Efecto de la adición de nanofibras de carbono en las propiedades mecánicas y de durabilidad de materiales cementantes

En este artículo se han estudiado los cambios en las propiedades mecánicas de los morteros de cemento Portland debido a la adición de nanofibras de carbono (NFC). Se han determinado las resistencias a flexotracción y a compresión de los morteros en relación a la cantidad de NFC añadidas a la mezcla, al tiempo de curado y a la porosidad y densidad de los mismos. Además se han investigado los niveles de corrosión de barras de acero embebidas en pastas de cemento con NFC expuestos al ataque por carbonatación y por ingreso de cloruros. El aumento en el porcentaje de NFC añadido se traduce en un aumento la intensidad de corrosión registrada y una mejora de las propiedades mecánicas.

Fire behaviour of tabique walls

This paper present a study on the behaviour of tabique walls, concerning its fire resistance. This work is based on the experimental analysis of real scale tabique panels. Such walls were made in pine wood with an earth-based mortar finishing. In order to assess the earth-based mortar thickness effect on the fire resistance of the wall, three specimens were tested with three different mortar thicknesses of 15 mm, 10 mm and 5 mm. The earth-based mortar was previously analysed in the laboratory. The wooden structures were constructed based on traditional tabique technique. The experimental models were tested in a fire-resistance furnace, according to the ISO 834 standard fire. Temperatures were recorded using two data acquisition systems (spot measuring and field measuring). Fire resistance of test elements is expressed as the time during which the appropriate criteria have been satisfied so that one can predict the time before collapse, increasing both people and property safety. The obtained results are of great importance as they allow to improve the knowledge on tabique walls behaviour subjected to fire conditions. Two performance criteria were verified: the integrity criteria and the insulation criteria.

Estimation of water retention curve from mercury intrusion porosimetry and van Genuchten model

The water retention curve (WRC) is a hydraulic characteristic of concrete required for advanced modeling of water (and thus solute) transport in variably saturated, heterogeneous concrete. Unfortunately, determination by a direct experimental method (for example, measuring equilibrium moisture levels of large samples stored in constant humidity cells) is a lengthy process, taking over 2 years for large samples. A surrogate approach is presented in which the WRC is conveniently estimated from mercury intrusion porosimetry (MIP) and validated by water sorption isotherms: The well-known Barrett, Joyner and Halenda (BJH) method of estimating the pore size distribution (PSD) from the water sorption isotherm is shown to complement the PSD derived from conventional MIP. This provides a basis for predicting the complete WRC from MIP data alone. The van Genuchten equation is used to model the combined water sorption and MIP results. It is a convenient tool for describing water retention characteristics over the full moisture content range. The van Genuchten parameter estimation based solely on MIP is shown to give a satisfactory approximation to the WRC, with a simple restriction on one. of the parameters.

Ionic and molecular diffusion in cementitious materials

The work described in this thesis is an attempt to provide improved understanding of the effects of several factors affecting diffusion in hydrated cement pastes and to aid the prediction of ionic diffusion processes in cement-based materials. Effect of pore structure on diffusion was examined by means of comparative diffusion studies of quaternary ammonium ions with different ionic radii. Diffusivities of these ions in hydrated pastes of ordinary portland cement with or without addition of fly ash were determined by a quasi-steady state technique. The restriction of the pore geometry on diffusion was evaluated from the change of diffusivity in response to the change of ionic radius. The pastes were prepared at three water-cement ratios, 0.35, 0.50 and 0.65. Attempts were made to study the effect of surface charge or the electrochemical double layer at the pore/solution interface on ionic diffusion. An approach was to evaluate the zeta potentials of hydrated cement pastes through streaming potential measurements. Another approach was the comparative studies of the diffusion kinetics of chloride and dissolved oxygen in hydrated pastes of ordinary portland cement with addition of 0 and 20% fly ash. An electrochemical technique for the determination of oxygen diffusivity was also developed. Non-steady state diffusion of sodium potassium, chloride and hydroxyl ions in hydrated ordinary portland cement paste of water-cement ratio 0.5 was studied with the aid of computer-modelling. The kinetics of both diffusion and ionic binding were considered for the characterization of the concentration profiles by Fick's first and second laws. The effect of the electrostatic interactions between ions on the overall diffusion rates was also considered. A general model concerning the prediction of ionic diffusion processes in cement-based materials has been proposed.

