| Abstract [eng] |
In this work, the influence of KOH and the complex additive KOH and Al2O3 on the crystallization of dibasic calcium silicate hydrates in the CaO-SiO2-H2O system with the molar ratio CaO/SiO2 = 1.9–2.1 was examined. The synthesis carried out under hydrothermal conditions at 200 ℃ saturated water vapor environment for 4, 8, 16, 24, 48 and 72 hours. Synthesized product was characterized by powder X-ray diffraction analysis, simultaneous thermal analysis, Fourier-transform infrared spectroscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. In the CaO-SiO2-K2O-H2O system with molar ratio CaO/SiO2 = 1.9 different concentrations of 3 to 7 %, KOH was found to have a positive effect on the reaction of the starting compounds Ca(OH)2 and quartz (SiO2) at the beginning of the synthesis, as they were not detected already after 4-8 hours. In addition, under the action of K+ ions, the dibasic calcium hydrosilicate - α-C2S hydrate - was rapidly formed in the system during the first hours of isothermal curing, which began to recrystallize to other calcium silicate hydrates and calcium silicate of various bazes killalaite, trabzonite and C8S5 with increasing duration of synthesis (after 16 to 72 hours). Meanwhile, in CaO-SiO2-K2O-Al2O3-H2O system using a complex additive KOH and Al2O3 (Al2O3/(SiO2 + Al2O3) = 0.025 + 5 % KOH) quartz reduced by 4 hours and Ca(OH)2 only by 16 hours. It was determined that Al3+ ions had a negative effect on the crystallization process of α-C2S hydrate, however it promoted the formation of the mentioned compounds and a new hydrogranate compound – katoite, which stayed stable during isothermal curing to 72 hours. It was found that in the higher alkalinity system (CaO/SiO2 = 2.1), regardless of the additive used, quartz reacted within 8 hours, and Ca(OH)2 was detected after 72 hours. It should be noted that the latter compound determined the stability of α-C2S hydrate in the system under all synthesis conditions. In this work, the thermal stability of synthetic dibasic calcium silicate hydrates in temperature range 25–1000 ℃ as well as the effect of these compounds on the early hydration of Portland cement were examined. It was found that during the thermal treatment of hydrothermal synthesis products in a high-temperature furnace various polymorphs of dicalium silicates x-, α-, α‘L-, β-, γ–2CaO∙SiO2 were formed. In addition, it was found that the steady-state intervals of the formed calcium silicates were influenced by K+ and Al3+ ions intervening in their crystal lattice. Meanwhile, it was found that different amount (5 to 15 %) of calcium silicate hydrates additive accelerate the hydration reactions of Portland cement during the first 72 hours of hydration, as the total amount of the heat evolution rate of Portland cement samples with mentioned additives was higher (332 to 352 J/g) than that of pure Portland cement (331 J/g). |
