Cement Paste Specimens Research Articles

Effect of AlF3 production waste on the processes of hydration and hardening of the alkali-activated Portland cement with sodium silicate hydrate

Generally, the early strength of hardened Portland cement paste is controlled, by changing the mineral composition of clinker and grinding fineness. By using clinker (calcium silicates) and liquid glass (sodium silicate) together, these materials have high degree of coagulation and rapid hardening. For this reason, it is important to regulate the setting times. One solution of this problem is the use of chemical admixtures. In this research, the admixture made from liquid glass and AlF3 production waste was suggested (the waste from phosphates fertilizers production). The cement system “Portland cement clinker–sodium silicate hydrate–AlF3 production waste” was characterized by X-ray diffraction, differential scanning calorimetry, measurements of hydration temperature and the compressive strength of hardened cement paste specimens. The microstructure of hardened cement pastes was evaluated by scanning electron microscopy. Reference specimens were made from pure Portland cement clinker. A possibility to regulate hydration rates both at the early stage of hardening and at the later ones by addition of this admixture was demonstrated. The effect of AlF3 production waste was described. By using this admixture, the hydration temperature increased, the hydration process delayed, and compressive strength of hardened Portland cement clinker paste samples increased, especially at later ages. This gave the opportunity for composition: clinker–liquid glass–AlF3 production waste to create slowly curing cement; which is suitable for long transportation and for massive constructions.

Properties of coal gangue-Portland cement mixture with carbonation

To promote the recycling of self-ignited coal gangue (CG) as a partial substitute for Portland cement (PC) and reduce carbon footprint of fossil fuel, carbonation curing was applied to PC-CG blend pastes as a novel strength enhancement method. Compressive strength, drying shrinkage and chloride ion permeability of PC-CG mixtures with or without carbonation curing were investigated and related mechanisms were demonstrated by X-ray diffraction (XRD), 29Si solid-state nuclear magnetic resonance spectroscopy (NMR), thermogravimetry-differential thermal analysis (TG-DTA), mercury intrusion porosimetry (MIP), scanning electron microscopy (SEM) and back scattered electron imaging (BSE) measurements. Results showed that carbonation curing behaves a better improvement effect on compressive strength of PC-CG pastes with the higher incorporation of CG. The accelerated carbonation curing can improve drying shrinkage and chloride ion penetration resistance of cement paste specimen due to the refinement of pore structure. The captured CO2 content reached around 10% of PC-CG blend by weight with this carbonation curing. On the other hand, up to 20% CG can be recycled to replace PC without sacrifice of strength. Therefore, this technology presents a great potential in both sequestration of carbon dioxide and recycling of coal gangue.

The variation of pore structure in cement paste exposed to sodium sulfate solution with an in-suit X-ray CT testing technology

In this paper, the variation of compressive strength of cement paste specimens under external sulfate attack was studied. Then the microstructure characteristics of the cement attacked by sodium sulfate solution was observed by the Scanning Electron Microscope (SEM) test. Subsequently, an in-suit X-ray CT testing technology was used to monitor the change of pore structure in cement paste specimens under different exposure time, and the Avizo software was used to calculate the porosity of cement paste specimens and porosities of different depth zones in cement paste specimen. The results show that the compressive strength increases firstly and then decreases. Owing to that ettringites are produced in cement paste specimens, and the number and the volume of ettringites increase with the increase of exposure time. Leading to that the porosity of cement paste specimens decreases first and then increases with the increase of exposure time. And the porosities of different depth zones undergo a process of decreasing first and then increasing from surface to interior in cement paste specimens. It indicates that the deterioration process is from the surface to the interior with the increase of exposure time.

3D meso-scale finite element modelling on cement paste corroded in sodium sulfate with X-ray CT technique

In this study, the cement paste corroded in sodium sulfate solution was regarded as a three-phase composites composing of unhydrated cement particles, reaction products and pores on meso-scale. First of all, based on the results of the in-situ X-ray Computed Tomography (X-ray CT) test, a three-dimensional (3D) meso-scale finite element (FE) model of the cement paste specimen was established. Subsequently, homogenization methods were used to calibrate the mechanical parameters (elastic modulus, Poisson's ratio and strength) of the model and the constitutive relations of unhydrated cement particles and reaction products were modified. Finally, the failure process of the cement paste specimens conducted by the uniaxial compression test was simulated. The results showed that the compressive strength of the cement paste specimens experienced a process of increasing first and then decreasing with the increase of corrosion time, caused by the combination of cement hydration and sulfate ion corrosion. Additionally, the simulation results and the uniaxial compression test results were in good agreement, indicating that the modified constitutive relations were effective and the 3D meso-scale numerical simulation can well predict the uniaxial compression test of cement paste specimen under the corrosion of sodium sulfate.

