Pavla Rovnaníková

Experimental analysis of coupled water and chloride transport in cement mortar

Abstract Coupled water and chloride transport in cement mortar is analyzed experimentally in the paper. Samples with initial moisture content corresponding to 45% relative humidity are subjected to one-sided sodium chloride-in-water solution uptake, and moisture profiles and chloride concentration profiles are determined in three chosen time intervals. In the evaluation of measured moisture profiles and chloride concentration profiles, diffusion model is employed. Moisture diffusivity is determined as function of moisture content and chloride diffusion coefficient as function of chloride concentration using two methods commonly used for analysis of moisture profiles, namely the double integration method and the Matano method. The highest values of both coefficients are obtained by Matano method for the curves corresponding to 24 h exposure to the solution, the lowest values by Matano method for 168 h, the results obtained by double integration method are in between. The complementary experiments with distilled water as penetrating liquid performed for the sake of comparison do not show this feature. It is observed that water transport in the initial time period is for the chloride solution faster and in later times slower than for distilled water. Therefore, the observed differences in the calculated apparent values of moisture diffusivity and chloride concentration coefficients are attributed to the fact that Cl− and Na+ ions are adsorbed on the pore walls faster than water molecules. This effect should be included into the mathematical models using ion binding isotherms.

THERMAL AND HYGRIC PROPERTIES OF PORTLAND CEMENT MORTAR AFTER HIGH-TEMPERATURE EXPOSURE COMBINED WITH COMPRESSIVE STRESS

Thermal conductivity λ, water vapor permeability δ, and liquid moisture diffusivity κ of cement mortar are measured on specimens subjected to four types of pretreatment, namely, unloaded, mechanically loaded to 90% of compressive strength, thermally loaded by subjecting to a temperature of 800 °C for 2 h, and loaded both mechanically and thermally. The values of λ and κ are found to depend very significantly on the loading mode. The maximum differences observed compared to the unloaded samples are almost one order of magnitude for λ and as much as three orders of magnitude for κ. In contrast, the values of δ are found to increase by only about 40% compared to the basic unloaded material. It is proposed that the observed large differences in λ and κ are due to the formation of cracks and the increase of total pore volume, which were shown by visual analysis and mercury porosimetry.

The effects of thermal load and frost cycles on the water transport in two high-performance concretes

Abstract Moisture diffusivity of two high-performance concretes used in concrete containment buildings of nuclear power plants is measured as a function of temperature up to 80°C, and the effects of high-temperature exposure up to 800°C and freeze/thaw cycles on its value are analyzed. The temperatures in the range of 0–80°C are found to have a moderate effect on the moisture diffusivities of both concretes, the typical increase being about 100–200% over the whole region. The high temperature exposure results in a much more significant increase of moisture diffusivity, up to three orders of magnitude, and the freeze/thaw exposure of 400 cycles to an increase up to one order of magnitude. As the main reason for the mentioned moisture diffusivity increase, the crack formation is identified. Both thermal decomposition accompanied by release of substantial amount of gaseous substances and water freezing in the porous system can damage the internal microstructure of the matrix due to the significant pressure increase in the porous body. The magnitude of changes of moisture diffusivity is found to be affected by the quality of aggregates rather than by the quality of cement. High quality quartzitic aggregates with very low porosity and very good mechanical properties perform much better than other more porous siliceous aggregates such as opal, and than the calcareous aggregates such as chalk.

APPLICATION OF A MICROWAVE IMPULSE TECHNIQUE TO THE MEASUREMENT OF FREE WATER CONTENT IN EARLY HYDRATION STAGES OF CEMENT PASTE

A microwave impulse method is designed and employed to the monitoring of residual moisture content in early hydration stages of cement paste. Complementary experiments consisting in determination of the times of beginning and end of setting and of the course of bending strength, compression strength, Youngs modulus and hydration heat production during the early hydration period are performed in order to find possible correlation between the measured moisture changes, development of mechanical parameters and hydration heat release. It is observed that the fastest decrease of residual moisture content roughly agrees with the early stages of the hardening process, i.e., with the time period between the end of setting and the moment when first measurable values of mechanical parameters are obtained.

Computational analysis of hygrothermal performance of renovation renders

A combined computational-experimental approach for service life estimate of surface layers of historical building made from renovation renders is presented in the paper. The experimental part is aimed at estimation of the durability of two commercial systems of double layered renovation renders in terms of their frost resistance. The computational part includes calculation of number of freeze-thaw cycles that may occur in surface layers during a reference year. To achieve this, a diffusion-type model of coupled heat and moisture transport is used. The computations are performed on historical load-bearing structure made of sandstone, brick or arenaceous marl, finished by two different hydrophobic renovation render systems.