Properties and performance of polymer modified cements and mortars

Polymer modified cements and mortars have become popular for use as patch repair materials. General evidence suggests that these materials offer considerable improvements compared to traditional mortars although the mechanisms for this are not fully understood. This work elucidates the factors which govern some properties and performance of different polymer systems. In view of the wide range of commercial systems available, investigations concentrated on the use of three of the most commonly available groups of polymers. These were: (1) Styrene Butadiene Rubber (SBR), (2) Acrylics and, (3) Ethylene Vinyl Acetates (EVA). The later two were in the form of both emulsions and redispersible powders. Experiments concentrated on: (1) Rheological behaviour of polymer modified cement pastes; (2) Workability of polymer modified mortars; (3) Influence of curing conditions on the pore size distribution and diffusion of chloride ions; (4) Bond strength of polymer modified cement and mortar patches; and (5) Microscopic examination and semi-quantitative analyses of the bulk and interfacial microstructures. The following main conclusions were reached: (1) The addition of polymer emulsions have a considerable influence on the workability of fresh cement pastes, the extent of this depending on the type of system used. (2) The rheological parameters of fresh polymer modified mortars can be established using a two-point workability test which may be used when comparing the properties of different systems at constant workability. (3) Curing conditions affect the properties of polymer modified systems and a wet/dry curing regime was essential for good adhesion of these materials to mortar substrates. (4) In contrast, the wet/dry curing regime resulted in a curing affected zone at the surface of patch materials. This can result in a much coarser pore structure and enhanced diffusion of e.g. chloride ions. (5) The microstructure of polymer modified systems was very different compared with the unmodified cement/mortar and varied depending on curing conditions.

The nitrate to ammonia and ceramic (NAC) process for the denitration and immobilization of low-level radioactive liquid waste (LLW)

Hazardous radioactive liquid waste is the legacy of more than 50 years of plutonium production associated with the United States' nuclear weapons program. It is estimated that more than 245,000 tons of nitrate wastes are stored at facilities such as the single-shell tanks (SST) at the Hanford Site in the state of Washington, and the Melton Valley storage tanks at Oak Ridge National Laboratory (ORNL) in Tennessee. In order to develop an innovative, new technology for the destruction and immobilization of nitrate-based radioactive liquid waste, the United State Department of Energy (DOE) initiated the research project which resulted in the technology known as the Nitrate to Ammonia and Ceramic (NAC) process. However, inasmuch as the nitrate anion is highly mobile and difficult to immobilize, especially in relatively porous cement-based grout which has been used to date as a method for the immobilization of liquid waste, it presents a major obstacle to environmental clean-up initiatives. Thus, in an effort to contribute to the existing body of knowledge and enhance the efficacy of the NAC process, this research involved the experimental measurement of the rheological and heat transfer behaviors of the NAC product slurry and the determination of the optimal operating parameters for the continuous NAC chemical reaction process. Test results indicate that the NAC product slurry exhibits a typical non-Newtonian flow behavior. Correlation equations for the slurry's rheological properties and heat transfer rate in a pipe flow have been developed; these should prove valuable in the design of a full-scale NAC processing plant. The 20-percent slurry exhibited a typical dilatant (shear thickening) behavior and was in the turbulent flow regime due to its lower viscosity. The 40-percent slurry exhibited a typical pseudoplastic (shear thinning) behavior and remained in the laminar flow regime throughout its experimental range. The reactions were found to be more efficient in the lower temperature range investigated. With respect to leachability, the experimental final NAC ceramic waste form is comparable to the final product of vitrification, the technology chosen by DOE to treat these wastes. As the NAC process has the potential of reducing the volume of nitrate-based radioactive liquid waste by as much as 70 percent, it not only promises to enhance environmental remediation efforts but also effect substantial cost savings. ^

