1. Experimental study and thermodynamic modelling of the temperature effect on the hydration of belite-ye’elimite-ferrite cementsMaruša Mrak, Frank Winnefeld, Barbara Lothenbach, Andraž Legat, Sabina Dolenec, 2024, original scientific article Abstract: This study focuses on the kinetics of hydration, phase assemblage, microstructure and mechanical properties after various hydration times of two different BCSA cements at 5, 20, 40 and 60 ◦C and compares experimental data with thermodynamic modelling. Different curing temperatures change the type of hydrates and their amounts. Ye’elimite and gypsum in belite-ye’elimite-ferrite cements are almost fully reacted after 24 h of hydration at 20, 40 and 60 ◦C, while not at 5 ◦C. The hydration of belite is slower than the one of ye’elimite, but reaches a high degree of hydration after 150 days which is increasing with temperature. Less ettringite is present at elevated temperatures due to its increasing solubility, while more monosulfate is observed. Furthermore, with increasing temperature siliceous hydrogarnet forms at the expenses of strätlingite as well as more C S H is found as more belite reacts resulting in higher compressive strength. Dense and homogenous microstructure is observed at 5 ◦C, while it is more heterogeneous at higher temperatures. The presence of more ye’elimite resulted in higher ettringite and str¨atlingite formation and a higher compressive strength, while more belite yields more C S H in the hydrates and lower compressive strength.
Keywords: belite-ye’elimite-ferrite cement, temperature, hydration, thermodynamic modelling
Published in DiRROS: 11.12.2023; Views: 169; Downloads: 141
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2. RILEM TC 247-DTA round robin test : sulfate resistance, alkali-silica reaction and freeze-thaw resistance of alkali-activated concretesFrank Winnefeld, Gregor J. G. Gluth, Susana Bernal, Maria Chiara Bignozzi, Lorenza Carabba, Sundararaman Chithiraputhiran, Alireza Dehghan, Sabina Dolenec, Katja Dombrowski-Daube, Ashish Dubey, Vilma Ducman, Yu Jin, Karl Peterson, Stephen Dietmar, John L. Provis, 2020, original scientific article Abstract: The RILEM technical committee TC 247-DTA ‘Durability Testing of Alkali-Activated Materials’ conducted a round robin testing programme to determine the validity of various durability testing methods, originally developed for Portland cement based-concretes, for the assessment of the durability of alkali-activated concretes. The outcomes of the round robin tests evaluating sulfate resistance, alkali-silica reaction (ASR) and freeze–thaw resistance are presented in this contribution. Five different alkali-activated concretes, based on ground granulated blast furnace slag, fly ash, or metakaolin were investigated. The extent of sulfate damage to concretes based on slag or fly ash seems to be limited when exposed to an Na2SO4 solution. The mixture based on metakaolin showed an excessive, very early expansion, followed by a dimensionally stable period, which cannot be explained at present. In the slag-based concretes, MgSO4 caused more expansion and visual damage than Na2SO4; however, the expansion limits defined in the respective standards were not exceeded. Both the ASTM C1293 and RILEM AAR-3.1 test methods for the determination of ASR expansion appear to give essentially reliable identification of expansion caused by highly reactive aggregates. Alkali-activated materials in combination with an unreactive or potentially expansive aggregate were in no case seen to cause larger expansions; only the aggregates of known very high reactivity were seen to be problematic. The results of freeze–thaw testing (with/without deicing salts) of alkali-activated concretes suggest an important influence of the curing conditions and experimental conditions on the test outcomes, which need to be understood before the tests can be reliably applied and interpreted.
Keywords: alkali-activated materials/geopolymers, sulphate resistance, alkali silica reactivity, freeze-thaw resistance, Rilem TC
Published in DiRROS: 17.08.2023; Views: 225; Downloads: 163
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3. The influence of calcium sulfate content on the hydration of belite-calcium sulfoaluminate cements with different clinker phase compositionsMaruša Mrak, Frank Winnefeld, Barbara Lothenbach, Sabina Dolenec, 2021, original scientific article Abstract: The influence of different amounts of gypsum on the hydration of a belite-rich and a yeʼelimite-rich belite-calcium sulfoaluminate clinker (BCSA) was investigated. The hydration kinetics, phase assemblages and compressive strength development of cements prepared using yeʼelimite/ calcium sulfate molar ratios of 1, 1.5 and 2 were studied. Besides ettringite and monosulfate, aluminium hydroxide, strätlingite, C-S-H, iron-containing siliceous hydrogarnet and hydrotalcite were present as hydration products. Increasing the amount of gypsum increased the ratio of ettringite to monosulfate formed in the cement paste, lowered the amount of pore solution, delayed the dissolution of belite and ferrite, decreased the formation of strätlingite and, in the case of the yeʼelimite-rich BCSA, led to an increase in compressive strength. Increased amounts of belite in the clinker led to the formation of higher quantities of C-S-H, at the expense of straätlingite and a lower compressive strength, as belite has a lower degree of reaction than yeʼelimite and due to the formation of more C-S-H and stratlingite compared to the more space-filling ettringite. The thermodynamic model established for BCSA cement hydration agrees well with the experimental data. Compressive strength directly correlated with bound water from thermogravimetric analyses and inversely correlated with the porosity calculated from thermodynamic modelling.
Keywords: belite-calcium sulfoaluminate cement, gypsum, hydration, thermodynamic modelling
Published in DiRROS: 28.07.2023; Views: 275; Downloads: 211
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