Cement Paste Pore Research Articles

Effect of chelator on carbon sequestration and mechanical properties of CO2-cured cement pastes with different pore structures

Chelator can accelerate the mineralization reaction of cement-based materials, playing a positive role in reducing CO2 emissions into the atmosphere. Cement pastes with different pore structures were prepared by adjusting the water cement ratio, and effects of chelator on the microstructure, phase composition, carbon sequestration and mechanical properties of CO2-cured cement pastes with different pore structures were investigated in this study. The results indicated that chelator could promote CO2 transport in dense cement paste with more gel pore and transition pore and medium dense cement paste with many transition pore and capillary pore, and induce mineralization reaction to repair larger pores in loose cement paste with more capillary pore and megalopore (LCM). Chelator had the most significant effect on improving the performance of cement paste of LCM, increasing the carbonation sequestration rate and compressive strength of cement paste cured for 48 h by 19.0 % and 28.1 %, respectively.

Effect of Poly(ethylene glycol)–Poly(propylene glycol) Triblock Copolymers on Autogenous Shrinkage and Properties of Cement Pastes

This study investigates the hydration, microstructure, autogenous shrinkage, electrical resistivity, and mechanical properties of Portland cement pastes modified with PEG-PPG triblock copolymers with varied molecular weights. The early age properties including setting time and hydration heat were examined using the Vicat test and isothermal calorimetry. The hydration products and pore size distribution were analyzed using thermogravimetric analysis (TGA) and nitrogen adsorption, respectively. Mechanical properties and electrical resistivity were evaluated using the compressive strength test and electrochemical impedance spectroscopy (EIS). It was shown that the addition of the copolymers reduced the surface tension of the cement paste pore solution due to the presence of a hydrophobic block (PPG) in the molecular structure of the copolymers. The setting time and hydration heat were relatively similar in the control paste as well as the pastes modified with the copolymers. The results showed that copolymers were able to reduce the autogenous shrinkage in the paste due primarily to a reduction in pore solution surface tension. TGA showed a slight increase in the hydration degree of the paste modified with the copolymers. The compressive strength was reduced in the pastes modified with the copolymers that showed an increased volume of air voids. The addition of copolymers did not affect the electrical resistivity of the pastes except in the case where there was a large volume of air voids, which acted as electrical insulators.

Performance study of modified aluminate cement under in-situ conversion conditions of shale oil

The resource potential of in-situ conversion of shale oil is enormous, and it is a strategic alternative resource for China's oil and gas industry. However, extreme high temperatures under in-situ conversion conditions of low maturity shale oil can lead to a decline in the mechanical strength of cement. In this paper, the influence of modified sodium hexametaphosphate on the hydration behavior of aluminate cement and its application performance at 650 °C was systematically studied, and its macro performance and microstructure were deeply explored. As a result, it was found that the sodium hexametaphosphate can significantly reduce the permeability of aluminate mudstone, but not significantly improve the strength, at the same time, it was found that the permeability of aluminate mudstone modified by micro-silica composite sodium hexametaphosphate is significantly reduced, and the compressive strength is increased. After treated at 650 °C, the compressive strength of 5.0% sodium hexametaphosphate modified aluminate mudstone is the highest (47.19 MPa), while the compressive strength of micro-silica composite sodium hexametaphosphate modified aluminate mudstone shows a downward trend. The aluminate brine debris has obvious transformation of hydration products before and after treatment at 650 °C, including mainly C3AH6 and AH3 into C12A7 and CA. Among them, C3AH6 and AH3 mainly undergo thermal decomposition at 180–400 °C, and the pores of aluminate mudstone increase attribute to the transformation of crystal form. The C2ASH8 generated by aluminate mudstone at 50 °C due to micro-silica is helpful to improve the microstructure. But, the decomposition of C2ASH8 after 650 °C treatment will also lead to the increase of cement paste pores.

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.