Desenvolvimento de cerâmica vermelha utilizando rejeitos da construção civil

The Industry of the Civil Construction has been one of the sectors that most contribute to the pollution of the environment, due to the great amount of residues generated by the construction, demolition and the extraction of raw material. As a way of minimizing the environmental impacts generated by this industry, some governmental organizations have elaborated laws and measures about the disposal of residues from the building construction (CONAMA - resolution 307). This work has as objective the reutilization of residues compound of sand, concrete, cement, red bricks and blocks of cement and mortar for the production of red ceramic, with the objective of minimizing costs and environmental impacts. The investigated samples contained 0% to 50% of residues in weight, and they were sintered at temperatures of 950°C, 1000°C, 1050°C, 1100°C and 1150°C. After the sinterization, the samples were submitted to tests of absorption of water, linear retraction, resistance to bending, apparent porosity, specific density, XRD and SEM. Satisfactory results were obtained in all studied compositions, with the possible incorporation of up to 50% of residues in ceramic mass without great losses in the mechanical strength, giving better results to the incorporation of 30% of residues in the fabrication of ceramic parts, such as roofing tiles, bricks masonry and pierced bricks

Estudo de revestimentos inorgânicos com inibidor de corrosão aplicado na superfície do concreto

The corrosive phenomenon on reinforced concrete structures is one of the most founded pathologies on the coastal area. With the objective to prevent the process development, or even, retard its beginning, it was studied the application of inorganic covering over concrete surfaces, after its cure, as well as, evaluate the efficiency of the covering applied on the concrete in reducing its porosity of concrete preventing the entrance of aggressive agents to preserve the integrity of the existing armor inside it, comparing the result obtained with the body-of-proof reference, that didn´t receive covering protection. On the concrete production it was used Portland Cement CP II 32, coarse aggregate, fine aggregate and water from the local distributive. Two types of covering were used, one resin based of silicon and solvent and other white cement based, selected sands and acrylic resin. The concrete mixture adopted was 1:1,5:2,5 (cement, fine aggregate, coarse aggregate) and 0.50 water/cement ratio. With the concrete on fresh state was made the experiment test to determinate the workability. On the hardened state was made the concrete resistance experiment, absorption of water and electrochemical experiments, through polarization curves. Also was held optical microscopy and Scanning Electron Microscopy experiments to analyze the layer of the covering applied to the concrete surface and the interface between the concrete and the layer. The obtained results shows that the covering applied to the concrete surface didn´t affect the resistance towards compression. On the absorption of water occurred a diminution of the percentage absorbed, improving the concrete development by making it more impermeable towards the entrance of aggressive agents. The electrochemical experiment results confirmed the water absorption results; the body-of-proof covered presented larger protection towards the development of corrosives process and retarded the evolution of the corrosive phenomenon

In-situ Molibdenum X-ray powder diffraction study of the early hydration of cementitious systems on a humidity chamber

The durability of cement-based construction materials depends on the environmental conditions during their service life. A further factor is the microstructure of the cement bulk, established by formation of cement hydrates. The development of the phases and microstructure under given conditions is responsible of the high strength of cementitious materials. The investigation on the early hydration behavior of cements and cementing systems has been for a long time a very important area of research: understanding the chemical reactions that lead to hardening is fundamental for the prediction of performances and durability of the materials. The production of 1 ton of Ordinary Portland Cement, OPC, releases into the atmosphere ~0.97 tons of CO2. This implies that the overall CO2 emissions from the cement industry are 6% of all anthropogenic carbon dioxide. An alternative to reduce the CO2 footprint consists on the development of eco-cements composed by less calcite demanding phases, such as belite and ye'elimite. That is the case of Belite-Ye’elimite cements (BY). Since the reactivity of belite is not quick enough, these materials develop low mechanical strengths at intermediate hydration ages. A possible solution to this problem goes through the production of cements which jointly contain alite with the two previously mentioned phases, named as Belite-Alite-Ye’elimite (BAY) cements. The reaction of alite and ye'elimite with water will develop cements with high mechanical strengths at early ages, while belite will contribute to later values. The final goal is to understand the hydration mechanisms of a variety of cementing systems (OPC, BAY and pure phases) as a function of water content, superplasticizer additives and type and content of sulfate source. In order to do so, in-situ laboratory humidity chambers with Molybdenum X-ray Powder diffraction are employed. In the first 2h of hydration, reaction degree (α) of ye'elimite had been decreased for superplasticizer.

