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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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    Effect of polyethylene glycol addition on metakaolin-based geopolymer
    (Elsevier, 2016-06-01) Mikhailova, Olesia; Rovnaník, Pavel
    Polyethylene glycol is a non-toxic water soluble polymer often used in many industrial applications. The aim of this paper is to study the effect of polyethylene glycol with relative molecular weight ranging between 400–20000 on the properties of geopolymer mortars composed of metakaolin and sodium silicate. Polyethylene glycol was added in the amount of 0.5 to 10% by mass of metakaolin. After 28 days of curing at ambient conditions, different tests were carried out: physico-chemical (density, porosity), mechanical (flexural and compressive strength) and structural (SEM). The results showed that a maximum compressive strength of 23.9 MPa and a maximum flexural strength of 2.9 MPa were achieved by adding 10% PEG 400.reference values with higher amount of MWCNTs.
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    Effect of carbon nanotubes on the mechanical fracture properties of fly ash geopolymer
    (Elsevier, 2016-06-01) Rovnaník, Pavel; Šimonová, Hana; Topolář, Libor; Schmid, Pavel; Keršner, Zbyněk
    Fly ash geopolymer is amorphous aluminosilicate material which is considered as alternative to Portland cement concrete. One of the limiting factors of its utilization is an increased shrinkage and related deterioration of fracture properties. This paper reports on a study of the application of multi-walled carbon nanotubes (MWCNTs) to improve the fracture properties of fly ash geopolymer. The amount of MWCNTs added varied in the range of 0.05–0.2% of the mass of fly ash. Mechanical fracture properties were determined via evaluation of three-point bending fracture tests. Specimen response during fracture tests was also monitored by means of acoustic emission, and this method was also used for the determination of cracking tendency occurring during the hardening process. Results show that the addition of MWCNTs increases the elastic modulus and compressive strength of fly ash geopolymer. However, basic fracture parameters (fracture toughness, fracture energy) firstly decreased with very small addition of MWCNTs and were regained or slightly exceeded the reference values with higher amount of MWCNTs.
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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.
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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.