specifically to the variable ratio of the mass fractions of iron oxide and iron hydroxide-oxide, which is affected by the magnitude of the applied corrosion current density. Based on this hypothesis, a corrosion-induced cracking model for virtual impressed-current testing is presented. The simulation results obtained with the proposed model are validated against experimental data, showing good agreement. Importantly, the model can predict corrosion-induced cracking under natural conditions and thus allows for the calculation of a newly proposed crack width slope correction factor, which extrapolates the surface crack width measured during accelerated impressed current tests to corrosion in natural conditions.

Amorphous calcium (alumino) silicates are the main component of industrial byproducts (e.g., blast furnace slag and fly ash) and can be generated by grinding silicate minerals that are abundant in steel slag and carbonated calcium silicate binders. To promote the production of CO₂-activated building material from diverse materials, this study investigated the carbonation of synthetic calcium silicate glasses, model compounds of these amorphous silicates. A dissolution-controlled carbonation mechanism was revealed, in contrast with a nucleation-controlled counterpart for the carbonation of wollastonite, a model compound of silicate minerals. The former mechanism is governed by the number of Q³ species in the silica layers on carbonated particle surfaces serving as sites for ionic exchange. The latter is evidenced by well-aligned CaCO₃ nuclei precipitating on wollastonite particles under high CaCO₃ supersaturation. Overall, high Ca/Si ratios favor carbonation. At Ca/Si = 1, the calcium silicate glass shows faster carbonation kinetics than wollastonite at early ages.

Recently, the widespread use of high‑potassium limestone has led to a sharp increase in the K₂O content in clinker, resulting in high‑potassium cement clinker. When mixed with water, high‑potassium cement clinker releases a significant amount of K⁺ ions in a short period, adversely affecting the performance of cement. Changes in the silica modulus can modify the proportion of silicate minerals and flux minerals that act as carriers of K⁺ ions, which in turn affects the solidification and the hydration rate of K⁺ ions. This study aims to investigate the influence of varying silica modulus in high‑potassium clinkers on the solid solution and hydration release rate of K⁺ ions and its influences on the mineral composition and hydration performance of high‑potassium clinkers. The results indicate that an increase in silica modulus leads to a decrease in the solid solubility of K⁺ ions in clinkers, resulting in a reduction in the ratio of C₂S-α′_L/C₂S-β and the ratio of C₃A-o/C₃A-c. Additionally, an increase in the silica modulus delays the sulfate depletion peak, retards the release of K⁺ ions, inhibits the transformation of AFt to Ms, and ultimately increases the compressive strength at 28 days.