Carbonation Shrinkage Research Articles

SMALL-GRAIN CONCRETE BASED ON METALLURGY WASTE

The paper presents the approach to the metallurgical waste recycling introducing them into the composition of fine-grained concretes. Blast-furnace produced by OOO „EVRAZ ZSMK‟ is used as a recyclable waste. According to the X-ray diffraction patterns, the blast-furnace is mainly represented by such components as iron and calcium hydrated silicates, quartz, calcite and carbon. It is found that during the hydrothermal treatment of fine-grained concrete, carbonate-containing additives facilitate the formation of hydrated calcium silicates containing carbonate groups the composition of which is close to the that of hydrosilicate phases. The formation of these products in the concrete production contributes to the strength properties improvement in the material obtained and reduces the possibility of late formation of similar phases during its service. This minimizes the carbon dioxide corrosion and shrinkage deformation accompanying this process. It is shown that the use of blast-furnace as fine aggregate in the concrete mix increases the material density and strength by 14 % and 11.2 %, respectively. The water absorption decreases by 5 %, which positively affects the frost resistance of finegrained concretes. The obtained fine-grained concrete provide can be used to produce the M250 concrete grade.

Mitigation of Drying and Carbonation Shrinkage of Cement Paste using Magnesia

Mitigation of Drying and Carbonation Shrinkage of Cement Paste using Magnesia

Influence of chemical-mineralogical properties of limestone on the shrinkage behaviour of cement paste and concrete made of limestone-rich cements

Shrinkage in concrete structures may lead to cracks which reduce the durability of concrete against aggressive mediums. In prestressed concrete elements shrinkage results in a significant loss of prestressing forces. Limestone-rich cements with limestone contents beyond the values of DIN EN 197-1 were developed in order to reduce the environmental impact of concrete. In this paper, the results of studies on the shrinkage behaviour of cement paste and concrete made of limestone-rich cements up to 70wt.-% are presented. Results indicated that shrinkage of cement paste and concrete with high limestone contents is strongly dependent on the amount and the chemical-mineralogical properties of limestone. In this context, methylene blue value (MB-value) and alkali oxide content of limestone were identified as the key parameters controlling the drying shrinkage of cement pastes. Depending on the type of limestone, concrete samples made of limestone-rich cements had either higher or lower drying shrinkage than reference samples made of CEM I 52.5 R with the same w/c-ratio. However, lower drying shrinkage deformation was observed in comparison to the reference concrete made of CEM I with a comparable compressive strength. Furthermore, it was ascertained that the contribution of the autogenous shrinkage and carbonation shrinkage to the total shrinkage deformation is almost negligible for cement paste and concrete made of limestone-rich cements.

Effect of accelerated carbonation conditions on the characterization of load-induced damage in reinforced concrete members

Reinforced concrete is widely used in the construction of buildings, historical monuments and also nuclear power plants. For various reasons, many concrete structures are subject to unavoidable cracks that accelerate the diffusion of atmospheric carbon dioxide to the steel/concrete interface. Carbonation at the interface induces steel corrosion that may cause the development of new cracks in the structure, and this is a determining factor for its durability. It is therefore important to accurately characterize the length of the load-induced damage along the steel/concrete interface in order to understand the effect of cracking on corrosion initiation and propagation. The aim of this paper is to present an experimental procedure that allows the load-induced damage length to be assessed. The procedure consists in subjecting specimens to accelerated carbonation and determining the length of the carbonated steel/mortar interface, which is assumed to be equal to the length of the damaged steel/mortar interface. Suitable conditions should therefore be found for the accelerated carbonation in order to obtain an accurate characterization of the damaged steel/mortar interface length. To this end, two carbonation concentrations (3, 50%) and several carbonation durations were tested. The results indicate that a strong carbonation shrinkage phenomenon develops at high carbon dioxide concentration and leads to new cracking along the steel/mortar interface. These cracks allow the carbon dioxide to spread along the interface over a length greater than the damaged length. This is not the case when the accelerated carbonation test is performed at lower carbon dioxide concentration. Consequently, accelerated carbonation at high carbon dioxide concentration (50%) cannot be used neither for the estimation of the length of the mechanically damaged steel/mortar interface nor for the carbonation-induced corrosion studies because it will lead to an overestimation of the size of the corroded area.

