Cement Paste Specimens Research Articles

Computational investigation of pore permeability and connectivity from transmission X-ray microscope images of a cement paste specimen

This study applied the Transmission X-ray Microscope (TXM) characterization techniques and permeability-solver computational program to investigate the transport properties of a microscale cement paste specimen. The TXM techniques allow fast-image acquisition of pore microstructure at a resolution of 30nm. The microscale cement paste specimen with 0.45water/cement ratio was specially prepared with a capillary tube. The pore microstructure of the microscale paste specimen was characterized by using the Advanced Photon Source at the Argonne National Lab. The image processing technique was conducted to identify the pore distribution in the captured images. The digital samples with different porosities were generated to compute the transport properties. The burning algorithm was employed to estimate the pore connectivity. The finite difference method with artificial compressibility relaxation algorithm was applied to simulate water transport in capillary pores with Stokes equation. The pore permeability was computed with the calculated average flow velocity using Darcy’s Law. The computed permeability results of digital samples were also compared with the predicted values from the Katz–Thompson equation to demonstrate the computational accuracy.

Estimation of the thermal properties of hardened cement paste on the basis of guarded heat flow meter measurements

In the present study, a new analysis method is proposed to measure thermal conductivity, thermal diffusivity, volumetric specific heat capacity, and specific heat capacity of cement-based materials based on the guarded heat flow meter apparatus. We tested hydrated cement paste specimens with 0.4 water-to-cement ratio and applied both steady and non-steady state data to compute thermal properties of the hydrated cement paste. Thermal conductivity and thermal contact resistance were determined as 1.28 W m−1 K−1 and 0.038 m−2 K W−1 using steady state data. Furthermore, using a computerized system of heat flow meters and non-linear regression, we used non-steady state data to measure thermal diffusivity, volumetric specific heat capacity and specific heat capacity of the hydrated cement paste as 4.77 × 10−7 m2 s−1, 2685.4 kJ m−3 K−1, and 1.28 kJ kg−1 K−1. The relative uncertainties of these properties fell within 2–7% range and the residual distributions were validated to follow a normal distribution based on quantile–quantile tests.

Local porosity distribution of cement paste characterized by X-ray micro-tomography

Porosity is one of the most important parameters for cement-based materials, which influences the mechanical property, transport property, and durability. The spatial and frequency distributions of local porosity of cement pastes are characterized using X-ray micro-tomography data and treating methods. The 3D spatial distributions for three cement paste specimens with different water cement (w/c) ratios show reasonable heterogeneity. The probability analysis also reveals this heterogeneity: the representative volume element (RVE) size based on porosity maps decreases with w/c ratio firstly, then increases with w/c ratio; and the heterogeneity on the characterized probe size or on the RVE size increases with w/c ratio. Average porosities obtained using the CT method are further compared with those by traditional methods.

Mechanical Properties and Durability of CNT Cement Composites.

In the present paper, changes in mechanical properties of Portland cement-based mortars due to the addition of carbon nanotubes (CNT) and corrosion of embedded steel rebars in CNT cement pastes are reported. Bending strength, compression strength, porosity and density of mortars were determined and related to the CNT dosages. CNT cement paste specimens were exposed to carbonation and chloride attacks, and results on steel corrosion rate tests were related to CNT dosages. The increase in CNT content implies no significant variations of mechanical properties but higher steel corrosion intensities were observed.

