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    On the effect of addition of carbon nanotubes on the electric conductivity of alkali-activated slag mortars
    (IOP Publishing, 2017-10-18) Kusák, Ivo; Luňák, Miroslav
    This paper presents basic electric properties of laboratory prepared alkali-activated composite materials on the basis of finely ground granular high furnace slag to which various quantities of carbon nanotubes (CNT) have been added. Impedance spectroscopy in the frequency range from 40 Hz to 1 MHz was used to measure the specimens. Electric resistivity versus frequency and electric resistivity versus CNT content relationships were examined on our specimens R&S ZNC vector analyser with DAK-12 coaxial probe (made by Speag) was used to carry out the measurements at higher frequencies (from 100 MHz to 3 GHz). Electric conductivity as a function of the frequency and as a function of the specimen CNT content was studied in this frequency range. Up-to-date instruments and a unique approach have evidently been employed to carry out non-destructive measurement of mortar materials.
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    Thermodynamic Stability of Ettringite Formed by Hydration of Ye’elimite Clinker
    (Hindawi Publishing Corporation, 2016-09-08) Fridrichová, Marcela; Dvořák, Karel; Gazdič, Dominik; Pekárková, Jana; Kulísek, Karel
    Although the stability of ettringite has been the subject of much research, the solution is not yet fully clear. Ettringite is generally considered to be stable up to a temperature of 110°C; however, some investigators claimed that ettringite may already decompose at even ambient temperatures. To prove these statements, ettringite was prepared by the hydration of ye’elimite, 3CaO·3Al2O3·CaSO4, and the system stored at laboratory temperature in two environments: in laboratory settings and in an environment of saturated water vapour.
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    Thermal behaviour of metakaolin/fly ash geopolymers with chamotte aggregate
    (MDPI, 2016-06-30) Rovnaník, Pavel; Halasová, Kristýna
    Geopolymers are generally appreciated for their good resistance against high temperatures. This paper compares the influence of thermal treatment with temperatures ranging from 200–1200 °C on the mechanical properties and microstructure of geopolymers based on two different aluminosilicate precursors, metakaolin and fly ash. Moreover, the paper is also aimed at characterizing the effect of chamotte aggregate on the performance of geopolymers subjected to high temperatures. Thermal treatment leads to a deterioration in the strength of metakaolin geopolymer, whereas fly ash geopolymer gains strength upon heating. The formation of albite above 900 °C is responsible for the fusion of geopolymer matrix during exposure to 1200 °C, which leads to the deformation of the geopolymer samples. Chamotte aggregate improves the performance of geopolymer material by increasing the thermal stability of geopolymers via sintering of the aggregate particles with the geopolymer matrix in the contact zone.
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    Properties of concrete with partial replacement of natural aggregate by recycled concrete aggregates from precast production
    (Elsevier, 2016-08-03) Nováková, Iveta; Mikulica, Karel
    Recycled concrete aggregates (RCA) can be used as a replacement of natural aggregates for a concrete production to save natural sources and also to decrease amount of demolition waste which has to be landfilled. Precast production generates some percentage of defected elements which are carted off and recycled with other demolition waste. In this study defected elements are separately recycled into RCA with beneficial properties and used directly into new mixtures for precast elements. Results from testing of RCA and application into new concrete mixtures are presented. It was proved that the replacement of raw aggregates by RCA up to 20% has no negative influence on physico-mechanical properties of concrete.
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    Behaviour of cement composites with lightweight and heavyweight aggregates at high temperatures
    (Budapest University of Technology and Economics, 2017-03-24) Bodnárová, Lenka; Peterková, Jitka; Brožovský, Jiří; Zach, Jiří; Válek, Jaroslav
    This paper presents new results of the research of the influence of individual input components done on the behaviour of cement based composite materials exposed to high temperatures. A new approach of this research lies in combining a fraction of basalt fine aggregate and an aggregate base on expanded clay for a composite material with lower volume weight and higher strengths and a resistance to high temperatures. In the field of dispersed reinforcement, new polypropylene fibers treated by low temperature plasma were tested. Degradation of the structure of designed mixtures was evaluated by comparing density changes, mechanical and acoustic properties before and after exposure to high temperatures. An optical microscope was used for determining the width of cracks caused by a thermal load. The contribution of this research was the use of non-destructive methods for diagnostics of quality and properties of designed concrete mixtures exposed to high temperatures.