Development and Validation of an NDT Based on Total Sound Signal Energy

Abstract This study proposes a new nondestructive testing (NDT) method for assessing the compressive strength of various materials. The method is based on the total sound signal energy, which is determined using the sound signal generated from impacting an object. A device was devised to generate an impact sound with a test object using a rotating, freefalling impact ball and subsequent repetitive impacts from the rebound action. First, to validate the method, soil cement, cement paste, pine wood, shale, and granite specimens were tested to examine the correlation between the direct compressive strength and the total sound signal energy. Next, the method was applied to a number of concrete test specimens with various sizes and strengths. The test results of the concrete specimens showed a direct relationship between the direct compressive strength and the total sound signal energy, which was dependent on the size of the specimen. Correlation equations between the total sound signal energy and the direct compressive strength were determined for different specimen sizes through regression analysis. The equations were then used to estimate the concrete compressive strength and validate the method. Statistical analysis indicated that the estimated compressive strength determined by the total sound signal energy quite reliably agreed with the directly measured compressive strength, regardless of the specimen size. It is expected that the new NDT method will play a meaningful role in nondestructively estimating the compressive strength of various materials in the future, including concrete.

Autogenous Healing of Early-Age Cementitious Materials Incorporating Superabsorbent Polymers Exposed to Wet/Dry Cycles.

This study experimentally investigated the autogenous healing performances of cementitious materials incorporating superabsorbent polymers (SAPs) after exposure to eight cycles of wet/dry conditions. In each cycle, cracked cement paste specimens with different SAP dosages were exposed to wet conditions for 1 h, during which capillary water absorption tests were conducted, and then exposed to dry conditions for 47 h. The test results reveal that the initial sorptivity values of the reference, 0.5% SAP, 1.0% SAP, and 1.5% SAP specimens after one cycle were decreased by 22.9%, 36.8%, 42.8%, and 46.3%, respectively, after eight cycles. X-ray micro-computed tomography analysis showed that the crack volume percentages filled with healing products were 1.1%, 1.6%, 2.2%, and 2.9% in the reference, 0.5% SAP, 1.0% SAP, and 1.5% SAP specimens, respectively. As the cycling was repeated, the reduction ratio of the initial sorptivity and the quantity of healing products were increased with increases in SAP dosage. Furthermore, more healing products were distributed near SAP voids than in other sections in the specimens. This study demonstrates that the incorporation of SAPs in cementitious materials can enhance the autogenous healing performances of materials exposed to cyclic wet/dry conditions.

Humidity Effect on the Photocatalytic Activity of Sustainable Cement-Based Composites

Introduction:In this study, a side-by-side comparison was performed to examine the effects of nano-TiO2(nT) on the photocatalytic activity of Portland cement (PC) mixtures and its effectiveness compared to similar replacement percentages of micro-TiO2(mT), both of which were 100% anatase.Methods:Cement paste specimens were prepared by progressive cement replacement with nT or mT. To simulate the effect of humidity in different seasons, the PC specimens were subjected to different moisture conditions, including normal conditions with 50% Relative Humidity (RH), saturated conditions with 100% RH, and oven-drying conditions with 0% RH. Their photochemical reaction (i.e., gas removal ability) with TiO2was evaluated and compared with the control specimens. Furthermore, the self-cleaning ability of the mixtures was examined by applying a drop of methylene blue dye to the specimen surface; differences in dye discolouration with time under different illuminations were noticed.Results and Conclusion:This paper describes the results of an attempt to understand the effect of humidity on the photocatalytic performance of PC composites that incorporate TiO2. The results show that PC mixtures modified with nano titanium are more effective in hot and dry conditions. In contrast, micro titanium mixtures appear to be favourable for wet conditions. Furthermore, the particle size (microversusnano), matrix voids and absorption values of the cement-TiO2composite were identified as the key factors affecting the photocatalytic performance.