Shrinkage and creep behavior of an alkali‐activated slag concrete

In this paper, experimental results about the shrinkage and creep behavior of an alkali‐activated slag concrete (AAS‐concrete) are presented. The autogenous shrinkage of AAS‐concrete is pronounced at a relatively high water to binder ratio of 0.41. The ultimate value of autogenous shrinkage is determined to be higher than that of high‐strength concrete (HSC). The observed self‐desiccation at a later concrete age can be one of the reasons for the autogenous shrinkage. In a drying environment of 65% relative humidity AAS‐concrete exhibits a significantly higher total shrinkage compared to normal‐strength concrete composed of Portland cement.In the contrary to normal and HSC carbonation shrinkage must not be neglected for AAS‐concrete in the early concrete age. The sum of drying and carbonation shrinkage develops much more rapidly for AAS‐concrete in the first weeks after the beginning of drying. Based on the experimental results the shrinkage behavior of AAS‐concrete cannot be predicted by conventional models. The creep behavior of the AAS‐concrete was investigated under sealed and unsealed conditions. The creep coefficients for both conditions are higher than those calculated when using conventional models.

Drying and carbonation shrinkage of cement paste containing alkalis

In this work, shrinkage performance of ordinary portland cement (OPC) paste containing various alkali salts was characterized at two drying conditions, namely: nitrogen gas and air. The results show that incorporation of alkalis dramatically increases shrinkage magnitude, but reduces shrinkage kinetics of OPC, regardless of source and type of alkalis (e.g. Na+ or K+). The amount of alkalis bound in the solid hydrated phases, rather than the free alkalis remaining in the pore solution, is crucial in controlling the shrinkage performance of OPC. It is suggested that the alkali enrichment in OPC increases the visco-elastic/visco-plastic compliance (reduce creep modulus) of its solid skeleton under drying-induced internal stresses. This phenomenon is likely to be attributed to the alkalis binding in calcium–silicate–hydrate (C–S–H), which promotes the packing of C–S–H nanoparticles. Carbonation results in shrinkage (i.e. carbonation shrinkage) in plain OPC, but expansion in OPC with alkali enrichment. The overall volume change of OPC due to carbonation may be a result of competition between dissolution-induced shrinkage and crystallization-induced expansion.

ASSESSMENT OF CRACKING RESISTANCE OF CELLULAR CONCRETE PRODUCTS UNDER MOISTURE AND CARBONISATION DEFORMATIONS WITH STRESS RELAXATION

Objectives. On the basis of the experimental, theoretical and field studies, an engineering calculation method was developed for assessing the cracking resistance of external enclosing constructions made of cellular concrete, with the maximum gradient development of moisture and carbonisation forced deformations along their thickness, taking into account the relaxation of the shrinkage stresses. In this regard, the aim of the work is to provide technological measures at the manufacturing stage in order to increase the operational cracking resistance of the construction's outer surface layers by reducing the moisture and carbonation shrinkage of cellular concrete by introducing a large or fine porous aggregate in calculated amounts. Methods. A number of analytical equations were applied to establish the dependence of the shrinkage of heavy concrete of conventional hardness on the amount of aggregate introduced and its elasticity modulus, water-cement ratio and cement consumption, as well as the concrete's moisture content. Results. Knowing the volumes of the structural aggregate and the cellular concrete mass, as well as their modulus of elasticity, the shrinkage reduction factor of the cellular concrete was calculated with the addition of a lightweight porous aggregate. Subsequently, the shrinkage deformations of concrete in the surface layer of the outer enclosing construction, maximising crack resistance due to moisture exchange and carbonation influences under operating conditions, were defined, taking into account the relaxation of tensile stresses due to creep of concrete. Conclusion. Theoretical calculations, based on the recommended method of assessing the cracking resistance of cellular concrete enclosing constructions under moisture exchange and carbonisation processes, taking into account the relaxation of shrinkage stresses, showed that in order to exclude the appearance of cracks in wall panels 280 mm thick made of 700 kg/m3 gas ash concrete with elasticity modulus of 2500 MPa, it is necessary to have 70-80% of keramzite or granulated slag, and 50-60% of stone crumb (granite or marble crushed stone) of the volume of cellular concrete in the surface layer of 30-50 mm.

