Pore Structure Of Cement Paste Research Articles

Research on the Permeability and Pore Structure Distribution Characteristics of High-Performance Mortar for Surface Treatment of Bridge Piers and Columns

In marine environments, bridge piers and columns are prone to corrosion caused by harmful media, particularly chloride ions. This corrosion can lead to cracking of the steel bars in the protective layer of the bridge piers. To enhance the corrosion resistance of concrete in bridge piers, this article introduces the use of nanoclay-modified cement mortar. This innovative material offers high-performance surface treatment options that can effectively slow down the erosion process of harmful media and reduce the risk of bridge pier column cracking. To evaluate the ion erosion resistance of this nanoclay-modified cement mortar, we conducted detailed experiments on the pore structure of cement paste. The pore structure of cement paste with different dosages of nano-kaolinite clay and the dispersion method was studied using mercury intrusion porosimetry (MIP). The fractal dimension of the pore surface area of the net cement paste was calculated from the fractal model based on thermodynamic relationships of the pore structure-related parameters obtained with mercury pressure experiments. The relationship among the multiple fractal dimensions, pore structure parameters, dispersion mode, and permeability is explored. The results show that the addition of nano-kaolinite clay particles can improve the internal pore structure of cement materials. When 1.5% nano-kaolinite clay is mechanical dispersed, the total specific pore volume and the most probable pore size are reduced by 47.83% and 56.87%, respectively, compared with the control group. The fractal dimension image of cement-based materials with nano-kaolinite clay has a range of singular points and does not have fractal characteristics in this range. Nano-kaolinite clay has a significant effect on the fractal dimension of pore size range I. The fractal dimension of the whole pore size range is not suitable for the analysis of permeability, and the fractal dimension calculated by selecting less than the critical pore size range has a good correlation with permeability.

Effect of maintenance environment on the mechanical property and pore structure of cement paste mixed by seawater

This study aims to examine the effects of macroscopic mechanical properties and pore structure of cement paste mixed with seawater and explore the relationship between pore structure and macroscopic properties. The changeable law of mechanical properties and pore microstructure of hardened cement paste under seawater mixing is methodically investigated based on the compressive and flexural strengths using X-ray diffraction (XRD), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP) tests. The relationship between the fractal dimension of the pore structure of cementitious materials and the concentration of chloride ions in simulated seawater, the curing age, and the fly ash dose was obtained based on the thermodynamic fractal model. The obtained results reveal that with the increase in chloride ion concentration of the simulated seawater, the compressive and flexural strengths of the samples substantially grow in the pre-maintenance period, and the strength of the samples tends to stabilize at the age of 28 days. For the case of the chloride ion concentration of 1.3%, the compressive strength of the cement slurry exhibits a growth of 24.08%, 24.83%, and 28.81% compared to the control cement sample after curing at 5 °C, 20 °C, and 50 °C for 3 days, respectively. Additionally, the flexural strengths in order are 41.86%, 46.57%, and 38.89% higher than those of the control group. The interaction of Cl- with C3A, C4AF, or AFm in seawater mixed at high-temperature to generate Friedel's salt stimulated the volcanic ash reaction of fly ash, promoted the generation of C-S-H gels, and improved the internal pore structure of cementitious materials. The results indicate that at 50 °C curing, the most probable pore diameter of cement paste with 1.3% chloride ion concentration is 39.51% less than that of the control cement sample, and its harmless pores are 23% higher than that of the control group. At 5 °C curing, the total specific pore volume of cement paste with 1.3% chloride ion concentration is 30.18% less than that of the control cement sample, and the total specific pore volume is 22.9% less than the control group for the fly ash content of 25%. The obtained results reveal that there exists a fairly low correlation between the fractal dimension of the pore surface area of the cement paste and porosity, compressive strength, and flexural strength. However, the achieved correlation with the mean pore diameter, median pore diameter, and pore surface area is good, which are all above 75%. The fractal dimension fitting curve of high-temperature curing possesses a range of singular points.

