Pore Structure Of Cement Mortars Research Articles

Synthesis, performance and mechanism of shrinkage-reducing agents with water-reducing function for cement-based materials

Three shrinkage-reducing agents (SRAs) possessing a water-reducing capacity with different structures were synthesized by aqueous free radical copolymerization. Their structures were characterized by Gel Permeation Chromatography (GPC), specific anion charge density, and Fourier Transform Infrared Spectrometer (FTIR) measurements. Their abilities to reduce surface tension and their adsorption and dispersing effectiveness in cement paste as well as their impacts on cement hydration and compressive strength, autogenous shrinkage, drying shrinkage, and pore structure of cement mortar were investigated. Results clearly showed that the three synthesized SRAs with a water-reducing function reduced the surface tension of pore solution, with the water-reducing SRA with butyl exerting the most significant effect due to its lower molecular mass. They enhanced the flowability of cement paste because of their adsorption on cement grains. The water-reducing SRA with dimethyl amino exhibited higher adsorption amounts on cement grains due to higher charge density, resulting in higher initial flowability of cement paste and more significant hydration retardation. Their presence decreased the autogenous shrinkage of cement mortar by reducing the surface tension of pore solution, delaying the cement hydration, and decreasing the capillary pore volume. Their addition also mitigated the drying shrinkage due to the synergistic effect of surface tension reduction, pore structure coarsening, and porosity (2.5–50 nm) reduction. Among the three, the water-reducing SRA with butyl demonstrated a higher capability of reducing the drying shrinkage and had a favorable effect on the compressive strength of mortar at later ages.

A new method for improving the resistance of cement-based materials to carbonic acid water corrosion: Carbonation curing and further water curing

Accelerated carbonation curing has several advantages compared with traditional moisture curing in terms of decreasing the duration time of early curing and improving the mechanical properties, dimensional stability and durability of concrete. This study investigated the effect of accelerated carbonation curing and further water curing on corrosion behavior, corrosion products, and pore structure of cement mortar and paste samples exposed to carbonic acid water corrosion. The results indicate the accelerated carbonation curing reduces corrosion depth and mitigates the decline in compressive strength caused by carbonic acid water corrosion. The accelerated carbonation curing can lead to the decalcification of C-S-H gel, and the formation of a large amount of CaCO3 and silica gel (SiO2·γH2O) on the surface of the C-S-H gel, which play a crucial role in the corrosion resistance of carbonic acid water. CaCO3, firstly, fills pores and optimizes the pore structure, slowing down the diffusion rate of carbonic acid water. Meanwhile, an essential "protective layer" forms on the surface due to abundant CaCO3, effectively preventing further corrosion of internal C-S-H gel, CH, C2S, and C3S. At the later age of corrosion, the carbonic acid water can lead to the decalcification shrinkage of CaCO3, and the deterioration of pore structure, which adversely affects the mechanical properties of cement-based materials.

Effect of magnesium hydroxide on the mechanical properties and pore structure of cement mortar after high temperature exposure

Magnesium hydroxide (Mg(OH)2) is commonly used as flame retardant chemical additive in polymers. In this study, the effect of Mg(OH)2 on pore structure and mechanical properties of cement mortar is quantitatively investigated through compression tests, NMR tests, and SEM observations. The results show that the compressive strength and elastic modulus increase first and then decrease with the content of Mg(OH)2, and the optimum addition of Mg(OH)2 is about 10 %. The addition of magnesium hydroxide improves the high-temperature resistance and mechanical properties of cement mortar. Compared with the compressive strength before heating, the strength loss varies from 0.13 % to 4.66 % after exposure at 200 °C, that of loss ranges from 26.16 % to 34.53 % after exposure at 400 °C, and that of loss ranges from 80.98 % to 86.19 % at 600 °C. The content of Mg(OH)2 makes the strength of specimens after high-temperature exposure higher than that of specimens without addition. The re-curing process makes the compressive strength of the specimens increase, with the highest increase rate of strength at 400 °C. The pore growth rate of the specimens after high-temperature exposure decreases with Mg(OH)2, and the specimen with 15 % Mg(OH)2 has the smallest pore growth rate. Through SEM observations, mortar with the content of Mg(OH)2 can improve the repair ability and strength of mortar after being exposed to high-temperature. The total pore fractal dimension of the specimens after exposure to different temperatures was found to have a negative correlation with the content of magnesium hydroxide. The study demonstrates the potential of Mg(OH)2 as an inorganic flame retardant for improving the mechanical properties and high-temperature resistance of cement mortar.

