Cement and Concrete Research最新文献

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.

Fineness, clinker mineralogy and sulfate carrier control intensity and kinetics of Portland cement hydration over time. In this study lab cements with defined particle size distribution were prepared from clinkers with varied C₃A content and sulfate carrier (interground/blended). A novel sample preparation procedure enabled the systematic increase of cement fineness, resulting in varying sulfate demands relative to the C₃A surface area, without altering the bulk composition of the binders. The samples were analyzed using isothermal calorimetry (IC) over 72 h, along with X-ray diffraction (XRD), semi-adiabatic calorimetry, and solution analyses. At low fineness, all lab cements exhibited similar IC patterns. However, when high-C₃A clinker was ground to higher fineness, rapid SO₃ consumption led to uncontrolled C₃A hydration and delayed the main C₃S hydration by several hours. Interestingly, blended sulfate carriers did not inhibit alite hydration, despite apparent early sulfate depletion. A new model is proposed to enhance the understanding of clinker and sulfate interactions in cement production.

This study aims to evaluate the influence of natural fiber leachates on cement pastes for advanced bio-composite production. Cement samples are made by mixing cement with leachates from oil palm and coir fibers at three concentrations. Through Nuclear Magnetic Resonance T₁-T₂ relaxations, fresh cement samples are evaluated for hydration reaction and microstructure. Results show that oil palm fiber leachates prolong plateaus in T₁ and T₂, suggesting a retarding effect on cement hydration. Subsequently, increased T₁ values and an extra T₂ component are observed which suggest coarser pores. Additionally, the leachates increase water fractions, reflecting reduced cement hydration degree. With increasing leachate concentration, the retarding effect and extra larger pores become apparent. Conversely, coir fiber leachates exhibit negligible influence. This difference is attributed to the less saccharides. These saccharides function through simultaneous adsorption, nucleation, complexation and precipitation. In conclusion, coir fibers demonstrate better compatibility with cement than oil palm concerning leachates.

Understanding the sorption of corrosive ions in cement requires a complete comprehension of charge reversal at the C-S-H/electrolyte interfaces. However, this charge phenomenon remains incompletely understood. We develop a mean-field molecular theory to revisit charge reversal behaviors by investigating how ions relax at the interfaces – whether bound or mobile – while considering ion–surface and ion–ion interactions. As a feature of our theory, we allow divalent calcium ions to adopt two binding configurations – bridging and non-bridging modes – with the ionized silanol sites, highlighting the necessity of multivalent ion condensation for charge reversal. Conversely, we demonstrate that the product of bulk concentration and the exponential of the electrosteric energies governs the accumulation of mobile ions at the interfaces, where a cancellation between them causes nonmonotonic behaviors for the mobile ions. In short, comprehending how ions compact the interfaces enables our theory to capture published experimental and simulation results, facilitating a deeper understanding of the charge reversal phenomenon.