Effect of surface curing condition on the humidity field and moisture transfer in concrete

In order to study the influence mechanism of different curing methods on the humidity field and scientifically design the surface curing methods, the humidity field of C30, C50 and C80 concrete cured with plastic film (PF), controlled permeability formwork liner (CPFL) and moisture retention curing film (MRCF) was measured by the designed testing device. And the pore structure of cement paste in the surface area of concrete with different curing methods was analyzed by mercury intrusion pore test (MIP). The theoretical calculation model for concrete humidity field considering the influence of surface curing method was also established. The results show that: CPFL curing improves the compactness of the concrete surface area by eliminating excess water and gas, reducing the rate of water loss and humidity decrease during the later drying process. It is more effective for the low strength concrete; MRCF curing extends the humidity saturation period and improving the final humidity level. It is more effective for the high strength concrete; PF curing creates an almost absolute humidity environment to prevent the moisture transfer during the curing period; The established model can accurately describe the impact of three different curing methods on the humidity changes for different strength concretes. It can be used to quantitatively analyze the humidity changes of concrete under different curing and environmental conditions. This study systematically clarifies the effect mechanism of different types of surface curing methods on the humidity field of concrete for the first time. And it will provide theoretical support for the scientific selection of concrete curing methods and accurate prediction of humidity field.

Preparation of cellulose fibre-sheets and its impact on pore connectivity of cement paste during early hydration

This study was aimed at evaluating the size and microstructure of cellulose fibres modified via ultrafine ball-milling methods and investigated the effects of these fibres on the rheological properties, compressive strength, hydration behaviour, and pore structure of cement paste in the early stage of hydration by scanning electron microscopy (SEM), isothermal calorimetry, X-ray diffraction (XRD), and thermogravimetric analysis (TG), low-field nuclear magnetic resonance (LF NMR) and electrical resistivity. The results showed that the ultrafine ball-milling method with a velocity of 500 r/min effectively produced cellulose fibre-sheets (CFS) by rupturing intermolecular hydrogen bonds. And their microstructure was transformed from a “fibrous structure” to a “fibre-sheet structure”, which improved the rheological properties and early compressive strength of cement by bridging effect between the cement particles, hydration products, and CFS. Meanwhile, CFS provided more nucleation sites for hydration products accelerating the early hydration rate without changing the structure of hydration products. Compared to pure cement paste, the cumulative heat released from the CFS-cement paste with 0.3% CFS increased by 14%, reaching 113.34 J/g over 20 h. Besides, CFS altered hydration product distribution and increased product content, thus rapidly decreasing the pore connectivity of cement paste. Hydrating for 12 h, the cement paste containing 0.3% CFS had a reduction in pore connectivity from 0.25 to 0.07, which decreased by 72% in contrast with that of pure cement paste.

Macro and micro properties of the cement-based material mixed with coral powder and GGBFS under seawater condition

On island, the use of discarded coral reef is a way to resolve the lack of raw material for preparing concrete. Discarded coral reef can not only be used as aggregate, but be ground to powder to replace cement. Since the cement-based material prepared with discarded coral reef is exclusively used on island, the property of the cement-based material exposed to seawater condition should be studied. In this study, discarded coral reef is ground into coral powder (CP). CP is used together with ground granulated blast furnace slag (GGBFS) to replace Portland cement (PC) partially to prepare a mixture with good performance and the minimum use of PC. The compressive strength, solid phase composition and pore structure of the cement-based material exposed to seawater condition are investigated systematically. The results show that the addition of CP decreases the fluidity of the cement mortar since it has a more rough surface than other raw materials. Although the use of CP decreases the compressive strength of cement mortar, the coupled use of GGBFS can largely mitigate the strength loss of the cement mortar by comparing 9 mixtures. When the specimens are exposed to seawater, the strength of cement mortar only decreases slightly, but the pore structure of cement paste is densified. It suggests that the use of CP and GGBFS together in the concrete can meet the construction of island; the compressive strength of the sample made with 25% CP and 25% GGBFS is around 54 MPa after exposed to seawater to 1 year.

