This article presents test results of cement paste and binders with admixture of hydrophilic or hydrophobic nanosilica. The aim of the study was to determine the influence of nanosilica type and mixing method on compressive strength, porosity, and bulk density of cement paste, also on hydration heat of cement binders. The binder compounds were mixed in high speed mixer in order to provide the highest possible dispersion of nanoparticles in the binder before adding it to mixing water. Two mixing methods were studied. The admixtures increased the reactivity of cement binders. Both nanosilica types increased early compressive strength by 25% in comparison with control series. The increase in 28-day compressive strength was observed with the admixture of hydrophilic nanosilica. The differences in dynamics of binders rate of hydration and development of cement pastes compressive strength denote different reaction mechanisms of both types of nanosilica. Application of higher rotation speeds does not guarantee satisfactory mixing of the binder components. For compressive strength enhancement of cement paste prolonged mixing time occurred to be more important.
A catalytic combustion of organic admixtures of air belongs to the basic technologies of gas purification. A macrokinetics of admixtures combustion over the porous catalysts was described. The theoretical approach is in agreement with standard description of macrokinetics of the catalytic processes. The relationship between the fundamental magnitudes: observed process rate r*, reaction rate r in the kinetic zone, and a coefficient of the surface utilization η in the form r*= r · η have been described. These magnitudes combines the Thiele module φ. A kinetics equation for the isothermal and non-isothermal conditions was provided. The influence of mass and heat transfer in the catalyst grain on the course of the process was described by means of the surface utilization coefficient η. An equation describing this coefficient for both isothermal and non-isothermal conditions was given. The second part of this work concerns the application of theory. When the composition of purified gas is continuously varied, a quantitative approach is rather impossible. The theory was used for the qualitative analysis of process on the basis of the experimental results. A fulfillment of the first-order kinetics means that the degree of admixtures conversion does not depend on their initial concentrations. A non-isothermicity of the catalyst grain is expressed in such a way that the process rate observed over the large porous grains of the catalyst can be higher than the reaction rate in the kinetic zone. A temperature deference between the catalyst grains and flowing gas causes that the reactor can be stably operated at varied concentrations of admixtures and temperature over a relatively wide range. It was also demonstrated that the flammable admixtures may advantageously influence the conversion of hardly combustible admixtures
Admixtures are commonly used nowadays in the mix composition of concrete. These additions affect concrete properties and performance especially creep deformations. This paper shows the effect of admixtures on creep of concrete. In fact, creep deformations have prejudicial consequences on concrete behaviour; an incorrect or inaccurate prediction leads to undesirable consequences in structures. Therefore, an accurate estimation of these deformations is mandatory. Moreover, design codes do not consider admixtures’ effect while predicting creep deformations, thus it is necessary to develop models that predict accurately creep deformations and consider the effect of admixtures. Using a large experimental database coming from international laboratories and research centres, this study aims to update the Eurocode 2 creep model by considering the type and percentage of admixtures using Bayesian Linear Regression method. The effect of two types of admixtures is presented in this paper; the water reducer and silica fume.
The properties of expansive concretes made of two types of cement: Portland cement CEM I and blast furnace slag cement CEM III were tested. The expansion of the concrete was caused by using an expansive admixture containing aluminium powder added in an amount of 0.5; 1 and 1.5% of cement mass. It was found that the compressive strength of concrete with CEM I decreased after using an expansive admixture in the amount of more than 0.5% of the cement mass. The compressive strength of concrete with CEM III decrease after addition of admixture in the entire range of dosages used. On the basis of electrochemical measurements, it was found no influence of an expansive admixture on corrosion of reinforcing steel. The use of an expansive admixture causes a slight increase in the effective diffusion coefficient of chloride ions in concrete.