Reactivity of nitrate and organic acids at the concrete–bitumen interface of a nuclear waste repository cell

This study investigates the fate of nitrate and organic acids at the bitumen–concrete interface within repository cell for long-lived, intermediate-level, radioactive wastes. The interface was simulated by a multiphase system in which cementitious matrices (CEM V cement paste specimens) were exposed to bitumen model leachates consisting of nitrates and acetic acid with and without oxalic acid, chemical compounds likely to be released by bitumen. Leaching experiments were conducted with daily renewal of the solutions in order to accelerate reactions. The concentrations of anions (acetate, oxalate, nitrate, and nitrite) and cations (calcium, potassium) and the pH were monitored over time. Mineralogical changes of the cementitious matrices were analysed by XRD. The results confirmed the stability of nitrates in the abiotic conditions of the experiments. The action of acetic acid on the cementitious matrix was similar to that of ordinary leaching in the absence of organic acids (i.e. carried out with water or strong acids); no specific interaction was detected between acetate and cementitious cations. The reaction of oxalic acid with the cementitious phases led to the precipitation of calcium oxalate salts in the outer layer of the matrix. The concentration of oxalate was reduced by 65% inside the leaching medium.

塩化カルシウム水溶液によるポーラスコンクリートの劣化に関する研究

Main purpose of the present research is to clarify the chemical deterioration mechanism of porous concrete by CaCl2 solutions as a deicer. In the present study, cement paste specimens were fabricated with ordinary Portland cement and soaked in high concentration CaCl2 solution. Observation of deterioration process and X-ray diffraction analysis were conducted on both chemical deterioration products and binder exfoliated from aggregate surfaces. As a result, it was found that CaCl2 solution penetrated into binder, then produced deterioration product either in the interfaces of binder and aggregates causing the interface peeling, or right under the carbonated surface of binder causing stratified peeling. In both cases, collapse of porous concrete structure has been finally found. Also, it was found that surface of binder has high resistance against the CaCl2 attack in the existence of CaCO3 which was produced from hydration product Ca(OH)2.

低水結合材比におけるセメント硬化体の力学特性に及ぼす高温履歴の影響

高温負荷が低水結合材比のセメント硬化体の力学特性に及ぼす影響を検討するため、普通ポルトランドセメント、高炉セメントB種、フライアッシュセメント用いたセメントペースト及びモルタルを、3種類の異なる温度履歴で養生した。混和材添加のセメントペーストは、高温負荷で水和反応が促進され常温養生に比べ材齢28日強度が増加し、水銀圧入試験からも組織の緻密化が確認された。モルタルに高温負荷を継続的に与えると、いずれも常温養生に比べ材齢28日強度が低下し、自己乾燥による骨材界面の損傷が理由に考えられた。初期高温履歴を与えると、モルタルの材齢91日強度は材齢28日より小さく、材齢経過により強度が低下する可能性が示唆された。

Evaluation of Recementation Reactivity of Recycled Concrete Aggregate Fines

The use of recycled concrete aggregate (RCA) as a drainage material in exfiltration trenches and base and subbase layers is becoming increasingly common. During crushing, stockpiling, transporting, and placing, however, RCA may produce fines that could recement and clog the pavement drainage system. Many previous studies indicated that not only the structural capacity but also drainage of pavement subsurface layers can be greatly affected by the properties and gradation of the aggregate used. This study evaluated the recementation reactivity of RCA fines that could eventually impair drainage performance. RCA fines passing the No. 200 sieve (less than 75 μm) were produced through the simulation of an in-place aggregate abrasion process, and cement paste cylinders were cast with complete replacement of cement with RCA fines. The hydration properties of the paste samples with RCA fines were evaluated by means of heat of hydration, pH variation, and Vicat needle penetration in their early age. Compressive strength tests and petrographic examinations such as scanning electron microscopy, energy dispersive X-ray, and X-ray diffraction were also performed to mechanically and chemically verify the recementation reactivity of the hardened paste with RCA fines. Test results demonstrated that recementation of RCA fines was modest, and therefore the compressive strength of the paste specimens with RCA fines was minimal compared with that of ordinary cement paste specimens. The microstructural and chemical composition analyses also indicated that the recementation reactivity of RCA fines was negligible.

