Understanding the changes in the pore solution upon carbonation is crucial with respect to comprehending the mechanisms of reinforcement corrosion in carbonated mortar or concrete.
In this paper we ...used Cold Water Extraction (CWE), a rapid leaching method, on profile ground powder from partially carbonated Portland cement and Portland-fly ash cement mortars to study changes in the pore solution composition.
Carbonation decreased the free alkali metal content measured with CWE, which matched with the portlandite profiles determined by thermogravimetry and the carbonation depth detected with pH indicator.
The alkali metal uptake by carbonation products was confirmed by SEM-EDS and thermodynamic modelling. Besides the decrease in the alkali metal concentration in the pore solution upon carbonation, we observed for both binders a considerable increase in chlorine and sulphur concentrations. This can further accelerate corrosion in carbonated concrete.
The effect of fly ash on the hydration of calcium sulfoaluminate cement was investigated. Increasing fly ash contents accelerated the hydration of calcium sulfoaluminate cement due to the filler ...effect. Dissolution rims around fly ash particles after 90days indicated a reaction degree of the fly ash of approximately 20 to 30% as estimated by various independent methods. The contribution of fly ash to the hydration reactions resulted in the formation of C-S-H, in an increase of the strätlingite content and in the destabilization of monosulfate.
The mortar with 7.5 mass-% fly ash reached a higher compressive strength than the reference without fly ash when the water to cement ratio was kept constant. Up to 15 mass-% of fly ash could be added without strength loss.
There exists limited information regarding the effect of temperature on the structure and solubility of calcium aluminosilicate hydrate (C–A–S–H). Here, calcium (alumino)silicate hydrate (C–(A–)S–H) ...is synthesised at Ca/Si=1, Al/Si≤0.15 and equilibrated at 7–80°C. These systems increase in phase-purity, long-range order, and degree of polymerisation of C–(A–)S–H chains at higher temperatures; the most highly polymerised, crystalline and cross-linked C–(A–)S–H product is formed at Al/Si=0.1 and 80°C. Solubility products for C–(A–)S–H were calculated via determination of the solid-phase compositions and measurements of the concentrations of dissolved species in contact with the solid products, and show that the solubilities of C–(A–)S–H change slightly, within the experimental uncertainty, as a function of Al/Si ratio and temperature between 7°C and 80°C. These results are important in the development of thermodynamic models for C–(A–)S–H to enable accurate thermodynamic modelling of cement-based materials.
This study investigates the influence of epitaxial orientation on the phase structures and electrical properties of Barium Strontium Titanate (Ba1-xSrxTiO3, BST) thin films by employing a ...thermodynamic model. A comprehensive analysis is conducted, including the construction of strain-concentration and temperature-strain phase diagrams for (001)-, (110)-, and (111)-oriented BST thin films. The results demonstrate that BST thin films with high-index (110) and (111) orientations have a greater tendency to stabilize the low-symmetry ferroelectric phase compared to the low-index (001) orientation. Subsequently, the impact of epitaxial orientation on the piezoelectric and electrocaloric properties is explored based on the constructed phase diagrams. Significantly, enhanced piezoelectric and electrocaloric effects closely related to the epitaxial orientation are observed at the different phase boundaries for BST thin films. These findings have profound implications for future experimental investigations and the design of devices incorporating BST thin films with precise epitaxial orientations and innovative functionalities.
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The carbonation of Portland cement, metakaolin and limestone mortars has been investigated after hydration for 91days and exposure to 1% (v/v) CO2 at 20°C/57% RH for 280days. The carbonation depths ...have been measured by phenolphthalein whereas mercury intrusion porosimetry (MIP), TGA and thermodynamic modeling have been used to study pore structure, CO2 binding capacity and phase assemblages. The Portland cement has the highest resistance to carbonation due to its highest CO2 binding capacity. The limestone blend has higher CO2 binding capacity than the metakaolin blends, whereas the better carbonation resistance of the metakaolin blends is related to their finer pore structure and lower total porosity, since the finer pores favor capillary condensation. MIP shows a coarsening of the pore threshold upon carbonation for all mortars. Overall, the CO2 binding capacity, porosity and capillary condensation are found to be the decisive parameters governing the carbonation rate.
Hydration of a commercial white calcium aluminate cement (CAC) at 23 °C was modified by silica fume (SF) addition in varying amounts. The process was followed by heat flow calorimetry, quantitative ...in-situ XRD analysis and Gillmore needle experiments supplemented by pore solution analysis, thermodynamic modelling, and 1H-TD-NMR measurements. Lower SF/cement ratios accelerate the hydration of CAC. Higher ratios trigger an intermediate heat flow event, which is correlated to increased setting rates. This intermediate event (IE) initiates an induction period of constant duration, which appears later with increasing SF/cement ratios. Results show the IE is caused by an initially hindered CA dissolution, in which dissolved silicon provided by SF plays a crucial role. Increasing the Si concentration in the pore solution leads to a further retardation of the IE and eventually prevents the entire hydration reaction if a critical amount is reached. A detailed model explaining the observed behavior is proposed.