Influence of sillimanite sand and corundum sand on the mechanical property, durability and pore structure of cement mortar

The present study investigates the effects of sillimanite sand and corundum sand on the mechanical property, durability, and pore structure of cement mortar. Normal sand in the cement mortar mix was replaced with sillimanite sand or corundum sand at 25, 50, 75, and 100 % by volume. Further, a binary blend of sillimanite sand and corundum sand was considered to study the dual aggregate effect on cement mortar. The incorporation of 75 % corundum sand in the mortar improved the compressive strength and flexural strength by 177.7 % and 41.5 %, respectively. The higher hardness, specific gravity, and rough surface texture of corundum sand improved the mechanical properties. Further, corundum sand was found to increase the bulk density and reduce the number of permeable pores as well as the sorptivity, shrinkage, and pore connectivity in the mortar. The properties of mortar prepared using sillimanite sand and binary blend (sillimanite sand and corundum sand) were superior to those of the control sample. The average particle size of sillimanite sand was 2.40 times lower than that of normal sand. The lower particle size of the sillimanite sand formed a dense microstructure, which enhanced both the strength and the durability. It is concluded that sillimanite sand and corundum sand are feasible alternatives to normal sand in the design of engineered cement mortars.

Application of electro-injection method in improving cement mortar durability

An electrochemical method for improving the durability of cement mortar by injection of nanoparticles (alumina (Al2O3)-coated silica (SiO2)) is presented. In this method, nanoparticles are injected into cement mortar pores under an external electrical field, refining the pore structure of cement mortar. The efficiency of electro-injection nanoparticle treatment with different applied electric fields was evaluated by monitoring the evolution of cement mortar resistivity. The improvement of cement mortar durability after electro-injection of nanoparticles was assessed by examining the resistance to water absorption, carbonation, sulfate attack and chloride diffusion. The microstructure of cement mortar after the electro-injection treatment was analysed using scanning electron microscopy (SEM), mercury intrusion porosimetry (MIP), X-ray diffraction (XRD) and differential thermo-gravimetric analysis (DTG). The results show that cement mortar durability can be improved by the injection of nanoparticles under external electric field. The efficiency of electro-injection nanoparticle treatment increased with the increase of applied electric field. The resistance to water absorption, carbonation, sulfate attack and chloride diffusion were significantly improved by using the electro-injection method. The improvement in cement mortar durability can be attributed to the filling effect of nanoparticles.

Feasibility of producing cement-based sacrificial materials with strontium ferrite: A preliminary study

The safety of nuclear power plant can be improved via core catcher, and cement-based sacrificial materials are widely used in most core catchers because of their simple construction process and the low manufacturing cost. This paper carried out a preliminary investigation to explore the feasibility of using strontium ferrite to prepare cement-based sacrificial materials. The effects of strontium ferrite on the room temperature and high temperature properties of cement mortar were systematically studied in the work. In addition, the damage evolution of cement mortar containing strontium ferrite subjected to elevated temperatures was also identified according to the damage mechanics theory. It was found that, (1) the addition of strontium ferrite led to the deterioration of the mechanical properties, microstructure and pore structure of cement mortar; (2) it was feasible to prepare cement-based sacrificial materials containing strontium ferrite that met the requirements on basic properties like mechanical properties, free water content, volume stability, and the high temperature integrity by controlling the content of strontium ferrite; (3) the correlation between the internal damage of cement mortar and the temperature could be described by Weibull Distribution Model, which provided a basis for the identification and evaluation of serious nuclear power accidents.

Mechanical strengths, drying shrinkage and pore structure of cement mortars with hydroxyethyl methyl cellulose

This paper aims at investigating the long-term mechanical strengths and drying shrinkage, as well as pore structure of cement mortars with hydroxyethyl methyl cellulose (HEMC). The results show that the 3-d flexural strength and compressive strength of cement mortars tend to decrease with the HEMC content increasing. At 28 d to 360 d, the strengths keep decreasing with the HEMC content increasing at 0–1%, and then increase from the minimum value with the HEMC content increasing at 1–3%, still being much lower than those of the control (0% HEMC). Throughout the curing age, both the tensile bond strength and the drying shrinkage value gradually increase with the HEMC content, to be nearly twice at 3% HEMC than those of the control. HEMC significantly delays the 3-d hydration of cement pastes, but has negligible retardation effect on the hydration after 28 d, to bring the similar late hydration degree of cement pastes with HEMC or not. With the HEMC content increasing in the range of 0–1%, incorporating HEMC brings the sharp increase of macro pores, large capillary pores and porosity, and decreases the interconnected pores to bring many more closed pores. Further increasing the HEMC content to 3%, macro pores and porosity decrease, large capillary pores slightly increase and interconnected pores have a negligible change. Obviously, the understanding on the performance development of cement mortars with HEMC is conducive to the better application of HEMC in dry-mix mortars to meet different requirements.