Micro-nano bubble water process: Effect of micro-nano bubble water as mixing water on volume stability of shotcrete

As a kind of active water, micro-nano bubble water (MBW) can promote the early hydration of cement paste and improve the early mechanical strength of shotcrete. However, accelerating the hydration process of cement-based materials will increase their shrinkage deformation to a certain extent at early age. Therefore, this paper mainly studies the effect of MBW combined with aluminum sulfate alkali-free accelerator (AL) on the volume stability of cement-based materials within 180d. The results show that MBW as mixing water can reduce the autogenous and drying shrinkage of cement-based materials with AL, and also has a certain mitigation effect on water loss rate and internal humidity (IH) drop under dry conditions. It can also reduce the ring constraint shrinkage cracking time and crack width. MBW can reduce the cement paste pore size distribution of 10–50 nm, and reduce the surface tension of the aqueous solution, thereby reducing the capillary tensile stress. This paper provides some theoretical guidance for micro-nano bubble water in practical engineering applications.

Internal hydrophobization of cement-based materials by means of silanes

The paper shows the opportunity of application of an organosilicon compound such as triethoxyoctylsilane (OTES) containing long, hydrophobic alkyl sidechains (octyl groups – C8H17) attached to the silicon atoms, as the admixtures for internal hydrophobization of cementitious building materials characterized by a porous internal structure. The study included the cement paste, mortar and concrete with three different hydrophobic admixtures based on triethoxyoctylsilane, but with miscellaneous concentration of the triethoxyoctylsilane in the admixture and different dosage in the fresh mix. The impact of organosilicon hydrophobic admixtures on the mechanical properties such as compressive strength, capillary water absorption of the cement mortar and concrete and the microstructure of cement paste was investigated. The hydrophobic, silicon-based admixtures decrease the capillary water absorption coefficient significantly. Even by 81.1% in case of cement mortar. The mechanical strength of cement mortar is also decreased, by 23.6%. The noticeable changes in the internal structure of pores of cement paste only for one admixture is observed.

Preparation and Hardened Performance of Bentonite-Induced Porous Magnesium Oxysulfate Cement Paste.

Porous magnesium oxysulfate (MOS) cement pastes were successfully fabricated by injecting presaturated bentonite into modified MOS cement paste. Their pore structure and hardened performance were investigated. The results indicated that the 20MgO-MgSO4·7H2O-18H2O system modified by citric acid (C6H8O7⋅H2O) and ethylene diamine tetraacetic acid was suitable to fabricate porous MOS cement paste. Bentonite slurry led to significant refinement of pores, generating nanosized pores in MOS cement pastes. When volume replacement of bentonite slurry in MOS cement paste rose between 0 and 60%, pore size corresponding to the peak in the pore size distribution curve of MOS cement-based materials decreased from 180.0 nm to 22.8 nm and then increased to 163.0 nm, and the porosity linearly increased from 21.1% to 58.1%. These small pores caused the successful preparation of porous MOS cement paste with dry bulk density of 760–1650 kg/m3, compressive strength of 7.8–69.8 MPa, and thermal conductivity of 0.25–0.85 W/(m·K).

Triaxial behavior of a stabilized and a highly porous oil well cement paste at different saturation and drainage conditions

Well cement is the most common barrier material in wells for geothermal and hydrocarbon production. As cements are exposed to hydrostatic loads and periods of deviatoric loading in wellbore environments, it is important to understand the mechanical behavior of cement under relevant conditions. We study effects of porosity, saturation, confining pressure and drainage conditions on the mechanical behavior of class G well cement using two basic formulations, one which produces a highly porous cement paste. The high-porosity cement exhibited lower strength and reduced elastic moduli compared to the stabilized formulation. The elastic moduli for both formulations were reduced with increasing confining pressure, and the most pronounced effect of confinement was increased ductility and pronounced strain hardening behavior of the two cements, likely due to compaction. Under saturated and undrained conditions, the stabilized cement exhibited an increase in stiffness and essentially brittle failure even at 20 MPa confining pressure. The porous cement showed less sensitivity to drainage conditions. We attribute this observation to possible generation of internal micro-cracks and dislocations instead of a macroscopic failure plane. The results contribute to increased understanding of the mechanical response of conventional and porous cements under relevant confining pressures and different saturation and drainage conditions.