Carbon composite foams with improved strength and electromagnetic absorption from sucrose and multi-walled carbon nanotube

Multi-walled carbon nanotube (MWCNT) reinforced carbon composite foams with interconnected cellular structure were prepared by thermo-foaming of MWCNT dispersions in molten sucrose followed by carbonization. The foaming and foam setting time decreases with an increase in the MWCNT concentration. The carbonization shrinkage, foam density and compressive strength showed a decreasing trend after reaching a maximum at a MWCNT concentration of 0.5 wt% indicating MWCNT agglomeration beyond 0.5 wt%. The maximum compressive strength (4.9 MPa) and specific compressive strength (21 MPa/g/cm3) obtained at 0.5 wt% MWCNT concentration corresponded to an increase of 189 and 133%, respectively. The cell walls and struts of the carbon composite foams contained micropores produced by an in situ activation and large mesopores produced by local shrinkage of sucrose polymer within the MWCNT agglomerates. The carbon composite foam showed the highest EMI shielding effectiveness and specific shielding effectiveness of 39 dB and 166 dB/g/cm3, respectively, at a MWCNT concentration of 2.5 wt%. The dielectric loss due to the interaction of electromagnetic waves with walls of the mesopores created by the local shrinkage of sucrose polymer within the MWCNT agglomerates contributed to the electromagnetic absorption.

Concrete Carbonation Monitoring by Nonlinear Ultrasonic Spectroscopy

The aim of this paper is to evaluate the possibility of using the nonlinear ultrasonic spectroscopy with a single exciting harmonic frequency for concrete carbonation monitoring. Carbonation of concrete is related with the corrosion of steel reinforcement and with carbonation shrinkage. Due to the conditions in the laboratory were as close as possible to practical terms, we used reinforced concrete samples. For the research, the concrete beams with one standard reinforcing bar passing through the centre of the sample were made. Prepared samples were exposed to an atmosphere with increased carbon dioxide content. The measurements using the nonlinear ultrasonic spectroscopy with a single exciting harmonic frequency were realized before and after carbonation of concrete.

Impact of carbonation on unsaturated water transport properties of cement-based materials

In unsaturated conditions, the durability of concrete structures is strongly dependent on the evolution of the amount of free water within concrete porosity. Reliable durability assessment of concrete structures in relation to their environment thus requires accurate unsaturated water transport description as well as reliable input data. The effect of carbonation on water transport remains poorly studied and data are lacking. It was then the purpose of this article to acquire all the data needed to describe unsaturated water transport in carbonated cementitious materials (porosity, water retention and unsaturated permeability). Four hardened pastes made with four different binders were carbonated at 3% CO2 to ensure representativeness with natural carbonation. Beyond the modification of the water retention curve and porosity clogging, significant microcracking due to carbonation shrinkage was observed. The consequence on permeability highlighted a competition between porosity clogging and microcracking that was dependent on the initial mineralogical composition.

Monitoring the Evolution of Accelerated Carbonation of Hardened Cement Pastes by X-Ray Computed Tomography

The evolution of carbonation front progress, crack spatial distribution, and pore structure parameters of hardened cement paste have been monitored in situ by three-dimensional (3D) X-ray computed tomography (XCT) at different accelerated carbonation ages of 0, 3, 7, 14, and 28 days, respectively. The gray-value histograms of cement paste are a function of carbonation time, and the carbonation degrees at different times can be estimated by the volume fractions of uncarbonated and carbonated parts. Many cracks generate along the carbonation direction because of carbonation shrinkage. With the extension of the carbonation time, the volume and surface of cracks become larger. The porosity markedly decreases when pore volume is between 0.01 and 0.1 mm 3 as the carbonation time increases. DOI: 10.1061/(ASCE)MT .1943-5533.0000610. © 2013 American Society of Civil Engineers. CE Database subject headings: Cement; Carbonation; Cracking; Porous media; Radiography; X rays. Author keywords: X-ray computed tomography (XCT); Hardened cement paste; Accelerated carbonation; Gray values; Crack distribution; Pore structure.