Carbonation curing of cement mortars incorporating carbonated fly ash for performance improvement and CO2 sequestration

Carbonation treatment of fly ash (FA) is proposed as a promising method for stabilization of hazardous residues and CO2 sequestration. This paper aims to investigate the performance of cement mortar incorporating carbonated FA and the synergistic effect with carbonation curing. The results show that the content of free CaO and the hydration heat were reduced apparently by carbonation treatment. The synergy of carbonation treatment and carbonation curing compensated for the strength loss caused by FA incorporation, but can also delayed strength development brought by pozzolanic effect. CO32- on the surface of FA introduced by carbonation treatment competed with Cl− for binding AFm, consequently lead to decreased resistance of chloride ion penetration. Carbonation curing mitigated this degradation by improving the pore structure of cement mortar significantly. In summary, carbonation curing can be an appropriate complement to carbonation treatment of FA for performance improvement of cement-based materials and more efficient CO2 sequestration.

Study on properties and pore structure characteristics of cement-based materials with limestone powder

In order to study the effect of limestone powder as an auxiliary cementing material replacing part of cement on the properties and pore structure of cement mortar, the working performance, mechanical properties and pore structure characteristics of cement mortar with 0%, 10%, 20% and 40% substitutions of limestone powder were tested. The experimental results show that with the increase of the amount of limestone powder, the fluidity of mortar increases continuously and the water retention also becomes better. The addition of limestone powder improves the workability of mortar, but with the increase of limestone powder, the compressive strength of mortar decreases continuously. The results of low field NMR show that the porosity of cement mortar increases firstly and then decreases with the increase of limestone powder content. Compared with the reference mortar, its porosity is improved to some extent, however the pore throat distribution indicates that the increase of porosity is caused by the increase of small pores between 0.01μm and 0.16μm. The harmful macropores between 0.16μm and 0.63μm in mortar are reduced, which indicate that the addition of limestone powder can effectively improve the pore diameter distribution of mortar.

Effects of polycarboxylate superplasticisers with various functional groups on the pore structure of cement mortar

The air-entraining capabilities of polycarboxylate superplasticisers (PCEs) with different functional groups were systematically analysed. Using different molecular PCEs, the evolution process from bubbles in fresh cement mortar to pores in hardened cement mortar was investigated, considering their effects on mortar workability and mechanical performance. The results showed that PCEs with various functional groups have different air-entraining capabilities. PCEs with ester groups decreased the surface tension and significantly promoted the air-entraining capability, resulting in a notable increase of air content in fresh cement mortar. PCEs with sulfonic acid groups increased the surface tension and remarkably decreased the air content in the fresh cement mortar. A higher air-entraining capability benefits the dispersing properties of PCEs and therefore the PCEs with ester groups performed better in dispersibility in comparison with the PCEs with sulfonic acid groups. The stability of bubbles entrained in fresh cement mortar leads to differences in the pore structure of hardened cement mortar. The PCEs with ester groups diminished the size of large bubbles, leading to significantly decreased porosity of the hardened mortar. PCEs with sulfonic acid groups in the backbone barely optimised the pore structure, with a large amount of large pores remaining in the hardened mortar. Decreased porosity and pore size hence led to higher compressive strength of the hardened cement mortar.

New approach for evaluating chloride ion penetration of mortar in cold environments

The aim of this paper was to use the surface fractal dimension (Ds) to evaluate the chloride ion penetration of cement mortar in extremely cold regions. Pore structure is a critical factor that influences the permeability of cement mortar. This study investigated surface fractal dimensions of the pore structure of cement mortars, which were subject to one-time freezing for 7 days after casting. A thermodynamic model was applied to calculate Ds on the basis of mercury intrusion porosimetry data. The results show that the pore structures of all specimens exhibit the scale-dependent fractal property, and three distinct fractal regions (I, II, III) are obtained. According to the regression analysis results of fractal dimension and pore structure parameters, it was found that the fractal dimension can characterise the complexity and chaos of pore structure and predict pore size distribution. Based on this, a linear regression between chloride diffusion coefficient (Dnssm) and Ds in fractal regions was carried out. The results show that Ds of micro pores (region I) have good correlation with Dnssm, up to 0.92908, which is more accurate than the porosity. An index of surface fractal dimension is proposed to evaluate the durability of concrete buildings in extremely cold regions.