Influence of ion chelator on pore structure, water transport and crack-healing properties of cement pastes incorporating high-volume fly ash and blast-furnace slag

This paper focuses on the influence of ion chelator (CA) on pore structure, water transport and self-healing ability of cement pastes incorporating high content fly ash (FA) and blast-furnace slag (BFS). The pore structure of pastes was measured by low-field nuclear magnetic resonance (LF-NMR). The water transport property was tested by water absorption test. The self-healing ability was assessed by water permeability and crack healing test. Results showed that the compressive strength of hardened pastes was promoted by CA, the increase rates in compressive strength of pastes were 21.9% (50%BFS), 15.7% (pure cement), 9.9% (30%FA) and 8.8% (16%FA24%BFS). CA decreased the proportion of small capillary pores (0.01–0.1 μm) in cement pastes which were more pronounced for the water absorption, the reduction rates of small capillary pores were 49.0% (50%BFS), 41.5% (pure cement), 20.2% (30%FA) and 13.0% (16%FA24%BFS). The self-healing performance of pastes with 50% BFS was most obviously improved that crack with width of 0.43 mm could be completely healed within 21d, while the relative water permeability coefficient dropped by around 80%, followed by pure cement, 30%FA and 16%FA24%BFS pastes. Notably, the morphology of the crack healing product in the blend pastes with FA and BFS was changed with the addition of CA, a large amount of lumpy and clustered calcite was generated, which promoted the compactness and content of healing products. The results of this study showed that CA could significantly improve pore structure, water transport and self-healing properties of high-volume BFS cementitious materials, which meant that CA had a better application prospect in high-volume BFS cement-based materials.

Is early drying shrinkage still determined by the mesopore content? A case study of cement paste with minerals

Long-term drying shrinkage is deemed to be relevant with the mesopore content of cement paste, so is early drying shrinkage still determined by the number of mesopores? This was not the case. Through combining the measurement of water loss and pore size distribution (PSD), this study provides an explanation of early-age drying shrinkage of cement paste with different types/contents of minerals (i.e., blended cement paste) added, and a corresponding prediction model was proposed in terms of its early shrinkage. The results show that during hydration, instead of the mesopore content, the water loss due to evaporation in capillary pores of cement paste was well correlated with its early drying shrinkage, which could be attributed to a small portion of mesopores rather than all mesopores have lost water at an early age. Meanwhile, a parameter rs, which was defined as the radius of the pores where the meniscus formed, was calculated by combining the results of PSD and water evaporation, and its reduction led to an increment of drying shrinkage of specimens. Finally, the outcomes obtained from the proposed prediction model fitted well with experimental data, revealing the effectiveness and accuracy of the model in foreseeing early drying shrinkage of blended 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.

Estimation of constituent properties of concrete materials with an artificial neural network based method

Multi-scale models are developed for heterogeneous concrete materials to estimate their macroscopic mechanical properties in terms of micro-structural data. One crucial challenge of those models is the identification of local properties of constituent phases. In this paper, we present an efficient method based on Artificial Neural Networks (ANN). Typical concrete materials are taken as example. A macroscopic analytical strength criterion is established from three steps of nonlinear homogenization procedure. The macroscopic strength of materials is determined as a function of the frictional coefficient and cohesion of solid cement particles at nanometer scale, intra-particle pores, inter-particle pores and aggregates (inclusions). The objective is to identify the nanoscopic frictional coefficient and cohesion of cement particle from measured macroscopic values of uniaxial compression and tensile strengths. For this purpose, a numerical method based on the ANN is developed. With the analytical macroscopic strength criterion, sensitivity studies are first realized to identify the most important micro-structural parameters influencing the macroscopic strength of concrete. A simplified analytical macroscopic strength criterion is then proposed. A large dataset is further constructed through the inversion of the analytical strength criterion by using the aggregates volume fraction, porosity, macroscopic uniaxial tensile and compressive strengths as input variables and the frictional coefficient and cohesion of cement particles as output unknowns. An ANN model containing four hidden layers and 100 neurons in each layer is constructed and trained by using this dataset. Various types of validation of the ANN model are performed. It is found that the proposed ANN based model can effectively predict the frictional coefficient and cohesion of porous cement paste at the microscopic scale with a very good accuracy.

