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    Development of the internal structure of an alkali-activated paste related to the length of curing
    (IOP Publishing, 2023-08-25) Kocáb, Dalibor; Bílek, Vlastimil; Nápravník, Petr; Kucharczyková, Barbara
    This paper deals with the influence of the length of the moist curing period on the development of the internal structure of a paste based on alkali-activated slag. Waterglass was used as the activator. For the purpose of experiment, the test specimens subjected to the experiment were divided into five sets with different curing regimes combining moist and air curing. The reference set was kept in an environment with relative air humidity 95% during the entire maturing period. The development of the internal structure of the alkali-activated paste was mainly monitored using the resonance method. The obtained results show that the curing regime applied to the test specimens significantly influences the development of the internal structure of the material. The development of the observed dynamic properties (dynamic moduli and Poisson’s ratio) shows the formation of damage to the internal structure of the paste after its exposure to the air. Interestingly, a massive decrease in dynamic modulus of elasticity immediately after exposure to air drying was not prevented, even by intensive moist curing of the paste for 28 days.
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    Effect of siliceous sand volume fraction on the properties of alkali-activated slag mortars
    (IOP Science, 2023-08-17) Hajzler, Jan; Bílek, Vlastimil; Kejík, Marek; Kucharczyková, Barbara
    One of the critical factors affecting the performance of alkali-activated slag (AAS) is the nature and dose of alkali activator. The activator type can play a significant role during the transition from pastes to mortars or concretes. Therefore, three basic sodium activators (water glass, carbonate, and hydroxide) of the same molarity of 4M Na+ were used to prepare AAS-based mortars with different volume fractions of siliceous sand. These were compared by means of workability, mechanical strength, and long-term shrinkage under autogenous conditions. The results were compared to those obtained on pastes with similar workability. Increasing the content of the sand tended rather to decrease the mechanical properties, while greatly decreased autogenous shrinkage. Nevertheless, the most remarkable differences for different activators were observed when comparing the mortars with pastes. The transition from pastes to mortars resulted in the highest reduction in both compressive and flexural strength for sodium hydroxide. The flexural strength of the mortars with sodium water glass and sodium carbonate even increased considerably in presence of sand.
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    Mechanical fracture characterization of alkali-activated slag mortars with standardized and natural sand
    (EDP Sciences, 2020-03-05) Šimonová, Hana; Kucharczyková, Barbara; Bílek, Vlastimil; Kocáb, Dalibor
    This paper is focused on the development of the physical, mechanical and fracture characteristics of alkali-activated slag (AAS) mortars during ageing. The two AAS mortars were selected for the purpose of the experiment. The difference was in a type of used sand: first mortar was made using the standardized siliceous sand with the maximum nominal grain size of 2 mm and the second mortar using the natural sand with a maximum grain size of 4 mm. The results of the elastic, fracture and strength parameters determined within the time interval of 3 to 330 days of ageing are summarized in the paper. All experimental results were evaluated for the prismatic specimens with dimensions of 40 × 40 × 160 mm and were expressed by the average value and sample standard deviation calculated from three independent measurements
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    MONITORING THE INTERNAL STRUCTURE BEHAVIOUR OF ALKALI-ACTIVATED SLAG PASTE: EFFECT OF THE CURING MODE
    (CZECH TECHNICAL UNIV PRAGUE, 2023-12-31) Nápravník, Petr; Kocáb, Dalibor; Bílek, Vlastimil; Lisztwan, Dominik; Kucharczyková, Barbara
    This paper deals with the monitoring of the internal structure behaviour of an alkali -activated slag (AAS) paste. The slag was activated with a 4M solution of sodium hydroxide. The behaviour of the internal structure of the paste was regularly monitored through the changes in the resonant frequency and the mechanical properties, until the paste reached the age of 90 days. The main aim of the article is to show the long-term maturation and degradation process of an AAS paste under different curing modes. The results obtained suggest that the curing mode of the specimens has a significant effect on the behaviour of the internal structure of the paste based on the AAS. The development of both the dynamic properties and the flexural strength indicates the occurrence of a higher porosity in the internal structure of the paste, especially when the free drying process is started earlier. Insufficient hydration of the binder system is also a likely cause of cracks. The reduction in the relative dynamic moduli values ranging from 50 % to 80 % was observed for drying specimens at the age of 90 days. What is very interesting is that the occurrence of cracks was not prevented even by intensive moist curing of the paste as, between the 21st and the 28th day of maturing, there was a significant decrease of about 20 % in the relative dynamic modulus of elasticity and also a 50 % reduction in the flexural strength.
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    The Effect of Specimen Size on Acoustic Emission Parameters and Approximate Position of Defects Obtained during Destructive Testing of Cementitious and Alkali-Activated Degraded Fine-Grained Materials
    (MDPI, 2023-05-04) Topolář, Libor; Kocáb, Dalibor; Hrubý, Petr; Jakubka, Luboš; Hoduláková, Michaela; Halamová, Romana
    Two sizes of test samples were selected to investigate the effect of size on the level of degradation. The smaller test specimens had dimensions of 40 × 40 × 160 mm, and the larger ones had dimensions of 100 × 100 × 400 mm. Both sizes of test specimens were always made of the same mortar. In one case, Blast Furnace Cement was chosen as the binder. In the other case, it was an alkali-activated material as a possibly more environmentally economical substitute. Both types of material were deposited in three degrading solutions: magnesium sulphate, ammonium nitrate and acetic acid. The reference set was stored in a water bath. After six months in the degradation solutions, a static elastic modulus was determined for the specimens during this test, and the acoustic emission was measured. Acoustic emission parameters were evaluated: the number of hits, the amplitude magnitude and a slope from the amplitude magnitude versus time (this slope should correspond to the Kaiser effect). For most of the parameters studied, the size effect was more evident for the more degraded specimens, i.e., those placed in aggressive solutions. The approximate location of emerging defects was also determined using linear localisation for smaller specimens where the degradation effect was more significant. In more aggressive environments (acetic acid, ammonium nitrate), the higher resistance of materials based on alkaline-activated slag was more evident, even in the case of larger test bodies. The experiments show that the acoustic emission results agree with the results of the static modulus of elasticity.