Utilizing halloysite nanotube to enhance the properties of cement mortar subjected to freeze-thaw cycles

This study investigates the impact of the long-term pozzolanic properties of halloysite nanotube (HNT) on the physical and mechanical properties, permeability, durability, and microstructural characteristics of cement mortar subjected to repeated freeze-thaw cycles. For this purpose, the two-year cured samples were exposed to 300 freeze-thaw cycles. Deterioration of the cement mortar surface was visually assessed and regularly quantified by mass loss and length change. Furthermore, helium porosity, electrical resistivity, and ultrasonic pulse velocity (UPV) were performed on cement mortar mixtures to elucidate the effect of repeated freeze-thaw cycles on permeability. In terms of mechanical properties, compressive and flexural strength tests were conducted; static elastic modulus and flexural toughness were also calculated. The frost durability of samples was analyzed using the relative dynamic modulus of elasticity and durability factor as the indexes to assess mortar's freeze-thaw resistance. Moreover, the microstructures of specimens before and after 300 freeze-thaw cycles were analyzed by scanning electron microscopy. Visual comparisons and experimental results demonstrated that the HNT introduction in cement mortars markedly enhances the frost resistance of plain mortar. It leads to the refinement of the cement paste pore structure and a reduction in its porosity due to the nucleating, filling, and long-term pozzolanic effects of HNT. It was found that the contribution of HNT's pozzolanic activity is the most important of these mechanisms. Therefore, less water penetration into the hardened cement composites occurs, which results in higher frost durability. The samples containing 2 wt.% HNT (by weight of cement) exhibited the lowest mass loss, expansion, porosity, and UPV loss due to repeated freeze-thaw cycles. The highest electrical resistivity and residual mechanical strengths also pertained to samples with 2 wt.% HNT. Hence, the HNT replacement dosage of 2 wt.% can be regarded as an optimal content for the cement mortars studied in this work. Finally, the two-way ANOVA revealed that HNT percentage is the factor with the greatest impact on mass loss, electrical resistivity, and compressive strength. In terms of length change, porosity, and flexural strength, the freeze-thaw cycle was the most statistically important factor.

Effects of recycled concrete powders on the rheology, setting and early age strength of cement paste

This work studies the effect of substitution rate and types of recycled concrete powders (RCPs) on the rheology over time, setting time, and 1 day compressive strength of cement paste. Two RCPs with distinct compositions, namely RCPI and RCPII, were adopted. Results showed that incorporating RCPs contributes to the hydration of cement paste in the first 5 h, and the initial zeta potential of cement paste is reduced after replacing cement by RCPs, which results in that utilizing RCPs increases the initial rheological properties of cement paste and its growth rate over time. Specifically, the paste with 30% RCPs exhibits 66.85–122.79% higher initial static yield stress than the pure cement paste, and the structuration rate in the paste with 30% RCPs is 10.26–141.03% higher than that of plain paste. Utilizing RCPs decreases the initial setting time. Utilizing RCPs is detrimental to the hydration reaction beyond about 8 h and pore structure of cement paste, the total porosity of paste with 30% RCPs at 24 h is 11.31–13.70% higher than that of plain paste, and the content of harmful and more harmful pores shows an increase of 19.30–30.43%, which results in a nearly 50% reduction in the 1 day compressive strength. Compared to the RCPI cement paste, the RCPII cement paste with the same RCP content exhibits lower initial rheology, higher growth rate of rheological parameters, lower total porosity, thus has relatively higher 1 day compressive strength.

Effect of Mineral Admixtures on the Mechanical and Shrinkage Performance of MgO Concrete.