Three-dimensional lattice modeling of concrete carbonation at meso-scale based on reconstructed coarse aggregates

A three-dimensional (3D) lattice type model was developed to simulate the concrete carbonation at the meso-scale. To construct the meso-scale concrete model, the 3D reconstruction technique was applied to obtain the shape of real coarse aggregates. Experimental data obtained from prior publications on cement paste, mortar, and concrete specimens were used to validate the developed lattice model of concrete carbonation. Based on a parametric study, it was then found that (1) the type and irregularity of the lattice network do not show obvious effects based on the simulation results of carbonation front, (2) a coarser lattice mesh will overestimate the carbonation process but the effect of the lattice mesh can be decreased by its refinement, (3) a two-dimensional (2D) simulation will underestimate the carbonation depth and the degree of underestimation increases as a function of the aggregate content, (4) the carbonation depth obtained from a 3D simulation can be 8–15% larger than that from a 2D simulation.

Fracture Properties of Cement Studied by Nanoindentation and FIB

The paper shows results of microscale experimental tests performed on cement paste specimens fabricated by focused ion beam milling. The specimens are prepared in the form of cantilever beams and loaded in bending by nanoindenter. The dimensions of specimens are in the order of a few micrometers which corresponds to the single phase size. Intact and notched specimens with a stress concentrator are tested. Tensile strength and fracture energy are derived for the hydration product by analyzing the nanoindentation data and with the aid of analytical and numerical modeling. Although small number of tests is performed good correlation of the results is reached with respect to the available literature and molecular dynamic simulations.

The effect of multi-walled carbon nanotubes on the rheological properties and hydration process of cement pastes

The properties of cementitious materials can be noticeably improved by the addition of carbon nanotubes (CNTs). It is important to detect the effects of the pellets containing multi-walled CNTs, dispersed in carboxyl-methyl cellulose, on the hydration process of fresh cement pastes, and physical or mechanical properties of hardened cement paste specimens, obtained without the use of commercially available surfactants and plasticisers, in order to use CNTs in cementitious materials more effectively. The influence of dispersed CNTs in different amounts (from 0 to 0.5%) on the pH and electrical conductivity values of water solutions and fresh cement pastes, as well as on the rheological properties, setting time and exothermic reaction of fresh cement pastes or on the physical and mechanical properties of hardened cement paste specimens was analysed in this work. It can be stated, that 0.005% is the marginal amount of CNTs, as amounts above 0.005% deteriorate the hydration parameters of cement pastes. This was confirmed by decreased values in electrical conductivity (approximately 17%), pH (approximately 6.8%) and dynamic viscosity (approximately 3 times) tests. Lower pH values lead to a significant retardation of the dissolution of cement minerals and this process is reflected in the setting time (prolonged by 110–130%) and maximum time (extended by 148%) or temperature (decreased by 18.4%) of exothermic reaction. Consequently, the mechanical properties of hardened cement paste specimens (after 28 days) decreased about 10%, as well as water absorption increased up to 10.6%.

Pre-failure indicators detected by Acoustic Emission: Alfas stone, cement-mortar and cement-paste specimens under 3-point bending

Acoustic Emission (AE) is the technique most widely used nowadays for Structural Health Monitoring (SHM). Application of this technique for continuous SHM of restored elements of stone monuments is a challenging task. The co-existence of different materials creates interfaces rendering “identification” of the signals recorded very complicated. To overcome this difficulty one should have a clear overview of the nature of AE signals recorded when each one of the constituent materials is loaded mechanically. In this direction, an attempt is here described to enlighten the signals recorded, in case a series of structural materials (natural and artificial), extensively used for restoration projects of classic monuments in Greece, are subjected to 3-point bending. It is hoped that obtaining a clear understanding of the nature of AE signals recorded during these elementary tests will provide a valuable tool permitting “identification” and “classification” of signals emitted in case of structural tests. The results appear encouraging. In addition, it is concluded that for all materials tested (in spite their differences in microstructure and composition) clear prefailure indicators are detected, in good accordance to similar indicators provided by other techniques like the Pressure Stimulated Currents (PSC) one.

Towards understanding stochastic fracture performance of cement paste at micro length scale based on numerical simulation

This work presents a study of stochastic fracture properties of cement paste at the micro length scale based on a combination of X-ray computed tomography (XCT) technique and discrete lattice type fracture model. Thirty virtual specimens consisting of pore, outer hydration products, inner hydration products and anhydrous cement particles were extracted from 3D images obtained through XCT from real cement paste samples. These virtual specimens were subjected to a computational uniaxial tension test to calculate their tensile strengths and elastic moduli. The predicted stochastic strengths were analysed using Weibull statistics, showing that specimens with lower w/c ratio yield higher strength and less variability. The strength-porosity and modulus-porosity relations were investigated based on existing empirical models. It was shown that existing models can predict the properties in the studied porosity range quite accurately, with the exponential model having the highest determination coefficient among all the models for both relations. Finally, by comparing the existing data in the literate, it is found that the smaller cement paste specimens have higher modulus/tensile strength ratio, which indicates that they are able to have more strain at the peak load.

