Advanced Cement Based Materials最新文献

The durability of type K and H cement based grouts under conditions potentially found to be in high level nuclear waste repositories was studied. Tests have been carried out to determine the effects of temperature on the hydraulic conductivity and the leaching resistance of the grouts. Measurements of mercury intrusion porosimetry and scanning electron microscopy with energy dispersive X-ray analysis have been used to investigate the changes in the pore structures of both grouts as function of leaching and permeating time. Type K and type H cement based grouts, made with low water to cementitious materials ratio, silica fume, and superplasticizer, were exposed to high temperature and pressure. Preliminary results indicate that both hydraulic properties and leaching behavior of the grouts investigated are affected by the increase in temperature. However, data show that leaching rate and hydraulic conductivity of the grouts decrease with time. The results showed clearly that chemical reactions, presumably accelerated by the elevated temperatures (100°C), led to the formation of a precipitate in the microcracks and on the surface of the leached specimens. This precipitate is likely the cause of the observed decrease in the hydraulic conductivity and leaching rate.

Deicing chemicals are mixed with corrosion inhibitors to reduce rebar corrosion in bridge decks. The corrosion inhibitors are thought to penetrate into concrete to the depth of the rebars and form the passive film on the rebar surface. In a previous study, it was found that corrosion-inhibitor-added deicing salts interacted with 3% NaCTadded concrete and produced precipitates through chemical reactions. The amounts of precipitates produced was dependent on the type and concentration of corrosion-inhibitor-added deicing salts. In this investigation, the precipitates formed by chemical reactions between concrete and corrosion-inhibitor-added deicing salts were identified by using chemical analysis and X-ray diffraction methods. The distributions of Cl⁻, SO₄²⁻, and PO₄³⁻ in concrete slabs ponded with corrosion-inhibitor-added deicing salts were determined by chemical analyses of powder samples obtained from the slabs. The major precipitates were calcium and/or magnesium phosphates as major chemical compounds and gypsum as a minor component. High concentrations of phosphate were observed at the top portion of concrete slabs when the deicing chemicals contained phosphate inhibitors. Voids were observed at the interface of aggregate and mortar in the concrete slabs tested with the deicing salts solutions containing corrosion inhibitors.

The ionic diffusivity of a concrete is a function of its microstructure at many length scales, ranging from nanometers to millimeters. The microstructure is largely controlled by the initial concrete mixture proportions and the ultimate curing conditions. Linking a property like ionic diffusivity to the microstructure then requires a multiscale approach. A multiscale microstructural computer model for ionic diffusivity has been previously developed. This model has been developed specifically to compute the chloride diffusivity of concretes with various mixture proportions and projected degrees of hydration. The three key parts of this model were dependent on large-scale supercomputer-magnitude simulations to: (1) determine the total volume of interfacial zones for a given aggregate distribution; (2) simulate the hydrated cement paste microstructure around a typical aggregate; and (3) compute the effect of the aggregates and interfacial zones on the overall diffusivity of the concrete. The key feature of this model is that one can approximately take into account the redistribution of cement paste between interfacial transition zone regions and bulk paste regions, and its important effect on overall concrete diffusivity. In the present article, we review the previously developed model and show how analytical equations can accurately replace the large scale computer simulations of parts (1) and (3). This accomplishment will make the model more usable by those who do not have access to supercomputer computing power.

A study to determine the role of Ca(OH)₂ in hydrated cement systems concerning the processes involved in the removal of water by solvent replacement methods was conducted. The length change characteristics of Ca(OH)₂ compacts containing varying amounts of water and immersed in large volumes of organic liquids (methanol, isopropanol, benzene, and acetone) were analyzed and compared with those of hydrated cement paste. Inferences regarding the relevance of the Bangham effect and possible chemical interaction with the solid as to the mechanisms responsible for length change were made. Dependencies of the latter on compaction pressure and solvent species are discussed. The implications of the results with respect to microstructural investigations of cement paste are suggested.

Extrusion is a plastic-forming method that is suitable not only for flat shapes, but also for structural shapes, such as I-sections, channels, pipes, and hollow and solid tubes. The advantage of introducing extrusion into cement product processing is that the materials are formed under high shear and compressive forces resulting in composites with improved performance. This article presents research results on extruded fiber reinforced cement pipe and flat sheet. A pipe die was designed and developed specifically for extrusion of cement based materials. Three different mix proportions were selected by varying the fiber type (polyvinyl alcohol and alkali-resistant glass), admixture type (silica fume, metakaolin, and latex), and their proportions to study their influence on the properties of extruded fiber reinforced cement pipes and thin sheets. Extruded cement pipes were tested for hydrostatic burst strength, bearing strength, and flexural performance. Thin sheets were tested for tensile and flexural performance. The test results indicated that polyvinyl alcohol fibers in the presence of silica fume and latex provide a suitable reinforcing effect, and silica sand provides good surface finish.

Small amounts of polyvinyl alcohol (PVA) were added to cement paste in an attempt to increase the aggregate-paste bond strength. Specimens consisted of ordinary Portland cement mixed with PVA/water solutions and cast against ground surfaces of limestone and granite. The aggregate-paste bond strength after curing was tested in a wet state by three-point bending. The morphology of the interfacial transition zone was observed with scanning electron microscopy, and the composition was analyzed with infrared spectroscopy. The addition of 1.4 wt% PVA based on the mass of cement increased the strength for both limestone-paste and granite-paste bonds. The strength increase was about five-fold for limestone and nearly two-fold for granite. The failure mode also changed, from pure adhesive failure without PVA to cohesive failure of the aggregate with limestone and to a mixed cohesive failure of the paste and adhesive failure with granite. The gain in bond strength with the addition of PVA seems to arise from suppression of the porous interfacial transition zone and an inhibition of calcium hydroxide nucleation on the aggregate surface.

This article outlines an experimental and numerical study on quasi-instantaneous and long-term deformations of high performance concrete subjected to sealed curing. For this purpose more than 100 cylinders and 400 cubes were made of eight concretes and studied in relation to creep and shrinkage, hydration, internal relative humidity, and compressive strength. One heat-cured concrete was studied at temperatures other than 20°C varying between −20°C and 60°C. Analyses were carried out of quasi-instantaneous deformation, short- and long-term basic creep, and autogenous shrinkage. Relationships were obtained between elastic modulus and creep compliance, and hydration, internal relative humidity, and compressive strength. New and original results are presented on relationships between autogenous shrinkage and internal relative humidity. Other results confirm and validate earlier findings of normal strength concrete regarding relationships between creep compliance, porosity, compressive strength, and maturity for high performance concrete. The project was carried out at Lund Institute of Technology between 1992 and 1996.

DLVO theory has been applied to cement suspensions containing admixtures. Two different batches of the same cement, different only in storage history, are compared. It is found that, although their general sedimentation behavior is similar, differences exist in the zeta potential and basic chemistry. Both the superplasticizer and water-reducing admixture result in all cases in a stable dispersion, contrary to the theoretical prediction that only a coagulated suspension should exist. This finding suggests that steric hindrance plays a larger role compared to electric repulsion in the deflocculation of cement pastes than previously believed. Zeta potential and sedimentation data for CaCl₂ and sugar are also presented.