Shrinkage deformation of concrete has been one of the difficulties in the process of concrete performance research. Cracking of concrete caused by self-shrinkage and temperature-drop shrinkage has become a common problem in the concrete world, and cracking leads to a decrease in the durability of concrete and even a safety hazard. Mineral admixtures, such as fly ash and mineral powder, are widely used to improve the temperature drop shrinkage of mass concrete; fly ash can reduce the temperature rise of concrete while also reducing the self-shrinkage of concrete, there are different results on the effect of mineral powder on the self-shrinkage of concrete, but the admixture of fly ash will reduce the strength of concrete, and mineral admixtures have an inhibitory effect on the shrinkage compensation effect of MgO expander(MEA). The paper investigates the effect of mineral admixtures on the mechanical and deformation properties of C50 mass concrete with a MgO expander(MEA), aiming to determine the proportion of C50 mass concrete with good anti-cracking properties under working conditions. The experiments investigated the effect of fly ash admixture, mineral powder admixture and MgO expander admixture on the compressive strength and deformation of concrete under simulated working conditions of variable temperature and analyzed the effect of hydration of magnesite in MgO expander and pore structure of cement paste on deformation. The following main conclusions were obtained: 1. When the concrete compounded with mineral admixture was cured under variable temperature conditions, the compounded 30% fly ash and mineral powder decreased by 4.3%, 6.0% and 8.4% at 7d age, and the compounded 40% fly ash and mineral powder decreased by 3.4%, 2.8% and 2.3% at 7d age, respectively. The incorporation of MEA reduced the early compressive strength of concrete; when the total amount of compounding remained unchanged, the early compressive strength of concrete was gradually smaller as the proportion of compounding decreased. 2. The results of concrete deformation showed that when the temperature rose, the concrete expanded rapidly, and when the temperature dropped, the concrete also showed a certain shrinkage, and the deformation of concrete basically reached stability at 18d. 3. The compounding of 30% fly ash and mineral powder As the compounding ratio decreases, the deformation of concrete increases, and the 28d deformation of concrete with a compounding ratio of 2:1 is 280 × 10-6, while the final stable deformation of concrete with a compounding ratio of 2:1 in compounding 40% fly ash and mineral powder is the largest, with a maximum value of 230 × 10-6, respectively. Overall, the concrete with a total compounding of 30% and a compounding ratio of 2:1 has the best shrinkage resistance performance.

Influence of hoop restraint on microstructure and phase composite of cement paste filled steel tube under external sulfate attack

The deterioration of cement-based material caused by external sulfate attack (ESA) is a major durability problem affecting concrete structures. This study aims to investigate the effect of hoop restraint of steel tube on microstructure and phase composition of cement pastes prepared with ordinary Portland cement (OPC) and high sulfate-resistance Portland cement (SRPC) under ESA for 540 days. The effects of hoop restraint on the change in micromorphology, phase composition and microstructure of cement hydration products under ESA were investigated by SEM-EDS, XRD, 29Si and 27Al MAS NMR spectroscopy and low-field NMR. Moreover, the effects of hoop restraint on local mechanical properties of the cement pastes under ESA were evaluated by micro-hardness measurement. Results indicate that hoop restraint significantly inhibits the formation of micro-cracking and limits the permeation rate of sulfate ions, leading to a reduction in the amount of ettringite and gypsum crystals. 27Al NMR analysis shows that the hoop restraint has significant “delay effect” on the transfer of Al phase transition from monosulfoaluminate (AFm) and third aluminate hydrate (THA) to ettringite. 29Si NMR test reveals that hoop restraint can significantly slow down decalcification and dealuminization of C-(A)-SH gel, leading to a slow reduction in Al[4]/Si ratio of C-(A)-SH, together with the lower mean chain length (MCL) of the silicate chain. The MCL reduces by 34.7 % and 9.0 %, and Al[4]/Si ratio increases by 41.9 % and 8.3 % for OPC and SRPC pastes with steel tube compared to the equivalent free ones, respectively. Additionally, hoop restraint results in better quality pore structure of cement pastes with steel tube than the equivalent free ones.

Static and dynamic mechanical thermoanalyses of cement paste with highly dispersed graphene oxide

This study investigated the static mechanical properties of cement pastes with highly dispersed graphene oxide (GO), performed a dynamic mechanical thermoanalysis (DMTA), and studied the microstructure of cement pastes after exposure to high temperatures (105, 200, 300, 450, 600, and 750 °C). First, GO was uniformly dispersed in the cement paste with the aid of polycarboxylate-based superplasticizers. Generally, the incorporation of dispersed GO can enhance the residual compressive strengths of the cement pastes. For pastes with 0.04% GO (by weight) exposed to temperatures of 105, 200, 300, 450, 600, and 750 °C, the strengths are 14.84%, 4.83%, 13.35%, 15.71%, 11.91%, and 64.49% higher than that of pastes without GO, respectively. The DMTA results reveal that cement pastes with 0.04% GO have a high storage modulus and low loss factors in the temperature range of 25–350 °C, which improve their thermal resistance. Moreover, the addition of dispersed GO refines the pore structure and decreases the total porosity at high temperatures by suppressing the coarsening of the pore structure of cement pastes.