A study of the transport properties of emulsified asphalt powder-modified cement mortar

Emulsified asphalt powder (EAP) is a redispersible powder material prepared by spray drying process using emulsified asphalt as raw material that has been developed following the successful utilization of emulsified asphalt in the ballastless track structures of high-speed railways and in road and bridge engineering. In this study, the influence of EAP on the surface properties, transport properties and pore structure of cement mortar was investigated. The results revealed that EAP can improve the surface properties and reduce the transport properties of cement mortar. Furthermore, it was observed that there are intrinsic links between different transport property parameters of cement mortar incorporating EAP, and that the reduction of transport properties is essentially related to the increase of hydrophobicity and the improvement of pore structure afforded by EAP. The capillary water absorption and the drying mass loss of EAP-modified cement mortar decreased with the reduction of critical pore diameter, indicating that the addition of EAP beneficially reduced the transport properties of cement-based materials, thereby improving their durability.

Multiscale pore structure analysis of nano titanium dioxide cement mortar composite

The effects of nano-Ti O2 (NT) on the multiscale pore system and morphology of cement mortars were studied by X-ray computed tomography (X-CT), scanning electron microscopy (SEM), and Brunauer-Emmett-Teller (BET) method. The obtained results from X-CT showed that microscale pores (from 20 μm to 200 μm) of mortars mixed with 2.5 wt% NT were significantly refined. When the content of NT is more than 2.5 wt%, the total pore volume and the number of pores begin to increase. However, when the amount of NT exceeds the optimal value, e.g., (2.5 < NT < 5%), the recorded 2D porosity (void area fraction), are still lower than the reference sample. Meanwhile, the microscopy results proved that, by adding 2.5 wt% NT, the pores of cement mortars were best filled, resulting in a more homogenous morphology. At the nanoscale, the BET results revealed a clear trend of increasing surface area and pore volume of the NT cement mortars, over the entire range of NT percentages, i.e., from 0 wt% to 10 wt%. This could stem from the formation of nanosized needle-shaped products in the presence of the nanoparticles. Further, the permeability of NT cement mortars was measured using a high-pressure core holder and the results showed that 2.5 wt% NT inclusions could reduce the permeability of the cement mortars by 32%. However, adding more NT (>2.5 wt.%) led to higher permeability. This is in line with the microscale pore analysis from X-CT. It can be concluded that adding a certain amount of NT (around 2.5 wt%) can modify the pore structure of cement mortars by changing the harmful microscale pores (permeability related) to the nano-sized benign pores, leading to a much stronger durability of cement-based materials.

Curing temperature: A key factor that changes the effect of TiO2 nanoparticles on mechanical properties, calcium hydroxide formation and pore structure of cement mortars

There are several studies about the effects of addition of TiO2 nanoparticles in mortar properties, but they were made at a given temperature. However, are the effects of this addition independent of curing temperature? The present research focused on the influence of TiO2 nanoparticles at three curing temperatures: 5 °C, 20 °C and 45 °C. Four different mortar mixtures were investigated: a reference mortar (without nanomaterial) and three mortars with different percentages of TiO2 (0.25%, 0.5% and 1%) by the total amount of cement for the reference mortar. For each curing temperature, the effect of TiO2 on (i) compressive and flexural strength at 7 days, (ii) calcium hydroxide (CH) formation at 1, 3 at 7 days and (iii) pore structure have been studied. The effect of TiO2 on compressive and flexural strength depends strongly on curing temperature: the higher the curing temperature, the lower the positive effect of TiO2. If this fact is not considered when curing temperature is higher than the standard (20 °C), improvements due to TiO2 substitution would not be as considerable as it is expected. TiO2 promotes CH carbonation during sample preparation. Results suggest that promotion of hydration is not the only effect of TiO2, which can contribute to enhance strength. A potential mechanism of these nanoparticles to enhance strength could be the modification of pore structure; according to 3D X-Ray microscope results, at 20 °C, TiO2 makes pores smaller, more homogeneous and spherical.

The Influence of Crystallization Additives on the Porosity of Polypropylene Fibre-reinforced Cement Mortar

The phenomenon of self-healing cement materials has been described in the professional literature for decades. One self-healing mechanism is based on the presence of crystallization additives in concrete or mortar; when a defect appears in the material, the crystallization additives (CA) are activated, and new crystallization products will fill the pore or crack. The activation of the crystallization additives occurs only under certain conditions. The intensity of crystallization is affected by the nature of the environment within the crack or pores, and in order for the crystallization to occur, the cracks or pores must contain an aqueous solution with sufficient ion content to react with the crystallization additive. This paper discusses the possibility of using crystallization additives to reduce the porosity of cement mortar reinforced with polypropylene (PP) fibres, describes the effect of crystallization additives on the pore structure of cement mortar, and evaluates the influence of the medium on pore size distribution in cement mortar.