Acquisition of Capillary Pore Structure by X-Ray CT and Visualization of Flow by Numerical Analysis

In concrete, air, water, and ions are passed mainly through pores inside the material. The purpose of this study is to visualize a flow in pore of the cement paste by a model based on the measurement. In this study, X-ray CT images are used to ascertain the pore structure. Using the X-ray, a CT image of cement paste specimens with two kinds of water/cement ratios (W/C) and a three-dimensional image of the pore structure are represented using about 2 μm voxel mesh. The voxel mesh is used as an analysis mesh in voxel finite element method (FEM) and seepage analysis. A typical pore model was taken out of the three-dimensional image of pore structure obtained by X-ray CT scanning and the fluid analysis was carried out by using the typical pore model.

Accelerated biodegradation of cured cement paste by Thiobacillus species under simulation condition

Biodegradation is one of the most important types of cement deterioration. Complex microbial populations take part in the biodegradation process of cement-based materials. Studies in this field show that the sulfur-oxidizing bacteria, including Acidithiobacillus thiooxidans, due to sulfuric acid formation, play a key role in this process. In this study, with the accelerated leaching process of calcium hydroxide of cement paste, cured under running tap water and exposed to sterile biogenic sulfuric acid for 6 days, the surface pH of the cement was reduced to a more favorable level for bacterial growth. In this case, the growth of Thiobacillus proceeded in the presence of cured cement paste specimens. After 90 days of exposure to a semi-continuous culture of A. thiooxidans with its pH less than 2 and continuous removal of damaged layers the compressive strength, length and mass of the samples dropped by 96%, 11% and 43%, in the order given. The mechanism of degradation and the structure of degraded specimens were analyzed by test laboratory techniques such as, XRD, SEM and EDAX analyses.

Comparative study of three sodium phosphates as corrosion inhibitors for steel reinforcements

A comparative study was performed on the inhibition of steel reinforcement corrosion using three soluble phosphates: sodium monofluorophosphate (Na2PO3F), disodium hydrogen phosphate (Na2HPO4) and trisodium phosphate (Na3PO4). Tests were carried out using ordinary Portland cement (OPC) paste specimens and OPC mortar specimens with embedded steel reinforcements. The corrosion inhibitors were deployed in two different ways: by immersion of OPC specimens in aqueous solutions containing the soluble phosphates (migrating corrosion inhibitor), and by addition of the phosphate powders to a fresh OPC paste (admixture corrosion inhibitor). After curing, the tested specimens were studied using X-ray diffraction, wavelength-dispersive electron microprobe analysis, linear polarisation resistance and electrochemical corrosion potential. A correlation was found between the phosphate content (by migration or admixture) in the OPC matrices and the steel corrosion rate.

Investigation of Thermal Properties of Cement Paste with Fluorescent Lamp Glass Waste, Glass Cullet and Coal/Wood Ashes

The thermal character of cement hydration reactions causes concrete to endure temperature changes during the first days after casting. The present research intends to evaluate the influence on the temperature of hydration of cement paste mixes with local industrial wastes such as lamp glass waste powder/suspension, glass cullet, bottom/fly coal/wood ashes substitution at level of 20% and 30%. The temperature of hydration of cement paste mixes was investigated by using specially designed equipment. The temperature of hydration inside of the cement paste specimens was continuously monitored at least up to 20 h after production at ambient air temperature 20±2°C. The results showed that cement paste substitution with lamp glass waste and coal/wood bottom ashes at level of 30% has influence on the peak temperatures of hydration with decrease from 58.8°C to 46.3°C and cement paste substitution with flint glass at level of 20% decrease temperature of hydration for around 19°C in comparison to control cement paste mix. DOI: http://dx.doi.org/10.5755/j01.sace.2.3.2781

Releases of bound chlorides from chloride-admixed plain and blended cement pastes subjected to sulfate attacks