The Effect of an Accelerator on Cement Paste Capillary Pores: NMR Relaxometry Investigations.

Nuclear Magnetic Resonance (NMR) relaxometry is a valuable tool for investigating cement-based materials. It allows monitoring of pore evolution and water consumption even during the hydration process. The approach relies on the proportionality between the relaxation time and the pore size. Note, however, that this approach inherently assumes that the pores are saturated with water during the hydration process. In the present work, this assumption is eliminated, and the pore evolution is discussed on a more general basis. The new approach is implemented here to extract information on surface evolution of capillary pores in a simple cement paste and a cement paste containing calcium nitrate as accelerator. The experiments revealed an increase of the pore surface even during the dormant stage for both samples with a faster evolution in the presence of the accelerator. Moreover, water consumption arises from the beginning of the hydration process for the sample containing the accelerator while no water is consumed during dormant stage in the case of simple cement paste. It was also observed that the pore volume fractal dimension is higher in the case of cement paste containing the accelerator.

Transmission micro-focus X-ray radiographic measurements towards in-situ tracing capillary imbibition fronts and paths in ultra-thin concrete slices

In the present study, we attempted to trace the imbibition fronts and paths in concrete by controlling specimen dimensions and imbibition liquids in transmission microfocus X-ray radiographic tests. Ultra-thin concrete specimens were specifically prepared for 2D imbibitions. CsCl was used as the agent to enhance X-ray attenuation. SEM/BSE/EDS tests were used to measure the micro morphology and element distributions at local sites. Results show that the designed experiments enable the acquisition of X-ray attenuation images with the enhanced contrast, which helps identify the capillary imbibition paths and fronts in the concrete. Liquid is more likely to penetrate through the interfacial transition zone and highly porous cement paste. CsCl solution also increases the contrast gradient of BSE images. Cs and Cl gradually distribute across the imbibition fronts. The proposed measurements offer a way to instantly, continually and visually probe capillary imbibitions in concrete and other porous materials.

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.

Effect of different lithological stone powders on properties of cementitious materials

Abstract As the by-product generated during the production of manufactured sands, stone powder is urgent to be utilized due to the high disposal cost and environmental pollution. The main objective of the research is to clarify the viability of stone powders as partial replacement of cementitious materials for a value-added recycling. The paste and mortar samples incorporating limestone, granite, tuff and quartzite powders with different fineness were prepared, whose properties including setting time, compressive strength, dry shrinkage, pore structure and hydration process were studied. The results showed that the limestone and tuff powders shorten the setting time, while the granite and quartzite powders prolong the setting time of the blended pastes. The compressive strength of the mortars blended with quartzite powder was notably improved with the increase of the specific surface area of stone powders. Stone powders with different specific surface areas could potentially decrease the dry shrinkage of the blended mortars. Increasing the fineness of stone powder could reduce the proportion of harmful pores in cement paste. The peak reaction rate and 72 h heat release were improved obviously when the specific surface areas of limestone, granite and quartzite powder were about 500 m2/kg and was about 700 m2/kg for tuff powder.

