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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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    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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    Electrical properties of alkali-activated slag composite with combined graphite/CNT filler
    (IOP Publishing Ltd., 2017-10-25) Rovnaník, Pavel; Míková, Maria; Kusák, Ivo
    Alkali-activated industrial by-products such as blast furnace slag are known to possess properties which are comparable to or even better than those observed for ordinary Portland cement. The combination of alkali-activated slag matrix with conductive filler introduces new functionalities which are commonly known for self-sensing or self-heating concrete. The present paper discusses the effect of the mixture of two different conductive fillers, graphite powder and carbon nanotubes (CNTs), on the electrical properties of alkali-activated slag mortars. Prepared samples were also tested for their mechanical properties and microstructure was investigated by means of mercury intrusion porosimetry and scanning electron microscopy. The percolation threshold for the resistance was reached for the mixture containing 0.1% CNTs and 8% graphite powder.
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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.