This paper reports results of a study conducted to evaluate the releases of bound chlorides from pastes made of ordinary Portland cement alone and a mixture of ordinary Portland cement and the partial replacements of cement with 10% silica fume (SF), 30% pulverised fly ash(PFA) and 50% ground granulated blastfurnace (GGBS) subjected to Na2SO4, K2SO4 and MgSO4 attacks. The pastes were prepared by mixing water, cementitious materials and sodium chloride considering three levels of water/cementitious material ratios and four levels of sodium chloride. Powder samples collected by triturating the cement paste specimens were immersed in 5.0% Na2SO4, K2SO4 and MgSO4 solutions, respectively for allowing releases of bound chlorides. The chloride concentrations and pH values of the leachates obtained by filtering the solutions were measured, and the chemical phases of insoluble powders in the solutions were analysed by X-ray diffractometer (XRD). It has been found that the bound chlorides are partially released by sulphate attacks to form free chlorides. The released chlorides are affected by the water–binder ratio, total chloride content, mineral admixtures and sulphates with different associated cation type. After the sulphate attacks, Friedel’s salt has transformed to ettringite, which contributes to the releases of bound chlorides. Besides, by means of the index of Cl−/OH−, the influences of various mineral admixtures and sulphate types on the corrosion risk of steel reinforcement have also been discussed.

Mechanical Behavior of White Concrete

In this study, possibility of the use of WPC 42.5 containing pyrophyllite in the production of white concrete was investigated. Normal and light-weight concrete samples were produced with WPC. In normal-weight concrete mixture, WPC and white aggregate were used. In light-weight concrete mixtures, WPC, FA, SF and SP were used. Setting time and soundness of cement paste specimens and compressive strength of light and normal weight white concrete specimens were determined. According to the test results, normal weight white concrete can be used in the construction of frames and light-weight white concrete can be used in the production of concrete/reinforce concrete panels.

Drying-induced changes in the structure of alkali-activated pastes

Drying of cement paste, mortar, or concrete specimens is usually required as a pre-conditioning step prior to the determination of permeability-related properties according to standard testing methods. The reaction process, and consequently the structure, of an alkali-activated slag or slag/fly ash blend geopolymer binder differs from that of Portland cement, and therefore there is little understanding of the effects of conventional drying methods (as applied to Portland cements) on the structure of the geopolymer binders. Here, oven drying (60 °C), acetone treatment, and desiccator/vacuum drying are applied to sodium silicate-activated slag and slag/fly ash geopolymer pastes after 40 days of curing. Structural characterization via X-ray diffraction, infrared spectroscopy, thermogravimetry, and nitrogen sorption shows that the acetone treatment best preserves the microstructure of the samples, while oven drying modifies the structure of the binding gels, especially in alkali-activated slag paste where it notably changes the pore structure of the binder. This suggests that the pre-conditioned drying of alkali activation-based materials strongly affects their microstructural properties, providing potentially misleading permeability and durability parameters for these materials when pre-conditioned specimens are used during standardized testing.

骨材粒子が超吸水性ポリマーの空間分布に及ぼす影響

内部養生材として使用される超吸水性ポリマー（SAP）の空間分布を点過程統計量によって定量的に評価した。その結果、セメントペースト供試体中のSAP粒子は粒径にかかわらずほぼランダムな分布であるのに対して、モルタル供試体中では凝集性を有した配置となった。凝集性を示す距離範囲はSAPの粒径によって異なり、小径のSAPの方がより近距離で凝集性が現れる。また、この凝集性により定常ポアッソン過程という帰無仮説は棄却されることがシミュレーションにより示された。これより、骨材粒子の存在はSAP粒子の分布を単純にランダムに制限する以上の効果を持つと考えられる。

Performance of waste glass powder (WGP) supplementary cementitious material (SCM) – Workability and compressive strength

Ecological and environmental benefits support the use of waste glass powder (WGP) as supplementary cementing material by the decrease of the amount of landfills, by the reduction of non-renewable natural resource consumption, by the reduction of energy demand for cement production (less cement is needed), and the reduction of greenhouse gas emission. Laboratory tests were carried out on cement paste specimens, in which waste glass powder (WGP) addition was used as a supplementary cementitious material. Cement was substituted with WPG at levels of 20% or 30% per mass. It was demonstrated that the WGP addition improves the workability of fresh pastes, and can be effectively used as cement replacement for compressive strength. It was also demonstrated that the particle size of the WGPs (specific surface area) has a stronger influence on the effectiveness of the cement replacement than the chemical composition. The effectiveness of the cement replacement increases as the specific surface area increases.

