Anatolijus Eisinas

In this study, the possibility of using cement bypass dust as a cement additive was investigated. The utilization of cement bypass dust remains a major problem in cement production, as huge amounts of it are stored in landfills. In this study, a hydrothermal treatment is proposed to modify the properties of this dust and to expand its use. Hydrothermal treatment with pure bypass dust and quartz was carried out to achieve a CaO/SiO₂ ratio of 1 to 2. Samples were synthesized at 200 °C for 2, 4, 8, and 24 h. To examine the influence of the hydrothermal treatment on cement properties, a sample with a CaO/SiO₂ ratio of 1, hydrothermally treated for 8 h, was selected. This study employed XRD, XRF, DSC-TG, and isothermal calorimetry. Most of the target synthesis products, e.g., tobermorite and calcium silicate hydrates, formed after 8 h of sample synthesis, during which quartz was added to bypass dust and a CaO/SiO₂ ratio of 1 was achieved. An examination of the composition of the liquid medium following hydrothermal processing showed that almost all chlorine passed into the liquid medium, while some K₂O remained in the solid synthesis product. The synthesized additive is an effective catalyst for the hydration of Portland cement. After a 28-day curing period, specimens incorporating modified bypass dust replacing up to 10% of the Portland cement by weight demonstrated compressive strengths comparable to, or surpassing, those of specimens composed exclusively of Portland cement.

This study aimed to investigate the possibility of using biomass fly ash as a cement additive after hydrothermal modification. The treatment of biomass fly ash involves additional milling and curing under various hydrothermal conditions. Variation of the CaO/SiO₂ ratio was achieved by mixing different samples of biomass fly ash. Samples with a CaO/SiO₂ ratio of 1 and 1.5 were selected for synthesis, and cement samples with different amounts of the synthesized additive were examined. The research employed the following methodologies: XRD, XRF, DSC-TG, ICP-OES, and isothermal calorimetry. It was found that in all the biomass fly ash samples, calcium silicate hydrates already formed after 2 h of hydrothermal synthesis at 200 °C. Analysis of the chemical composition of the liquid medium after hydrothermal maintenance revealed an exceptionally low release of heavy metals from the samples. The synthesised additive accelerated the initial hydration of Portland cement. In total, 5% to 10% of the weight of cement can be replaced by hydrothermally treated biomass fly ash without reducing the compressive strength class of Portland cement samples.

The main measure to reduce energy losses is the usage of insulating materials. When the temperature exceeds 500 °C, silicate and ceramic products are most commonly used. In this work, high-crystallinity 1.13 nm tobermorite and xonotlite were hydrothermally synthesized from lime and Ca–Si sedimentary rock, opoka. By XRD, DSC, TG and dilatometry methods, it has been shown that 1.13 nm tobermorite becomes the predominant compound in stirred suspensions at 200 °C after 4 h of synthesis in the mixture with a molar ratio CaO/SiO₂ = 0.83. It is suitable for the production of insulating products with good physical–mechanical properties (average density < 200 kg·m⁻¹, compressive strength ~0.9 MPa) but has a limited operating temperature (up to 700 °C). Sufficiently pure xonotlite should be used to obtain materials with a higher operating temperature. Even small amounts of semi-amorphous C–S–H(I) significantly increase its linear shrinkage during firing. It has also been observed that an increase in the strength values of the samples correlated well with the increase in the size of xonotlite crystallites. The optimal technological parameters are as follows: molar ratio of mixture CaO/SiO₂ = 1.2; water/solid ratio W/S = 20.0; duration of hydrothermal synthesis at 220 °C—8 h, duration of autoclaving at 220 °C—4 h. The average density of the samples was ~180 kg·m⁻¹, the operating temperature was at least 1000 °C, and the compressive strengths exceeded 1.5 MPa.