Desenvolvimento de um sistema misto de pasta à base de geopolímero e cimento portland para correção de perda de circulação

The materials engineering includes processes and products involving several areas of engineering, allowing them to prepare materials that fulfill the needs of various new products. In this case, this work aims to study a system composed of cement paste and geopolymers, which can contribute to solving an engineering problem that directly involves the exploitation of oil wells subject to loss of circulation. To correct it, has been already proposed the use of granular materials, fibers, reducing the drilling fluid or cement paste density and even surface and downhole mixed systems. In this work, we proposed the development of a slurry mixed system, the first was a cement-based slurry and the second a geopolymer-based slurry. The cement-based slurry was formulated with low density and extenders, 12.0 ppg (1.438 g/cm ³), showing great thixotropic characteristics. It was added nano silica at concentrations of 0.5, 1.0 and 1.5 gps (66.88, 133.76 and 200.64 L/m3) and CaCl2 at concentrations of 0.5, 1, 0 and 1.5%. The second system is a geopolymer-based paste formulated from molar ratios of 3.5 (nSiO2/nAl2O3), 0.27 (nK2O/nSiO2), 1.07 (nK2O/nAl2O3) and 13.99 (nH2O/nK2O). Finally, we performed a mixture of these two systems, for their application for correction of circulation lost. To characterize the raw materials, XRD, XRF, FTIR analysis and titration were performed. The both systems were characterized in tests based on API RP10B. Compressive strength tests were conducted after curing for 24 hours, 7 and 28 days at 58 °C on the cement-based system and the geopolymer-based system. From the mixtures have been performed mixability tests and micro structural characterizations (XRD, SEM and TG). The results showed that the nano silica, when combined with CaCl2 modified the rheological properties of the cement slurry and from the concentration of 1.5 gpc (200.64 L / m³) it was possible to obtain stable systems. The system mixture caused a change in the microstructure of the material by favoring the rate of geopolymer formation to hinder the C3S phase hydration, thus, the production of CSH phases and Portlandite were harmed. Through the mixability tests it can be concluded that the system, due to reduced setting time of the mixture, can be applied to plug lost circulation zones when mixed downhole

An experimental study on cracking evolution in concrete and cement mortar by the b-value analysis of acoustic emission technique

Notched three point bend (TPB) specimens made with plain concrete and cement mortar were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/s and simultaneously acoustic emissions (AE) released were recorded during the experiments. Amplitude distribution analysis of AE released during concrete was carried out to study the development of fracture process in concrete and mortar specimens. The slope of the log-linear frequency-amplitude distribution of AE is known as the AE based b-value. The AE based b-value was computed in terms of physical process of time varying applied load using cumulative frequency distribution (Gutenberg-Richter relationship) and discrete frequency distribution (Aki's method) of AE released during concrete fracture. AE characteristics of plain concrete and cement mortar were studied and discussed and it was observed that the AE based b-value analysis serves as a tool to identify the damage in concrete structural members. (C) 2012 Elsevier Ltd. All rights reserved.