Preparation of carbon foams by thermo-foaming of activated carbon powder dispersions in an aqueous sucrose resin

Thermo-foaming of activated carbon (AC) powder dispersions in an aqueous sucrose resin followed by carbonization has been studied to prepare carbon foams. The dispersions were characterized by viscosity measurements and sedimentation studies. The OH to OH condensation reactions, leading to the cross-linking of the sucrose polymer, were retarded by the AC powder. The AC particles adsorbed on the gas–liquid interface stabilized the gas bubbles that resulted in foaming of the poorly cross-linked sucrose polymer resin having low viscosity. The carbon produced by the carbonization of the sucrose polymer binds the AC particles as in reaction bonding. The carbon foams have an interconnected cellular structure. Density (0.138–0.22g/cc), cell size (0.62–3mm) and compressive strength (0.42–3.4MPa) of the carbon foams depends on the AC powder to sucrose weight ratio. Incorporation of the AC powder in the sucrose resin decreases the carbonization shrinkage that enables the preparation of large carbon foam bodies without warping. The carbon foam prepared at an AC powder to sucrose weight ratio of 0.1 shows the highest density and compressive strength and the lowest cell size.

Application of X-ray computed tomography in characterization microstructure changes of cement pastes in carbonation process

The microstructure characteristics and meso-defect volume changes of hardened cement paste before and after carbonation were investigated by three-dimensional (3D) X-ray computed tomography (XCT), where three types water-to-cement ratio of 0.53, 0.35 and 0.23 were considered. The high-resolution 3D images of microstructure and filtered defects were reconstructed by an XCT VG Studio MAX 2.0 software. The mesodefect volume fractions and size distribution were analyzed based on 3D images through add-on modules of 3D defect analysis. The 3D meso-defects volume fractions before carbonation were 0.79%, 0.38% and 0.05% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The 3D meso-defects volume fractions after carbonation were 2.44%, 0.91% and 0.14% corresponding to w/c ratio=0.53, 0.35 and 0.23, respectively. The experimental results suggest that 3D meso-defects volume fractions after carbonation for above three w/c ratio increased significantly. At the same time, meso-cracks distribution of the carbonation shrinkage and gray values changes of the different w/c ratio and carbonation reactions were also investigated.

Chemo-Micro-Dilatation Modeling of Carbonation Shrinkage Phenomenon of the Porous Mortars

In this paper, we investigate the CO2 carbonation on porous materials such as the cement paste mortar. Our scientific goal is to model the CO2 carbonation effect on the porous cement mortar by the micro-dilatation theory and to determine the chemo-mechanical stress development due to the chemical reactions. In fact, this phenomenon will modify not only the pH value of the cement paste but also the micro-structure due to collapsing pore-network. Based upon the micro-dilatation theory originally proposed by Cowin and Nunziato, four unknown parameters related to the micro-structure will be determined for the cement mortar using the semi-empirical method and numerical modeling. The experimental carbonation tests on two different types of cement paste (CEMI and CEMIII) have been realized to capture carbonated affected zones and to measure their shrinkage deformations.

Preparation of porous SiC ceramics from waste cotton linter by reactive liquid Si infiltration technique

Porous carbon and porous silicon carbide with homogenous microstructure are prepared from cotton linter and phenolic resin through mold-stacking, solidification, carbonization and liquid Si infiltration process. SEM observation reveals that the irregular interconnected pores of carbon preparation were originated from the random arrangement of cotton linter fiber and different carbonization shrinkage. After the liquid Si infiltration process, the porous carbon was converted into dense SiC/Si composites. The porous SiC was formed during the Si-removing process at higher temperature. The result shows that open porosity of porous silicon carbide increases as Si-removing time increase, while the bending strength and the fracture toughness decrease.

Improved properties of carbon fiber paper as electrode for fuel cell by coating pyrocarbon via CVD method

The fabrication of a pyrocarbon coated carbon paper and its application to the gas diffusion lay (GDL) of proton exchange membrane (PEM) fuel cell were described. This carbon paper was fabricated by using conventional carbon paper as the precursor, and coating it with pyrocarbon by pyrolyzing propylene via the chemical vapor deposition (CVD) method. For comparison, conventional carbon paper composites were also prepared by using PAN-based carbon fiber felt as the precursor followed by impregnation with resin, molding and heat-treatment. SEM characterization indicates that pyrocarbon is uniformly deposited on the surface of the fiber in the pyrocarbon coated carbon paper and made the fibers of carbon felt bind more tightly. In contrast, there are cracks in matrix and debonding of fibers due to carbonization shrinkage in the conventional carbon paper. Property measurements show that the former has much better conductivity and gas permeability than the latter. In addition, current density-voltage performance tests also reveal that the pyrocarbon coating can improve the properties of carbon paper used for electrode materials of fuel cell.