In this work, the thermal stability, microstructure, and catalytic activity in oxidation reactions of calcium silicate hydrates formed in the CaO-SiO₂-Cr(NO₃)₃-H₂O system under hydrothermal conditions were examined in detail. Dry primary mixture with a molar ratio of CaO/SiO₂ = 1.5 was mixed with Cr(NO₃)₃ solution (c = 10 g Cr³⁺/dm³) to reach a solution/solid ratio of the suspension of 10.0:1. Hydrothermal synthesis was carried out in unstirred suspensions at 175 °C for 16 h. It was determined that, after treatment, semicrystalline calcium silicate hydrates C-S-H(I) and/or C-S-H(II) with incorporated Cr³⁺ ions (100 mg/g) were formed. The results of in situ X-ray diffraction and simultaneous thermal analyses showed that the products were stable until 500 °C, while, at higher temperatures, they recrystallized to calcium chromate (CaCrO₄, 550 °C) and wollastonite (800–850 °C). It was determined that both the surface area and the shape of the dominant pore changed during calcination. Propanol oxidation experiments showed that synthetic semicrystalline calcium silicate hydrates with intercalated chromium ions are able to exchange oxygen during the heterogeneous oxidation process. The obtained results were confirmed by XRD, STA, FT-IR, TEM, SEM, and BET methods, and by propanol oxidation experiments.

The effect of synthetic CAH (130°C; 8 h; CaO/(SiO₂+ Al₂O₃) = 0.55; Al₂O₃/(SiO₂+ Al₂O₃) = 0.1, 0.15) with different crystallinity on the hydration kinetics of OPC at early stages of hydration was investigated. Also, the formation mechanism of compounds during OPC hydration was highlighted. It was determined that the synthetic CAH accelerated the initial reaction and shortened the induction period. Also, the second and third exothermic reactions begun earlier, and, during the latter reaction, the higher values of the heat flow were obtained in comparison with pure OPC samples. At later stages of hydration, synthetic CAH affect the OPC hydration as the usual pozzolanic additives; moreover, the larger values of cumulative heat were reached. It should be noted that the nature of synthetic CAH samples accelerated the dissolution of gypsum and stimulates the earliest C₃S hydration.

The adsorption capacity of clinoptilolite for Zn²⁺ ions in alkaline solutions has been examined. It has been proved that in the alkaline solution the intrusion of Zn²⁺ ions into its structure depends on the metal ion concentration in the initial solution. Within 30 s of the reaction, ~78 % of Zn²⁺ ions were incorporated into the structure of clinoptilolite when the initial concentration of the solutions was 0.3 g Zn²⁺/dm³. The ion exchange reactions proceed more intensively in a solution with a 1.4 g Zn²⁺/dm³ concentration, because after 30 s of reaction ~96 % of Zn²⁺ ions are adsorbed. The variation of the pH value when using the alkaline solution was not considerable. Independently of the solution pH value, the cation exchange reactions are specific to the chemisorption process. X-ray diffraction and simultaneous thermal analysis results confirmed that the crystal structure of clinoptilolite remains stable.DOI: http://dx.doi.org/10.5755/j01.ct.63.1.4527

The adsorption capacity of clinoptilolite for Zn²⁺ ions in acidic solutions has been examined. Adsorption experiments were carried out at 25 °C in a thermostatic absorber by stirring 1 g of clinoptilolite in 100 ml of Zn(NO₃)₂ solution for 120 min. The initial concentration of Zn²⁺ ions was equal to 0.3 and 1.4 Zn²⁺g/dm³. In the acidic solution the intrusion of Zn²⁺ ions into the clinoptilolite structure was found to proceed by two types of chemical reaction: substitution and addition. During adsorption, ion exchange occurs between divalent ions and single-charged ions, because Zn²⁺ ions replace 68 % of Ca²⁺, ~13 % of Na⁺, and 0.5 % of K⁺ ions as the remaining part of these ions are combined by clinoptilolite according to the addition reaction. It has been proven that, independently of the solution pH value, the cation exchange reactions are irreversible. The X-ray diffraction and simultaneous thermal analysis results have confirmed that the crystal structure of clinoptilolite remains stable after the adsorption process.DOI: http://dx.doi.org/10.5755/j01.ct.63.1.4517