The Study on Preparation of Kaolin-Acrylic Super Absorbent Polymer and its Internal Curing Effect in the Hydration Procedure of Cement Materials

In this paper, a new kind of superabsorbent polymer (SAP) is designed and prepared. Using sodium persulfate as initiator and N,N’-methylene bisacrylamide as Crosslinker, monomer of acrylic acid can be grafted into the skeleton of natural mineral kaolin with the method of microwave irradiation. The performance of water adsorption for SAP is tested in saturated Ca(OH)2 solution. Also, the internal curing effect of SAP to hydration procedure of cement paste is studied. The result shows that the compressive properties and flexural strength of cement paste specimens in early stage have significant improvement in contrast to those without superabsorbent polymer or excessive or a small amount of superabsorbent polymer, when superabsorbent polymer is accounted for 0.3% to 0.5% of the total cement.

Sealing process induced by carbonation of localized cracks in cementitious materials

The healing of cracks plays an important role with regard to the air- and water-tightness of concrete structures and the durability of cement based materials in general. This paper aims at further characterization and better comprehension of the healing phenomenon induced by the precipitation of calcite in localized cracks. The experimental program consisted in generating a localized crack in a cement paste or concrete specimen and healing the crack created by percolation tests with carbonated water or gas (CO2-air mixture). For tests with liquid, results show that the healing process depends on physical parameters like crack width, pressure gradient and carbonate content in the percolating fluid. For tests with gas, the supply of the crack with Ca2+ ions to form calcite depends on the moisture transport mode on the crack edges, conditioned by the relative humidity of the percolating gas mixture. A simplified model of the leakage rate evolution is proposed. It provides indications concerning the effect of each test parameter on the healing process and allows the experimental results to be reproduced. Application of the model to other tests from the literature shows its limits and gives guidance for future investigations.

Electrochemical performance of steel in cement extract and bulk matrix properties of cement paste in the presence of Pluronic 123 micelles

The influence of Pluronic P123 (PEO20-PPO70-PEO20) non-stabilized micelles (of 10 nm size) on the corrosion behavior of low-carbon steel in cement extract was studied, using electrochemical impedance spectroscopy (EIS) and potentio-dynamic polarization (PDP). Mercury intrusion porosimetry (MIP) was employed to derive the impact of admixed P123 micelles on porosity and pore-size distribution of cement paste. As far as steel corrosion resistance is concerned, a positive effect was observed, initially denoted to the presence of the polymer itself, rather than the presence of micelles. Further, the P123 micelles were found to result in increased corrosion resistance in the presence of 1 % and 3.5 % NaCl in the alkaline environment of cement extract. There was no significant influence on porosity and pore size distribution of the admixed in cement paste P123 micelles. The observed phenomena are related to self-assembly of the micelles only within higher ionic strength and the presence of chloride, in which case the critical micelle concentration is reduced. At micelles concentration of 0.024 g/l for the chloride-free cement extract (and the solid cement paste specimens, respectively), the medium actually contain unimers that have minimal impact on electrochemical performance and/or microstructural properties. In contrast, with increased ionic strength of the medium (1−3.5 % NaCl and altered ion concentrations resulting from the anodic/cathodic reactions within steel corrosion), the positive effect of 0.024 g/l micelles (and higher of 0.072 g/l) is more pronounced, i.e., increased corrosion resistance and anodic control with external polarization was observed.