Effect of Porosity and Water Absorption on Compressive Strength of Fly Ash based Geopolymer and OPC Paste

The fly ash based geopolymer is a promising binder by activation of fly ash with an alkaline activating solution. The fly ash based geopolymer prepared was characterized by several methods. The experimental result, studies effect of the porosity and water absorption on compressive strength of fly ash based geopolymer and Ordinary Portland Cement paste for comparison. The porosity studies were determined using the Brunauer, Emmett and Teller method included nitrogen adsorption / desorption plots. Then followed by water absorption and compressive strength tested at 7 and 28 days curing time. The result shows that the porosity of fly ash based geopolymer paste was in the lower surface area, pore volume and pore size compared to Ordinary Portland Cement paste. The small pore size of the fly ash based geopolymer had a significant proportion of a micropores whilst Ordinary Portland Cement paste pores were mostly mesopores. The highest compressive strength of fly ash-based geopolymer can be achieved up to 76.723 MPa at 28 days when less of pore size and water absorption. Therefore, the paste based on geopolymeric materials is a better durability and high resistance to aggressive environment compared Ordinary Portland Cement paste.

Chemical properties of pore solution of hardened cement pastes with mineral additions

This paper presents the influence of mineral additions in blended cement, or ‘green’ concrete, on the divalent ions of the cement paste pore solution. Because of the lack of data concerning the direct comparison between the pore solution of cement and mineral additions-based material, six types of hardened cement pastes were tested. For this purpose, Portland cement (CEM I or CEM V) and mineral additions (limestone filler, fly ash, blast-furnace slag and silica fume) were used. Results from chemical analyses by ionic chromatography show significant concentrations of divalent ions, such as sulfates and calcium, in the pore solution. In fact, the reference cement paste based on CEM I contains about 1.8 mmol/l of calcium and 1.6 mmol/l of sulfates. However, the use of mineral addition significantly modifies the pore solution. In particular, a substitution of cement by 10% of silica fume increases the calcium and sulfates concentrations to 12.2 mmol/l and 6.7 mmol/l, respectively. This change in the chemical composition, the pH and the ionic strength should be taken into account in multispecies transfer modelling and the thermodynamic equilibrium. The time evolution of the chemical composition, pH and ionic strength of the pore solutions was also investigated.

Effect of ion chelator on hydration process of Portland cement

Ion chelator is a novel crystalline additive which can chelate Ca2+ and migrate in concrete to promote self-healing of surface crack and internal pore. In order to further explore the effect of ion chelator on hydration process of cement, the hydration heat, chemically bound water, compressive strength, hydration products, pore-size distribution and microstructure of cement paste with ion chelator were investigated in this paper. Results of the experiments showed that the ion chelator accelerated the hydration of the cement paste. Incorporation of ion chelator in cement greatly enhanced the chemically bound water content and the mechanical property. Compared with control cement paste, the chemically bound water content and compressive strength of cement paste with 0.5 wt% ion chelator increased by 13.3% and 16.8% after curing for 28 days. According to the DTA-TG and XRD tests, the ion chelator can promote the hydration of C3S and C2S, which was conducive to the generation of calcium silicate hydrate gels. Furthermore, the addition of ion chelator improved the pore-size distribution and microstructure of cement paste, and a large number of floccules and needles appeared in the pores of cement paste.

Application of micro-CT to Mori-Tanaka method for non-randomly oriented pores in air-entrained cement pastes

The homogenization technique has played a key role in elucidating the mechanical, thermal, and chemical behaviors of composite materials. Specifically, the cement-based material is a representatively complex composite material consisting of various inclusions, such as pores, fibers, and aggregates. Recent studies have mathematically introduced the Mori-Tanaka method to express non-randomly oriented ellipsoidal inclusions. Therefore, the present study aimed to practically apply pore information obtained from micro-computed tomography (micro-CT) to the Mori-Tanaka method. Cement-paste samples were prepared with 0, 1, 2, 5, and 10 wt% of an air-entraining (AE) agent. The representative ellipsoidal shape and orientation distribution function (ODF) of the pores were obtained from the micro-CT, and this information was incorporated into the Mori-Tanaka method. The computation results revealed good agreement between the results of the Mori-Tanaka method and the finite-element method (FEM). Additional sensitivity studies using the Mori-Tanaka model allowed for quantifying the anisotropic degree of the pores in the AE-agent added cement pastes.