Investigating the potential of trass-cement binary blend for enhancing microscopic properties, macroscopic performance, and sustainability of cement paste

• Effects of trass on micro- and macro-level properties of cement paste were evaluated. • Blending cement with trass accelerated the C 3 S and C 3 A hydration. • Higher nonevaporable water contents were detected in blended sample compared to plain one. • The values of compressive strength activity index fluctuated between 106.7 and 124.6%. • Incorporating trass into cement can lead to a cleaner, cheaper, and less energy consumer construction. Due to some physical or chemical incompatibility issues between supplementary cementitious materials (SCMs) and cement, it is necessary to investigate the microscopic properties and macroscopic performance of cement composites containing such blends before their incorporation in construction. Hence, in this study, the hydration kinetics, degree of hydration, microstructural morphology, and pore structure of cement paste blended with trass, as well as its mechanical and transport properties, were profoundly evaluated. Moreover, the potential of trass for enhancing the sustainability of construction was also assessed. The physical properties of trass accelerated the C 3 S hydration, while its chemical composition led to the reduction of induction period, in addition to the acceleration of C 3 A hydration. Significant refinement of pore structure characteristics was found in blended sample after 90 days, when the threshold and critical pore diameters were reduced by respectively 27.7 % and 10.7 %, compared to those of the plain sample. This denser microstructure was reflected in superior electrical resistivity of the blended sample, which was 2.1 times of the plain one at 90 days. Furthermore, the inclusion of trass through refining the morphology of CH crystals in the microstructure, besides lowering the calcium hydroxide (CH) content by 12.1–63.3 % and increasing the nonevaporable water content by 2.1–8.0 %, led to 6.7–24.6 % higher compressive strengths in blended sample. Regarding the sustainability, at a similar 90-day compressive strength of 40 MPa, using trass results in an approximately 21.7 % less energy consumer, 20.3 % cleaner, and 15.8 % cheaper construction. Moreover, at equal levels of energy consumption or CO 2 emission, the blended sample has greater compressive strength and electrical resistivity than the plain one, which can be indicative of an extended service life. Accordingly, the utilization of trass is recommended since it has the potential to improve the sustainability both directly and indirectly.

Effect of Content and Fineness of GGBS on Pore Structure of Cement Paste

Effect of Content and Fineness of GGBS on Pore Structure of Cement Paste

Deterioration of performances and structures of cement pastes under the action of thermal cycling fatigue

Concrete structures in the atmosphere are continuously subjected to thermal cycle (TC) inducedby varying environmental temperature. However, the influencing mechanism of thermal cycling fatigue on the mechanical properties and microstructure of cement-based materials has not been thoroughly documented. To address this issue, the relationship between the macro-properties and microstructure of cement paste under TC was analyzed using micro-properties measurement and quantitative microcrack evaluation. In this work, ordinary Poland cement pastes with three different water-to-cement (w/c) ratios (0.4, 0.5, and 0.6) were exposed to TC in the range of 5–45 °C. The mechanical properties, pore structures, and micromorphology of cement pastes after different cycle times were measured. The result indicated that as the number of cycles increased, TC negatively affected the macro-mechanical characteristics, pore structure, and micromorphology of cement pastes. Cement pastes with greater w/c ratios were more susceptible to TC-induced deterioration. In addition, thermal cycling fatigue caused the propagation of microcracks including crack width, length, and density. Thermal cycling fatigue also coarsened the pore structure of cement pastes, especially the large pores (i.e., diameter > 100 nm). The underlying damage mechanism caused by TCis the coupled actions of fluid flow, thermal deformation, and thermal fatigue. Moreover, a negative linear relationship was established between microcrack density and Vickers hardness of cement pastes.