The effect of carbon fabric treatment on delamination of 2D-C/C composites

The influence of carbon fabric treatment on delamination in two dimensional carbon/carbon composites (2D-C/C) was studied. The surface chemical elements and surface morphology of both original and treated carbon fibres were examined by X-ray photoelectron spectroscopy and scanning electron microscope. The properties including interlaminar shear stress, density and open porosity were tested, and delamination defects were characterized using metallographic microscope. It has found that the concentration of surface oxygen element decreases, while carbon element increases after high temperature treatment, which indicates the activities of carbon fiber reduced. The 2D-C/C composites with treated carbon fabric are non-delaminable, while the non-treated carbon fabric reinforced 2D-C/C composites are delaminable during carbonization, which are largely attributed to various interfacial strength in polymer matrix composites, and different degree of matrix carbonization shrinkage.

Decalcification shrinkage of cement paste

Decalcification of cement paste in concrete is associated with several modes of chemical degradation including leaching, carbonation and sulfate attack. The primary aim of the current study was to investigate the effects of decalcification under saturated conditions on the dimensional stability of cement paste. Thin (0.8 mm) specimens of tricalcium silicate (C 3S) paste, white portland cement (WPC) paste, and WPC paste blended with 30% silica fume (WPC/30% SF) were decalcified by leaching in concentrated solutions of ammonium nitrate, a method that efficiently removes calcium from the solid while largely preserving silicate and other ions. All pastes were found to shrink significantly and irreversibly as a result of decalcification, particularly when the Ca/Si ratio of the C–S–H gel was reduced below ∼ 1.2. Since this composition coincides with the onset of structural changes in C–S–H such as an increase in silicate polymerization and a local densification into sheet-like morphologies, it is proposed that the observed shrinkage, here called decalcification shrinkage, is due initially to these structural changes in C–S–H at Ca/Si ∼ 1.2 and eventually to the decomposition of C–S–H into silica gel. In agreement with this reasoning, the blended cement paste exhibited greater decalcification shrinkage than the pure cement pastes due to its lower initial Ca/Si ratio for C–S–H gel. The similarities in the mechanisms of decalcification shrinkage and carbonation shrinkage are also discussed.

Preparation and Characterization of Highly Ordered Graphitic Mesoporous Carbon as a Pt Catalyst Support for Direct Methanol Fuel Cells

Mesoporous carbon with graphitic pore walls is highly desired in many electrochemical applications such as fuel cells and lithium ion batteries. In this study, ordered graphitic mesoporous carbon was prepared by chemical vapor deposition (CVD) of benzene in the pores of mesoporous SBA-15 pure-silica template without loading any catalytic species. Nitrogen adsorption, small-angle X-ray scattering, X-ray diffraction, Raman spectrometry, field-emission scanning electron microscopy, transmission electron microscopy, and thermogravimetric analysis techniques were used to characterize the samples. It was observed that the CVD method affords highly ordered mesoporous carbon with graphitic pore walls and low carbon shrinkage because of the high degree of infiltration of pyrolytic carbon. The catalytic performance of the mesoporous carbon as a support for Pt catalyst in room-temperature methanol oxidation was examined. Results show that the specific activity of the Pt catalyst supported on the mesoporous carbon is...

Calcium silicate structure and carbonation shrinkage of a tobermorite-based material

Carbonated autoclaved aerated concretes (AACs) show no shrinkage at a degree of carbonation approximately less than 20%. The 29Si MAS NMR spectrum showed that at a degree of carbonation less than 25%, the typical double-chain silicate anion structure of tobermorite-11Å was well maintained and interlayer Ca ions were exchanged with protons. This corresponded to the absence of carbonation shrinkage at a degree of carbonation less than 20%. When the degree of carbonation increased from 25% to 50% up to 60%, the double-chain silicate anion structure of tobermorite-11Å was decomposed and Ca ions in the Ca–O layers were dissolved, showing a possible mechanism of carbonation shrinkage.