Cement and Concrete Research最新文献

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.

The hydration processes of Portland cements (PC) and blends are complicated as there are many components with great heterogeneity at different length scales. Thus, 3D nanoimaging techniques with high spatial resolution and scanning large fields of view are needed. Here, synchrotron X-ray near-field ptychographic tomography is used to investigate four pastes within 0.2 mm thick capillaries: PC with CaCl₂, PC hydration enhanced by C-S-H nucleation seeding, PC partly substituted with metakaolin, and PC partly substituted with metakaolin and limestone. Data analysis emphasis has been placed on the characterization of amorphous components: (i) C-S-H and C-A-S-H gels; (ii) iron aluminium siliceous hydrogarnets; (iii) metakaolin; and (iv) aluminium carboaluminate, AFm-like. Synchrotron ptychotomography yields electron density and absorption coefficient tomograms and the resulting bivariate plots are instrumental for characterising these amorphous components. The attained spatial resolution, ∼220 nm, with very good contrast allowed us to determine nanofeatures including mass densities and spatial distributions of amorphous components. For instance, the C-S-H gel mass density differences between the two type of accelerated pastes are detailed.

This study employed core specimens taken from a 24-year-old concrete bridge to investigate the ITZ spatial variation. As the lowest hardness component in the concrete, the ITZ was determined by height differences with other components caused by polishing. An automatic method for identifying the ITZ was developed based on 3D images with horizontal and vertical resolutions 1.75 μm and 0.41 μm, respectively. The thicknesses of ITZ and height differences between ITZ and mortar were statistically analysed, and a copula joint distribution model was established to describe these characteristics. The results showed that thicknesses at beam end and midspan were 47.62 μm and 42.95 μm, and height differences were 24.38 μm and 19.98 μm, respectively, indicating that the ITZ degradation in beam end was more severe. However, the correlation coefficients for the joint distribution at beam end and midspan were nearly identical (0.7197 and 0.7111, respectively), showing a relative stable relationship between the ITZ thickness and hardness.

In this paper, we are studying the shrinkage and microstructural damages induced by drying in samples exposed to drying at very early ages. We measure for two model mortars the average and local shrinkage through microtomography images subtraction along with microstructure evolution through microtomography radiographs. Our measurements showcase the role played by the kinetics of propagation of drying-induced capillary stresses on shrinkage distribution. They moreover show the existence of two distinct types of microstructural damages in such exposed samples occurring at the onset of the sample desaturation after a period as short as a couple of hours. Our analysis allows for a first-order understanding of the scaling of the main stress, length and time scales of the problem with some material parameters such as compressibility and permeability. We finally extrapolate and discuss the consequences of the above features on standard concrete casting and on concrete 3D printing.

The fracture toughness of cement paste is difficult to quantify both by standard nanoindentation tests and by time-consuming and expensive measurements on micro-specimens milled with a focused ion beam. Here, a well-calibrated scratch test with simultaneous recording of acoustic emission signals was used to quickly and easily provide statistically relevant quantitative results for a wide range of scales (1–100 µm). The microscale fracture toughness for the main hydration products reached 0.54 ± 0.03 MPa$\cdot$m${}^{1/2}$ for the outer product, 0.64 ± 0.05 MPa$\cdot$m${}^{1/2}$ for the inner product, 0.66 ± 0.06 MPa$\cdot$m${}^{1/2}$ for Portlandite, and 1.24 ± 0.20 MPa$\cdot$m${}^{1/2}$ for clinker on well-hydrated sample. Two primary deformation mechanisms inherent in the micro-scratch process, material compaction and “ripping off”, were identified and their impact on fracture toughness and friction coefficient was quantified. Simulation of cracking and damage mechanisms, plus estimation of the otherwise unavailable tensile strength of compacted cement paste constituents, were successfully modeled using a Griffith-type fracture model.

This study aims to achieve an in-depth understanding of the manufacturing process of natural hydraulic lime (NHL) by assessing the influence of raw materials' chemical- mineralogical composition and the effect of the slaking process. NHLs with variable hydraulicity were manufactured using 56 raw materials from carbonate outcrops in Andalusia (Spain). This study shows that siliceous limestones with microcrystalline quartz generate hydraulic phases after calcination. However, when the amount of this reactive silica exceeds 18% by weight, CaO is not formed, and only calcium silicates appear. It was also found that slaking of NHL leads to partial hydration of the most reactive calcium silicates, reducing the expected reactivity of the lime. Instead, exposure of NHL quicklimes to environmental relative humidity promotes the formation of disordered portlandite and reduces the partial hydration of hydraulic phases. Our findings demonstrate that standard slaking can be replaced by alternative methods for the studied binders.

Mitigating the release of dihydrogen resulting from metal corrosion or water radiolysis is an important issue for the disposal of certain types of cemented radwaste packages. The approach investigated in this work consists in adding an oxide getter (γ-MnO₂/Ag₂CO₃) to the cement matrix. Since the efficiency of the getter decreases under wet environment, two self-desiccating binders (calcium sulfo-aluminate and magnesium potassium phosphate cements), are used to obtain significant desaturation of the pore network by the sole hydration reactions. The getter slightly influences the rate of cement hydration at early age, but has no effect afterwards. Sorption of ions released by dissolution of cement phases onto γ-MnO₂ is evidenced, as well as partial or total destabilization of silver carbonate. Nevertheless, the getter still enables to reduce strongly the outgassing of dihydrogen from mortars encapsulating Al-metal, which opens new perspectives to improve the conditioning of waste producing H₂ in a cement matrix.

Although the mechanism of pore formation in cement paste owing to high temperatures is a critical characteristic directly linked to fire resistance, research regarding the 3D characteristics of nano-scale pores within a consistent volume is limited. This study uses synchrotron X-ray nanoimaging to investigate the impact of heating at various temperatures (400, 600, and 800 °C) on the morphology, distribution, and volume changes of nano-scale pores in a consistent volume of ordinary Portland cement paste. The mechanical and hydration properties were assessed via compressive strength tests, X-ray diffraction, and thermogravimetry. After heating to 400 °C, new high-flatness pores formed, whereas heating to 600 and 800 °C resulted in pore coalescence and the formation blade-like pores developed around unhydrated cement particles. Elongated pores formed after heating to 600 °C, which resulted from the decomposition of Ca(OH)₂, leading to the structure being prone to internal cracking.