Study on the long-term chloride permeability of nano-silica modified cement pastes cured at negative temperature

This paper investigated the 56-day and 120-day chloride permeability of nano-silica (NS) modified cement pastes with different water-cement ratios (0.3, 0.35, and 0.4) cured at a negative temperature of −5 °C. The influences of water-cement ratio, negative temperature, and the improvement effect of nano-silica were highlighted. Experimental results indicated that negative temperature could result in serve and irreversible deterioration of the chloride permeability, and a high water-cement ratio can also aggravate the deterioration. Meanwhile, the incorporation of nano-silica can effectively improve the resistance to chloride permeability of cement pastes, and the improvement of nano-silica was more evident in the environment of −5 °C compared with the environment of +20 °C. Through mercury intrusion porosimetry (MIP) technology, three key parameters (total porosity, tortuosity, and threshold diameter) were determined, which exhibit a strong relationship with the chloride permeability of cement paste through the statistical analysis. The results of MIP and correlation analysis indicated that, unlike the previous studies, the influence of nano-silica on the long-term pore structure of cement pastes under negative temperature is not total porosity but internal pore structure. Nano-silica has little effect on improving the total porosity, but it can effectively increase the tortuosity and decrease the threshold diameter, which will hinder the diffusion of chloride ions in the cement matrix, thereby improving the chloride permeability resistance. Also, the micro-mechanism of nano-silica on the permeability of cement paste is different under different water-cement ratios. In the case of a low water-cement ratio, nano-silica mainly increases the tortuosity of pore structure. In the case of a high water-cement ratio, nano-silica mainly reduces the connectivity of pore structure.

Performance of cement-based materials containing calcined coal gangue with different calcination regimes

The storage of coal gangue (CG) takes up a lot of lands and pollutes the environment. Calcined CG has the potential to be used as supplementary cementitious material. In this paper, the CG from Yibin, Sichuan province of China were treated with different calcined temperature, holding times and cooling modes, and the microtopography, phase composition and pozzolanic reactivity of calcined CG were studied. Meanwhile, the fluidity, mechanical properties, hydration products, and pore structure of cement paste and mortar with different calcined CG were analyzed. The apparent color of coal gangue under different treatment methods has also been studied to promote its better application for a wider range in cement-based materials. Results showed that there exists an optimal calcination condition of calcined CG at 800 °C and holding time of 2 h. Rapid cooling and slow cooling reduced the activity of calcined CG. Calcined CG powder would reduce the fluidity of cement mortar. However, proper calcination temperature and sufficient holding time could eliminate it to some extent. It was also found that calcination temperature has a greater impact on the early age reactivity of CG when compared to different temperature treated CG powder, C8T2 sample has the best early activity. Compared to rapid cooling, slow cooling and water-cooling mode, room temperature cooling would result in higher reactive CG. CG powder after proper thermal treatment could refine the pore structure of the hardened cement paste in the later age (28 days), reduce the harmful pores content (>100 nm), and increase harmless pores content (<100 nm) in hardened cement paste. It was found that the CG powder of calcined at 800 °C for 2 h, cooling in water and room temperature might be more suitable as the supplementary cementing material combined the apparent color, pozzolanic reactivity, and preparation process.

Insights to compressive strength, impermeability and microstructure of micro-expansion steel slag cement under constraint conditions

This work aims at evaluating the influence of steel slag as partial replacement of cement on the mechanical properties and impermeability of cement-based materials under constraint conditions. The results show that, compared with the raw steel slag, the setting time of the wet-grinding steel slag system is shortened, and the free expansion rate is increased. When the wet-grinding steel slag content is 10%, the compressive strength of the specimen (constraint) after 28 days of curing is the highest, reaching 68.5 MPa, which is 9.3% higher than that of the free curing specimen, and the impermeability is also the best. Under the constraint conditions, the expansion strain produced by steel slag leads to the compact sample structure and reduced porosity, which is conducive to the mechanical properties and compactness of the pore structure of cement paste.