Effect of Fly Ash Content on the Carbonization Resistance of Cement Paste

Carbonation test was carried out on the cement paste specimens with fly ash content of 0% to70%. The change rules of pore structure of cement paste with different fly ash (FA) content before and after carbonation were studied by means of microscopic test, which included mercury intrusion porosimetry (MIP) test and Brurauer Emmerr Teller (BET) test. It is found that total porosity, harmful holes’ prosity (pore diameter>50nm), as well as most probable pore diameter and average pore diameter increase obviously when the content is above 40%, and a certain two order function relationship between the CO2 gas composite diffusion coefficient and the content of FA is presented.

Effects of sulfate on cement mortar with hybrid pozzolan substitution

Sulfate is one of the most important chemical risks which affect the durability of concrete and reinforced concrete structures. Therefore, this study investigates the effects of sulfate on blended cement mortars. In this paper, cement mortar specimens were prepared with the substitution of CEM I 42.5 R cement with Fly ash + Bottom ash + Blast-furnace Slag at the ratios of 5%, 10%, 15%, and 20% along with a control specimen without additives. These prepared cement mortar specimens were then cured for 2, 7, 28, 90, 180, and 360 days either in potable water or 10% sodium sulfate (Na2SO4) solution. Cement paste specimens were subjected to the initial setting, final setting, and volumetric expansion tests in accordance with the TS EN 196-3 standard. Cured for 2, 7, 28, 90, 180, and 360 days, cement mortars were subjected to compressive strength tests as per the TS EN 196-1 standard while length change tests were conducted as per the ASTM C 1012 standard.It was found that the compressive strength of cement mortars blended with 5% Fly ash + Bottom ash + Blast-furnace Slag cured in sodium sulfate for 360 days was approximately 2% higher than that of the cement mortar without additives. The length change of specimens obtained from cured in sodium sulfate solution shows best results in higher additive ratio. These all length changes ratio are greater than 0.087% ratio which is maximum length change expansion in potable water. This study suggests that 15% and 20% additive ratios are effective in reducing unfavorable effects of sulfate.

Influence of calcium leaching on chloride diffusivity in cement-based materials

The coupling action of calcium leaching and chloride attack is an important durability problem for the cement-based materials served in long-termly in the environmental water like seawater or groundwater containing chloride. This paper investigates the influence of calcium leaching on the chloride diffusivity in cement-based materials by the corrosion experiment, in which the hardened cement paste (HCP) specimens are immersed into three corrosion solutions with the same chloride concentration, such as 1 M sodium chloride solution, 1 M ammonium chloride solution and the mixed solution of 1 M sodium chloride and 3 M ammonium nitrate. Some measurements, like the phenolphthalein, saturation-drying weighing, mercury intrusion porosimetry (MIP), Scanning Electron Microscopy (SEM/EDS), X-ray diffraction analysis (XRD) and rapid chloride test (RCT), are used to analyze the leaching behaviors and its influence on the chloride diffusivity of HCP, characterized by leaching depth, pore structure, morphology, calcium-silicon ratio, chloride-silicon ratio, phase composition and free chloride ion content. Results show that, in the calcium leaching process, the compactness, calcium hydroxide content and calcium-silicon ratio of C-S-H gel of HCP decreases with the leaching time. The calcium leaching causes the reduction of the physically chloride-bound ability and the chemically chloride-absorbed ability in HCP, and it results in high chloride diffusivity.

Developing green cement paste using binary and ternary cementitious blends of low pozzolanic sewage sludge ash and colloidal nanosilica (short-term properties)

This study investigates the effect of small dosages of nanosilica (0.5 and 1.5% of the total binder) on physical, mechanical, durability, and microstructural properties of green cement paste containing sewage sludge ash (SSA) with low pozzolanic activity. Experimental results revealed that hardened performances of cement pastes were reduced significantly by incorporating SSA particles at higher replacement ratios (> 10%), whereas mechanical and durability properties of cement paste specimens were improved by the addition of nanosilica both at early and moderate ages. According to the microstructural investigation, the activity of SSA particles in the presence of nanosilica can be improved.