Research on the Mechanical Properties and NMR Characteristics of Cement Mortar during Freeze-Thaw Cycles

Low-field nuclear magnetic resonance (NMR) technology has the characteristics of nondestructive, rapid, and accurate. In the present paper, the mechanical properties and the size and distribution of pores of cement mortar during freeze-thaw cycles were studied by using the NMR technology for the first time. The change law of surface and quality, compressive strength, splitting tensile strength, and elastic modulus of cement mortar under 0, 25, 50, 75, and 100 freeze-thaw cycles were studied. And the changes of T2 spectra of cement mortar under different freeze-thaw environments were analyzed; the change rule between freeze-thaw cycles and the size of the pore within the cement mortar were also obtained. Moreover, the relationship between the mechanical properties and the pore structure of cement mortar was studied.

Effect of Using Micropalm Oil Fuel Ash as Partial Replacement of Cement on the Properties of Cement Mortar

This study investigates the effects of micropalm oil fuel ash (mPOFA) on compressive strength and pore structure of cement mortar. Various experimental techniques, such as compression test, isothermal calorimetry, mercury intrusion porosimetry, and X-ray diffraction, are performed to figure out the effect of using mPOFA as partial replacement of cement on the hydration of cement and determine its optimal replacement level to increase mechanical property of the mortar specimens. 10 wt.% of cement replacement with mPOFA is found to give the highest level of compressive strength, achieving a 23% increase over the control specimens after 3 days of curing. High K2O contents in mPOFA stimulate C3S in cement to form C-S-H at early ages, and high surface area of mPOFA acts as a nucleus to develop C-S-H. Also, small mPOFA particles and C-S-H formed by pozzolanic reaction fill the pores and lead to reduction in large capillary pores. In XRD analysis, a decrease in Ca(OH)2 and SiO2 contents with age confirmed a high pozzolanic reactivity of mPOFA.

Influence of a Crystalline Additive on the Porosity of Cement Mortars

The pore structure of a material radically infuences its properties, such as strength, freeze-thaw resistance, capillarity or resistance to elevated temperatures. The character of the pore structure is also limiting for the intrusion of aggressive oxides into the material, which affects its service life. The paper describes how a crystalline additive influences the pore structure of cement mortars. It describes the effect of secondary crystallisation and the influence of the additon of PP fibers on the apparent porosity and the internal structure of the mortar. It also discusses the influence of aggressive conditions on the mortar’s pore structure.

The influence of type of cement on the degradation of microstructure and transport properties of cement mortars exposed to frost induced damage

The purpose of the study is to understand how the cyclic water freezing (0, 25, 50, 75, 100 and 150 freeze-thaw cycles) impacts microstructure and transport properties of cement-based materials. Tests were conducted on cement mortars with different water/cement ratios (w/c=0.45 and 0.40) and on two types of cement (CEM I and CEM III) without air-entraining admixtures. The changes of pore size distribution and open porosity were investigated by means of mercury intrusion porosimetry. Additionally, the relationship between intrinsic permeability and the water absorption coefficient of cement mortar samples was analysed. The water absorption coefficient and gas permeability were determined using capillary absorption test and the modified RILEMCembureau method. The evolution of transport coefficients with growing number of freeze-thaw cycles were determined on the same sample. It was also established that change of pore structure (a decrease of small pore volume &lt;100nm and increase of larger pores &gt;100nm) induces an increase of water transport parameters such as permeability and water absorption coefficient. The higher gas permeability corresponds to the higher internal damage. In particular, it is associated with the change of cement mortar microstructure, which indicates damage of narrow channels in the pore structure of cement mortars.

Effect of sand content on strength and pore structure of cement mortar

The effects of four sand contents on the compressive, flexural and splitting-tensile strength of cement mortars were evaluated. Moreover, we experimentally investigated the pore structure of cement mortar brought about by changing the sand content and water/cement ratio. The changes in the pore structure were quantified by measuring the porosity and pore size distribution obtained by using mercury intrusion porosimetry (MIP) technique. The test results show that the strengths of cement mortar increase with increasing sand content. It is also suggested that the traditional water/cement ratio law can be applied to cement mortar with different sand contents, provided that a slight modification is introduced. Sand content is an important parameter influencing the pore structure of cement mortar. Moreover, there is a good relationship between the pore structure and strength of cement mortar.