CEMENT BRICK STRUCTURE FORMATION DEPENDING ON HARDENING TIME AND TEMPERATURE

Purpose: The aim of this paper is to explore the structural conditions of cement brick using the Rietveld refinement at various isothermal hardening. Investigations include the qualitative phase analysis, phase redistribution depending on hardening time and temperature, and identification of lattice parameters and phase composition of cement brick. Design/methodo­logy/approach: Due to a multidemensional process of concrete hardening and individual operating capabilities, it is expedient to study the hydration process, structure and phase composition depending on isothermal hardening of cement brick. As a result, mechanisms of a quick achievement of strength properties, age-induced critical damages, and lifetime of concrete products are identified. It is found that at a hardening stage these factors are more intensive. The grade CEM I 42.5B Portalnd cement without mineral additives was investigated. Cement paste was heated up to the initial isothermal temperature of 40, 50 and 70 °С respectively for 6, 6 and 9 h. The X-ray diffraction analysis was carried out by DRON-4-07 diffractometer which was adapted to a digital signal processing. Measurements were conducted using copper radiation (K_a) and Bragg-Brentano X-ray optical scheme. Specifications for the DRON-4-07 included 0.020 scanning step; 16–81° range for angles to be scanned; 30 kV voltage; and 25 mA current. Phase composition was investigated in a hardening cement brick in accordance with the accepted mode of thermal specimen treatment. 40, 50 and 70 °С heating lasted for 0, 3, 6, 19, 30, 43, 54, 67; 0, 3, 6, 19, 30, 43 and 0, 3, 17, 27, 41, 51 hours, respectively. The quantitative phase analysis with the Rieteveld method identified the  contribution of certain mineral lattices to the integral X-ray intensity. A non-linear least-square method was used to evaluate the difference between the integral and experimental X-ray intensities depending on various profile, structure and phase composition of lattices. The energy of major tobermorite and deliate phases was obtained using ab initio calculations. Research findings: The quantitative phase analysis for 40 °С hardening shows that tobermorite, deliate and O₂₀Si₄Ca₇ phases predominate in cement brick during 3 h hardening and further. Within 3–30 h hardening time, the amount of afwillite phase grows, and considerably decreases with time. After 67 h hardening, tobermorite, deliate, reinhardbraunsite and Ca₅H₂O₁₀Si₂ phases predominate, while afwillite phase is unstable. At 50 °С hardening, tobermorite, deliate, afwillite are major phases at the in initial stage. The amount of tobermorite, deliate, Ca₅H₂O₁₀Si₂ phases increases. The initial amount of Ca₅H₂O₁₀Si₂ phase decreases with time, and redistributes into Ca₅H₂O₁₀Si₂ phase. After 54 h hardening the amount of calcium hydroxide grows. Afwillite phase predominates at 70 °С hardening. Its amount is high prior to isothermal heating and non-uniform in cement paste. Further this amount decreases down to 26.76 %, while the amount of tobermorite, deliate, Ca₅H₂O₁₀Si₂ phases increases up to 20.33 %. The quantitative phase analysis with the Rieteveld method and experimental results indicate that the hardening time and temperature have a great effect on the content of hydrated silicates in the cement brick. It is shown that tobermorite, deliate and Ca₅H₂O₁₀Si₂ phases are major. Hydrated silicates, tobermorite and deliate appear at a saturation stage which is supported by results on the phase energies. Practical implications: Research results can be used for the improvement of existing technologies and the development of new ones concerning the time reduction of strength generation of different-type concretes applied in Siberia, the Extreme North, and the Arctic region. Originality/value: The obtained results will allow identifying a set of structural parameters and conditions for a quick achievement of the appropriate structural and mechanical properties of concretes. A multilevel investigation of different types of concrete with various Portland cement compositions is rather new. Theoretical rules on general laws of strength generation will underlie the scientific solutions on the production of products recommended for applications in Siberia, the Extreme North, and the Arctic region.

cement brick, Rietveld method, lattice parameters, crystal structure, quantitative phase analysis

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