A study of deterioration of cement paste due to acid attack using X-ray computed micro-tomography

Cementitious materials are highly vulnerable to rapid and serious deterioration in aggressive acidic environments. The aim of this paper is to understand the microstructure deterioration of cement paste due to acid attack using the non-destructive technique called X-ray computed micro-tomography (micro-CT). To investigate this deterioration, an accelerated static immersion based leaching test with periodic abrasive action was performed on cement paste specimens of size 10 × 10 × 60 mm, prepared using ordinary Portland cement with a water-to-cement ratio of 0·40. After 28 d of initial curing in saturated lime water, specimens were exposed to 1% sulfuric, 1% hydrochloric, 0·5 M acetic and 0·5 M citric acid solutions for a maximum period of 6 weeks. X-ray micro-CT was used for imaging the deteriorated specimens to investigate the changes in microstructure. Various parameters related to degradation were also deduced based on the image analysis performed on the CT images in order to evaluate the performance of the cement paste in various acid solutions. Parameters such as altered depth/area could be considered as primary indicators of attack, which can be best captured and analysed using X-ray CT.

An experimental investigation of hydration mechanism of cement with silicane

The article aimed mainly to research the influence of silicane on the hydration mechanism of cement paste containing different dosages silicane using Standard consistency water consumption, the setting time, fluidity, porosity and the degree of hydration test. Meanwhile, the Micro-structures mechanism of cement paste were researched by the Fourier transform-infrared spectroscopy (FT-IR), X-ray diffraction (XRD), Nitrogen-absorption and Scanning electron microscopy (SEM). Modified cement paste specimens were prepared by adding 1%, 3% and 5% silicane into control cement paste, respectively. the Standard consistency water consumption and the setting time were evaluated using Vicat apparatus. Non-evaporable water content was employed to assess the degree of hydration of the cement paste. The fluidity of cement paste specimens was characterized using flow table apparatus. Experimental results showed that for the silicane and mix designs used in the article, the incorporation of silicane generally increased the fluidity, porosity and setting time, however, decreased the degree of hydration and Standard consistency water consumption compared with the control cement paste. In addition, the analysis of XRD, FT-IR, Nitrogen-absorption test and SEM revealed the hydration mechanism and the connection between macroscopic and micro-mechanism of cement paste incorporating different dosages silicane.

Area of lineal-path function for describing the pore microstructures of cement paste and their relations to the mechanical properties simulated from μ-CT microstructures

The pore distribution of a cement paste strongly affects its mechanical behavior such as its stiffness and strength. Porosity is an influential parameter that can be used to identify the complex pore microstructures of cement paste, but it has limitations as a scalar parameter. In this study, the lineal-path function, a low-order probability function, is investigated as a supplement or an alternative parameter for describing the microstructural characteristics of cement paste microstructures. In particular, the area of the lineal-path function is used as a measure of the pore microstructural characteristics, which can be linked with its properties.A relatively new method for simulating crack propagation, the crack phase field model, is used to evaluate the stiffness and tensile strength of cement paste microstructures and the evaluated properties are linked to the proposed characterization parameters. The evaluation is performed on virtual specimens obtained from micro-level computerized tomography (μ-CT) images of real cement paste specimens. The validity of the microstructure-property relations obtained from the proposed characterization parameters and the crack phase field model are confirmed through the statistical analysis of dozens of specimens.It is concluded that the correlation between the area of the lineal-path function and the mechanical properties is very strong. The parameter could potentially be used as a supplementary or an alternative parameter to describe the pore microstructures of cement paste.

Use of tomography to understand the influence of preconditioning on carbonation tests in cement-based materials

Recognition of the rising amount of atmospheric CO2 has brought renewed interest in understanding the effects of carbonation on reinforced concrete performance. In laboratory testing, the specimens must be preconditioned to effectively study carbonation. This paper studied the influence of several preconditioning schemes on the carbonation profiles of cement paste specimens subjected to accelerated carbonation tests. The evolution of microstructure and moisture during carbonation were investigated accordingly. Bulk of the work was based on an extended X-ray attenuation method (XRAM), which relied on X-ray computed tomography (CT). A novel method was introduced to evaluate the extent of damage due to drying. Based on extent of damage, the paper recommends standard-cured specimens for carbonation tests as compared to water-cured specimens. Also, when comparing between oven drying and mass balancing, the latter was shown to be more suitable, as the inner moisture distribution becomes more uniform after this drying protocol, and less fluctuation of humidity will occur during carbonation.