The main reaction product in Ca-rich alkali-activated cements and hybrid Portland cement (PC)-based materials is a calcium (alkali) aluminosilicate hydrate (C-(N-)A-S-H) gel. Thermodynamic models ...without explicit definitions of structurally-incorporated Al species have been used in numerous past studies to describe this gel, but offer limited ability to simulate the chemistry of blended PC materials and alkali-activated cements. Here, a thermodynamic model for C-(N-)A-S-H gel is derived and parameterised to describe solubility data for the CaO–(Na2O,Al2O3)–SiO2–H2O systems and alkali-activated slag (AAS) cements, and chemical composition data for C-A-S-H gels. Simulated C-(N-)A-S-H gel densities and molar volumes are consistent with the corresponding values reported for AAS cements, meaning that the model can be used to describe chemical shrinkage in these materials. Therefore, this model can provide insight into the chemistry of AAS cements at advanced ages, which is important for understanding the long-term durability of these materials.
•Impact of the aluminum concentration on the synthesis of V2AlC is investigated.•Thermodynamic calculations are done to predict oxidation reaction pathway of V2AlC.•Oxidation thermal kinetic of the ...V2AlC is studied.•Kinetic triplets responsible for oxidation of the V2AlC are determined.•P4 & A4 nucleation mechanisms are identified in stage I & stage II of oxidation.
Nanolaminated V2AlC MAX phase has been synthesized through pressureless sintering by varying aluminum content. The oxidation stability of the V2AlC is studied under non-isothermal conditions through a TGA/DTA technique at multiple heating rates in the air atmosphere. It is observed that the oxidation of V2AlC occurred in two different stages. Thermodynamic calculations are also performed to predict the oxidation reaction pathway of V2AlC MAX phase. The kinetic triplets (activation energy, pre-exponential factor and reaction mechanism) are determined for both stages of oxidation. The Kissinger-Akahira-Sunose (KAS) and the Flynn-Wall-Ozawa (FWO) isoconversional kinetic methods are used to calculate the activation energy. The reaction mechanism is identified by employing the integral master plot method. The results indicated that the nucleation mechanism dominated the oxidation process in the V2AlC MAX phase. P4 and A4 nucleation reaction mechanisms are identified in stage I and stage II of oxidation, respectively.
It is of tremendous difficulty to draw a conclusive verdict on the complex process of mutual interference regarding the combined chloride-sulfate attack on concrete, where many have explored the ...topic, but contradicting results have been reported. In this paper, by taking advantages of the chemo-physical-mechanical method, based on the robust technique of thermodynamic modelling, the competitive antagonism effect in combined chloride-sulfate attack is investigated. Firstly, a range of reported tests are numerically modelled to demonstrate the effectiveness of the employed method in interpreting those opposing and diverse experimental outcomes. More importantly, a fresh conjecture incorporating a novel assessment criterion for the competitive antagonism effect is proposed and supported through a carefully designed illustrative example. Based on the numerical exploration, unique findings and implications for future engineering practices are revealed.
•A chemo-physical-mechanical method based on thermodynamic modelling is implemented.•The contradicting findings from reported experiments are numerically explored.•The competitive antagonism effect is time-dependent rather than a conclusive term.•The competitive antagonism effect is quantifiable with a novel assessment criterion.
•The compressive strength and flexural strength of shotcrete with SCMs at 40–80 °C were investigated.•The hydration products of shotcrete with SCMs at 40–80 °C were analysed via XRD, SEM-EDS, and ...TG.•The hydrate phase assemblage was simulated by thermodynamic modelling (GEMS).
The effect of temperature on the strength, hydration products and microstructure of shotcrete blended with or without 15 wt% FA, 8 wt% SF, or 15 wt% GBFS are investigated in the range of 20 to 80 °C. The characterization of hydration products, including the type and content of hydration products, C-S-H composition, microstructure and porosity were determined via XRD, Raman spectroscopy, SEM-EDS, and TG. In addition, the hydrate phase assemblages depending on temperature are simulated by thermodynamic modelling (GEMS). The results indicated that the hydration temperature of 40–80 °C promotes a fast hydration of shotcrete with or without SCMs within 28 days. After 28 days, the compressive strength of shotcrete decreases significantly when the curing temperature over 40 °C, however, the compressive strength of shotcrete blended with SCMs does not decrease. Above 60 °C, the porosity of shotcrete with or without SCMs increases after 28 days. At 70 °C, due to the accelerator and increased pozzolanic reaction of the SCMs, the composition of C-S-H of shotcrete with SCMs is shifted towards higher S*/Si, Al/Si, and Si/Ca ratio. In addition, the finer needle-like ettringites with the maximum width between ~0.390 μm and ~0.643 μm at 60–80 °C were observed. The observed variations of the phase with temperature are confirmed by the thermodynamic model.
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