Thermal and Hygric Parameters of Carbon-fiber-reinforced Cement Composites after Thermal and Mechanical Loading

The basic thermal and hygric parameters of two different types of carbon-fiber-reinforced cement composites are analyzed in this article. The thermal conductivity, specific heat capacity, moisture diffusivity, and water vapor diffusion resistance factor are determined as functions of thermal load and tensile load applied before the measurement as well as of the combination of both types of load. The tensile load up to failure is found to be not a very significant factor for all material properties analyzed except for the moisture diffusivity. On the other hand, the thermal load is observed to result already at 600 C in considerable changes in all investigated thermal and hygric properties except for the specific heat capacity. The combinations of thermal and tensile loads lead to similar results as the effect of the thermal load alone so that the domination of the thermal load is apparent. This is supposed to be due to the positive effect of randomly distributed carbon fibers that can reduce the damage of the pore structure by the tensile stress. The resistance of the materials studied to high temperatures expressed by the change of hygric and thermal properties after thermal load is found to be positively affected by the application of the high alumina cement and in the case of the Portland cement-based composite also by using the autoclaving procedure in the production process.

Coagulated silica - a-SiO2 admixture in cement paste

Amorphous silica (a-SiO2) in fine-grained form possesses a high pozzolanic activity which makes it a valuable component of blended binders in concrete production. The origin of a-SiO2 applied in cement-based composites is very diverse. SiO2 in amorphous form is present in various amounts in quite a few supplementary cementing materials (SCMs) being used as partial replacement of Portland cement. In this work, the applicability of a commercially produced coagulated silica powder as a partial replacement of Portland cement in cement paste mix design is investigated. Portland cement CEM I 42.5R produced according to the EU standard EN 197-1 is used as a reference binder. Coagulated silica is applied in dosages of 5 and 10 % by mass of cement. The water/binder ratio is kept constant in all the studied pastes. For the applied silica, specific surface area, density, loss on ignition, pozzolanic activity, chemical composition, and SiO2 amorphous phase content are determined. For the developed pastes on the basis...

Application of a-SiO2 Rich Additives in Cement Paste

The effect of a-SiO2 of various origin on the properties of cement paste with incorporated different silica containing materials is experimentally studied in the paper. For the applied a-SiO2 materials, basic physical and chemical properties are accessed, together with their chemical composition. Amount of amorphous phase of SiO2 in particular siliceous materials is determined using XRD analysis. Matrix density, bulk density, total open porosity, compressive and bending strength are measured for all developed pastes with incorporated a-SiO2 containing materials, together with initial and final setting time of fresh mixtures. The obtained data give evidence on a high and fast reaction activity of tested siliceous materials which results in a significant improvement of porosity and mechanical strength of a-SiO2 modified cement pastes.

Influence of various amount of diatomaceous earth used as cement substitute on mechanical properties of cement paste

Active silica containing materials in the sub-micrometer size range are commonly used for modification of strength parameters and durability of cement based composites. In addition, these materials also assist to accelerate cement hydration. In this paper, two types of diatomaceous earths are used as partial cement replacement in composition of cement paste mixtures. For raw binders, basic physical and chemical properties are studied. The chemical composition of tested materials is determined using classical chemical analysis combined with XRD method that allowed assessment of SiO2 amorphous phase content. For all tested mixtures, initial and final setting times are measured. Basic physical and mechanical properties are measured on hardened paste samples cured 28 days in water. Here, bulk density, matrix density, total open porosity, compressive and flexural strength, are measured. Relationship between compressive strength and total open porosity is studied using several empirical models. The obtained res...

Influence of Guar Gum Derivatives on Hardened Properties of Aerial Lime-Based Mortars

This paper studies the possibility of usage of the guar gum and its derivatives (carboxymethylhydroxypropyl guar (CMHPG) and hydroxypropyl guar (HPG)) as admixtures for aerial lime-based mortars. The influence on the properties of mortars was studied on the aerial lime –based mortars prepared with quartz fine grained sand and doses of admixtures ranging between 0,5 and 10‰. The hardened bulk densities, flexural and compressive strength, porosity, water absorption coefficient due to capillarity action and carbonation rate were studied. The addition of the hydroxypropyl guar lowered the bulk density (due to an air intake), improved workability, slightly increased the strength, slowed carbonation rate, and nobbled the water transport in the mortar. The addition of carboxymethylhydroxypropyl guar does not impact the bulk density, the strengths were increased similarly to HPG: it does not impact carbonation rate significantly, so the long term strengths were comparative with the HPG. The water transport was slightly better in lower doses and slightly worse in larger doses in comparison with the reference mortar. The pure guaran was found not to be beneficial for the lime mortars for its only advantage is in the water transport, where in any dose the transport was better than any other mortar, but the benefit of this is questionable.