Effect of Graphene Oxide on the Pore Structure of Cement Paste: Implications for Performance Enhancement

The effect of graphene oxide (GO) nanosheets on the spatial distribution of cement paste pores has been investigated. In this study, a scheme based on the radial distribution function is developed to analyze the spatial distribution of pores in GO-cement composites. By applying the developed scheme, the addition of 0.04% GO is found to reduce the spatial inhomogeneity of pores in cement paste by around 20% at all curing ages. The test results show that the compressive strength of the cement paste sample containing GO is increased by 30-40%, indicating the negative correlation between the spatial inhomogeneity of pores and mechanical properties of GO-cement composites. Besides the spatial homogenization effect of GO on pores, pore structure refinement by GO is also found to increase the mechanical properties of cementitious materials. The findings suggest the great potential of using GO to modify cementitious materials by homogenizing the spatial distribution of pores. The spatial distribution analysis scheme presented in this study can be used for the future structure-property study of nanoengineered cementitious materials with enhanced performance.

Early-age hydration process and autogenous shrinkage evolution of high performance cement pastes

This paper presented a comprehensive study on the early-age hydration process and autogenous shrinkage evolution of high performance cement pastes. The early-age hydration process of cement pastes was analyzed by combining 1H nuclear magnetic resonance relaxometry (1H NMR) with isothermal calorimetry. The hydration degree was contrastively analyzed by using 1H NMR and thermogravimetry analyses (TGA). Moreover, the relationship between the free water consumption process and autogenous shrinkage for cement pastes was revealed. Results showed that a magnetism-heat (M − H) model could be established according to the correspondence between free water consumption rate and hydration exothermic rate, which divided the early-age hydration process of cement pastes into four periods. The initial setting time was the onset that free water signal fraction started to drop, after this moment, the consumption rate of free water was increased and more hydration products were generated. After about final setting time, the weighted mean relaxation time of the free water sharply decreased, and the pore structure of cement pastes was gradually refined. The hydration degree (α) of cement calculated by 1H NMR and TGA showed good agreement, with the error of α values less than 2% after initial setting time. Furthermore, cement paste with a lower water/cement ratio showed higher autogenous shrinkage and lower free water signal fraction, but they did not show a good linear correlation with each other throughout the hydration process.

Trans-scale multi-physics coupling finite element model of concrete during freezing and thawing

This paper presents a trans-scale finite element model based on hydraulic-thermal-mechanical coupling equations of porous medium to simulate the behavior of concrete during freezing and thawing. Unlike previous models that regard concrete as homogeneous material, this paper aims to establish a macro-size concrete model with detailed micro-structure, the aggregates (mm) and air voids (μm) are randomly generated by the Monte Carlo method, and the interfacial transition zones (μm) with random thickness are built around the aggregates. The capillary pores (nm) of concrete is expressed by the relationship between the percentage of frozen ice and the radius of capillary pore. The simulation results reveal the mechanism of air voids protecting the concrete from freezing and thawing damage, and the mechanical field shows the freezing and thawing damage occurs preferentially at the interfacial transition zone, because the stress concentration due to the irregular distribution of aggregates and water resistance of aggregates. The temperature boundary conditions were also varied to study the influence of the temperature change rate on hydrostatic pressure and volume stress. The effect of the cement paste's pore structure, including the spacing of air voids, on the ability of concrete to resist freezing and thawing is presented.

Carbonation process simulation for cement-based materials based on microstructure by a cement hydration model

Based on three-dimensional distributions of cement particles, cement hydration was simulated to obtain the hydrate content and the pore structure of cement paste. Additionally, the porosity of mortar and concrete was obtained by considering aggregate inclusion and interfacial transition zones (ITZ). Then, a coupled model for heat transfer, moisture transport and carbonation in cement-based materials was established. The experimental carbonation results agreed well with the simulated results for mortars and concretes with different water-cement ratios. The carbonation reaction front was far beyond the complete carbonation depth. The aggregate dilution had a more significant effect on carbonation than the ITZ did.