Microcracking and fracture process in cement mortar and concrete: a comparative study using acoustic emission technique

This article reports the acoustic emission (AE) study of precursory micro-cracking activity and fracture behaviour of quasi-brittle materials such as concrete and cement mortar. In the present study, notched three-point bend specimens (TPB) were tested under crack mouth opening displacement (CMOD) control at a rate of 0.0004 mm/sec and the accompanying AE were recorded using a 8 channel AE monitoring system. The various AE statistical parameters including AE event rate , AE energy release rate , amplitude distribution for computing the AE based b-value, cumulative energy (I E) pound and ring down count (RDC) were used for the analysis. The results show that the micro-cracks initiated and grew at an early stage in mortar in the pre peak regime. While in the case of concrete, the micro-crack growth occurred during the peak load regime. However, both concrete and mortar showed three distinct stages of micro-cracking activity, namely initiation, stable growth and nucleation prior to the final failure. The AE statistical behavior of each individual stage is dependent on the number and size distribution of micro-cracks. The results obtained in the laboratory are useful to understand the various stages of micro-cracking activity during the fracture process in quasi-brittle materials such as concrete & mortar and extend them for field applications.

Age and curing effect on the bond strength of thin bed concrete masonry with polymer cement mortar

Bond characteristics of masonry are partly affected by the type of mortar used, the techniques of dispersion of mortar and the surface texture of the concrete blocks. Additionally it is understood from the studies on conventional masonry, the bond characteristics are influenced by masonry age and curing methods as well as dryness/dampness at the time of testing. However, all these effects on bond for thin bed masonry containing polymer cement mortar are not well researched. Therefore, the effect of ageing and curing method on bond strength of masonry made with polymer cement mortar was experimentally investigated as part of an ongoing bond strength research program on thin bed concrete masonry at Queensland University of technology. This paper presents the experimental investigation of the flexural and shears bond characteristics of thin bed concrete masonry of varying age/ curing methods. Since, the polymer cement mortar is commonly used in thin bed masonry; bond development through two different curing conditions (dry/wet) was investigated in this research work. The results exhibit that the bond strength increases with the age under the wet and dry curing conditions; dry curing produce stronger bond and is considered as an advantage towards making this form of thin bed masonry better sustainable.

Influence of Joint Thickness and Mortar-Block Elastic Properties on the Strength and Stresses Developed in Soil-Cement Block Masonry

Soil-cement blocks are employed for load bearing masonry buildings. This paper deals with the study on the influence of bed joint thickness and elastic properties of the soil-cement blocks, and the mortar on the strength and behavior of soil-cement block masonry prisms. Influence of joint thickness on compressive strength has been examined through an experimental program. The nature of stresses developed and their distribution, in the block and the mortar of the soil-cement block masonry prism under compression, has been analyzed by an elastic analysis using FEM. Influence of various parameters like joint thickness, ratio of block to mortar modulus, and Poisson's ratio of the block and the mortar are considered in FEM analysis. Some of the major conclusions of the study are: (1) masonry compressive strength is sensitive to the ratio of modulus of block to that of the mortar (Eb/Em) and masonry compressive strength decreases as the mortar joint thickness is increased for the case where the ratio of block to mortar modulus is more than 1; (2) the lateral tensile stresses developed in the masonry unit are sensitive to the Eb/Em ratio and the Poisson's ratio of mortar and the masonry unit; and (3) lateral stresses developed in the masonry unit are more sensitive to the Poisson's ratio of the mortar than the Poisson's ratio of the masonry unit.

Equivalent Model Of Expansion Of Cement Mortar Under Sulphate Erosion

The expansion property of cement mortar under the attack of sulfate ions is studied by experimental and theoretical methods. First, cement mortars are fabricated with the ratio of water to cement of 0.4, 0.6, and 0.8. Secondly, the expansion of specimen immerged in sulphate solution is measured at different times. Thirdly, a theoretical model of expansion of cement mortar under sulphate erosion is suggested by virtue of represent volume element method. In this model, the damage evolution due to the interaction between delayed ettringite and cement mortar is taken into account. Finally, the numerical calculation is performed. The numerical and experimental results indicate that the model perfectly describes the expansion of the cement mortar.