Papers by Keyword: Ettringite

Abstract: Normal-weight mortars with quartz sand filler and lightweight mortars with expanded perlite filler were prepared from a lime-metakaolin-gypsum binder. Mortar test samples were cured in a humid atmosphere and also in the laboratory ambient conditions. Development of hardening processes was studied by X-ray diffraction, thermal analysis and SEM-EDS observation. Compressive strength, drying shrinkage, bulk density and thermal conductivity of the mortars were determined. The presence of gypsum binder in the mixtures enabled formation of ettringite that caused expansion and reduced drying shrinkage of mortars with 20% content of gypsum binder.

Abstract: Under the background of shrinkage cracking of cement and concrete, ettringite is regarded as the most effective expansion source because of its outstanding expansion characteristics. In order to study the expansion mechanism of ettringite, the growth process and formation conditions of ettringite in composite cement system were studied by means of SEM, MIP and ICP. The effects of pore structure and ion concentration of pore solution on the morphology and expansion properties of ettringite were also analysed in this paper. The results show that the pore structure of cement paste directly affects the expansion properties of ettringite. It is verified that there is no obvious linear relationship between the expansion rate of ettringite and its quantity. The concentration of SO₄^2- and Al(OH)₄^- in pore solution is the determinant of ettringite formation rate and quantity in composite cement system at the initial stage of hydration reaction. The change of crystallinity of ettringite will be directly caused by pH.

Abstract: The main objective of this study is to investigate the effect of cementitious capillary crystalline waterproofing (CCCW) material on the water impermeability and microstructure of concrete. The water impermeability of concrete covered with or without CCCW material was tested according to the Chinese standard GB 18445-2012. The results indicate that concretes coated with CCCW material showed much higher water impermeability than blank ones, and the ratio of water impermeability pressure between them reached 275. The samples obtained in various depths of hardened cement paste specimens with or without CCCW coating were analyzed through scanning electron microscopy (SEM) and thermogravimetry-differential scanning calorimetry (TG-DSC), to study the differences in microstructure and hydration products. The results present that after a 28-day standard curing, there were lots of ettringite crystals and CaCO₃ formed in the paste in 1 cm from the coating, but the action depth of the CCCW coating could not reach 3 cm. The ettringite and CaCO₃ is precipitated in the pore structure of cement matrix and filling the voids, which leads to the significant enhancement in water impermeability.

Abstract: The stability of ettringite as high-watery mineral is highly dependent on the ambient temperature. Under standard laboratory conditions, onset of decomposition of this phase occurs at temperature of 80°C already and the theoretical temperature of the complete decomposition of ettringite is 180°C. Ettringite decomposition can occur at significantly different temperatures under humidity conditions other than the laboratory ones. Within the work verification of the possibility of synthetic preparation of ettringite by direct addition of aluminum sulfate, Al₂(SO₄)₃·18H₂O, and calcium hydroxide, Ca (OH)₂, as an alternative method to the yeelimite hydration procedure was carried out. The stability of the resulting systems was examined in two different environments, namely in a laboratory environment and the environment of saturated water vapour. The phase composition of the samples was determined by X-ray diffraction (XRD) analysis, thermal analysis and scanning electron microscopy (SEM).

Abstract: The aim of the work was to investigate the influence of extremely low and high temperature on the course of hydration of cement stone compared to the reference sample hydrated at normal conditions. The work was structured in such a way that Portland cement was first subjected to standard technological tests. Next, sets of test prisms using cement paste were prepared. This samples were immediately exposed to saturated water vapor in three temperature environments: 5 °C, 20 °C and 80 °C. The course of the hydration process during the first 28 days was monitored by the determination of the phase composition by XRD analysis and thermal analysis, and mediated by means of determination of tensile strength after bending and compressive strength. The low temperature did not have a negative effect on the final properties of the hardened cement stone. On the other hand, high temperature accelerated the initial phase of the hydration process, but negatively affected the resulting mechanical parameters

Abstract: This paper deals with the verification of the possibility of preparing synthetic ettringite in a way of direct addition of aluminum sulfate and calcium hydroxide as an alternative to the hydration of yeelimite. The stability of the resulting system was studied using the X-ray diffraction (XRD) and differential thermal analysis (DTA) in two different environments, namely in a laboratory environment and the environment of saturated water vapour. The evaluations of X-ray diffraction analysis and thermal analysis show that in the ettringite sample stored in the laboratory environment, evaporation of molecular water and subsequent transformation of ettringite into metaettringite or monosulfate can occur over time. Conversely, exposition of saturated water vapour environment can lead to saturation of the system by free calcium ions due to the slow dissociation of calcium carbonate, and thereby increase the amount of ettringite at the expense of gypsum and aluminum amorphous phase. To verify the above, it is recommended long-term monitoring of samples exposed in monitored environments.

Abstract: Fluidized bed ash, which is the youngest industrial waste can be considered on the basis of research, a high-quality secondary raw material base which can be easily used in the production of Portland cements and, in general, other hydraulic binders. Regarding the direct effect of anhydrite on the course of hydrating mixed cement with the fluidized fly ash and the consequent behaviour of the originated cement stone, its reaction with clinker materials on the mentioned Aft (Al₂O₃–Fe₂O₃-–tri) phase is clear. The creation of AFt phases, if they originate additionally, due to their morphology, is accompanied by significant volume changes which may lead to deformation up to the destruction of the cement stone. Some foreign publications show the possibility of the transformation of this thermo-dynamically unstable mineral into the further mineral from the group AFt phase, which is thaumasite, Ca₃Si (CO₃)(SO₄)(OH)₆^.12H₂O.

Abstract: In order to investigate the early hydration behavior of rapid setting and hardening belite sulfoaluminate cements, the methods of X-ray Diffraction, Scanning Electron Microscope, Compressive Strength test and Setting Times test were used to identify and quantify the hydration kinetics and microstructure of this new-found cements in China. The results showed that the main mineral compositions of high belite sulfoaluminate cement clinker included calcium sulfoaluminate (4CaO·3Al₂O₃·CaSO₄), belite (2CaO·SiO₂), ferrite phase, free gypsum and free lime. It was found that not only the setting time and compressive strength but also the composition of hydration products were influenced by anhydrite to some extent. Meanwhile, a mass of AFt and AFm generated along with the hydration process at different ages, overlapped, crossed and penetrated through calcium silicate hydrate gel and aluminum oxide to form a relatively dense structure which could contribute to the high strength of cement.

Abstract: In this article the self-hardening structure of stone based on calcium sulfate formation is described. Increase of strength of gypsum is possible by additional reinforcing of a stone ettringite crystals. The form and character of the formed crystals is defined by size рН. Dependence рН from the maintenance of additives was investigated in work. The limiting factor of formation of crystals of an ettringite of a necessary look is the maintenance of an additive. In work gypsum composites with the improved physical-mechanical properties on the basis of the modifying complex are received and investigated.

Abstract: The pure ettringite, (Ca₆Al₂(OH)₁₂(SO₄)₃· 32H₂O), was synthesized by chemical reaction with saturation calcium hydroxide and aluminum sulfate. The ettringite in different environments including various concentration of sodium chloride, 5% magnesium sulfate and carbonation for 3 days were synthesized. And then these ettringtie specimens were prepared under different pre-loads. The static modulus and phase composites were tested. The experimental result shown that the influence of chloride ions concentration on the static modulus of ettringite could be ignored. And the magnesium ion reduced the generation of ettringite, and promoted the formation of gypsum. The static modulus of ettringite increased with carbonation time due to the formation of calcium carbonate.