Papers by Keyword: Thaumasite

Abstract: Portland-limestone cement materials are susceptible to sulfate attack at low temperature and high humidity, because such conditions facilitate the formation of thaumasite, detriment to the structural integrity of calcium silicate hydrates (C─S─H). In this work, the effect of the cation associated with sulfates, concentration of sulfate solution, and limestone content in cement, were thermodynamically simulated. MgSO₄ solution is of higher risk, degrading extensively the structural integrity of C─S─H. Although this phase is partially preserved under the effect of Na₂SO₄ and K₂SO₄ solutions, extensive expansion and thaumasite formation occur. The sulfate content of the corrosive solution and the limestone content in cement are the factors mostly intensifying the attack caused by MgSO₄ and Na₂SO₄/K₂SO₄ solutions, respectively.

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: AFt phases – mainly ettringite and thaumasite, as products of sulfate attack in concrete, can cause serious damage to cement binder. The second mentioned, thaumasite CaSiO₃∙CaCO₃∙CaSO₄∙15H₂O, is connected to lower temperatures. It is less common and less known. For further research of this mineral, it is fundamental to prepare thaumasite in clear form in laboratory conditions. The aim of this work was to try several possibilities of thaumasite preparation - firstly by burning of mixes consisting of stoichiometric proportions of particular components in thaumasite and secondly by method in sugar solution and in MgSO₄ solution.

Abstract: The DFT method was used for modeling a partial decomposition of the structure of the thaumasite mineral. The four models with a consecutive decreasing of water content were prepared (T12 – 100 %, T9 – 75 %, T6 – 50 %, T3 – 25 %) and corresponding decomposition enthalpies were calculated. The results showed a good agreement with available experimental data for the decomposition reaction of the thaumasite structure.

Abstract: In this work, synthesis procedures to obtain thaumasite and different influences on its formation were revised. Theoretical background is compiled in this paper from published sources with focus on synthesis of pure thaumasite in laboratory environment.

Abstract: Limestone filler and aggregates are used widely in cement production and concrete mixing nowadays, which could be connected with thaumasite formation, and lead to a lack of durability further. This work deals with the sulfate minerals including of thaumasite, ettringite and gypsum in two types of cement pastes containing 35% w/w limestone powder immersed in MgSO₄ solution at 5°C±2°C for 15 weeks by Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD). Two types of cements were used: (i) ordinary Portland cement (P·O), (ii) typeII Portland cement (P·II). Test results show that thaumasite is present in two types of cement pastes, amount of thaumasite as well as amount of portlandite reacted with external SO₄^2- in P·II cement paste are more than those in P·O cement paste. It indicates that P·II cement is more susceptible to thaumasite formation than P·O cement containing the same amount of limestone powder, and more gypsum formation could contribute to thaumasite formation possibly during the external MgSO₄ attack at low temperature.

Abstract: To establish a quantitative identification method of thaumasite, internal doping method was used to accelerate thaumasite form of sulfate attack (TSA). A cement-limestone powder paste doped 10% of magnesium sulfate was immersed in water at (5±2) °C, while the blank was in 10% magnesium sulfate solution (by weight). Paste corrosion products were analyzed by infrared spectrum (IR), thermal analysis, nuclear magnetic resonance (NMR), and X-ray diffraction (XRD)/Rietveld refinement methods. The results show that the thaumasite formation was successfully accelerated by internal doping method. A lot of thaumasite formed after 6-15 months low temperature sulfate attack with gray mud-like material. IR analysis is able to be used to analysis thaumasite qualitatively, and Rietveld refinement was suitable to quantitative analysis of thaumasite. A rapid quantitative identification method of thaumasite including visual inspection, IR spectrum and XRD/Rietveld refinement was also established which will improve the accuracy and rapidness of TSAs research.

Abstract: The effects of initial high humid air-curing, standard water-curing and sealed-curing on thaumasite form of sulfate attack (TSA) of cement based materials were studied. The erosion products after three years attack in 5% MgSO4 solution were analyzed by means of X-ray Diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR). Initial high humid air-curing can delay the TSA of specimens for CaCO3 generated by carbonation and filled in the pores of the specimens, forming a compacted surface and restricting the intrusion of SO42-. In comparison, specimens cured in sealed condition occurred TSA most serious resulting from that large amount of inter-defects in specimens for the lower hydration degree, and SO42- ions could intrude into specimens more easily.

Abstract: The samples were prepared to study the influence of pH on the formation and stability of thaumasite using Fourier transform infrared and X-ray diffraction to analyze the composition of the samples. The erosion solutions with the pH arranged from 9.5 to 13.5 prepared by sodium bicarbonate, sodium hydroxide and 5% sodium sulfate. The experimental data for three years has been given in the paper. The result of the research indicates that the value of pH at 10.5 is the optimum pH for the forming of thaumasite and when the pH comes up to 11.5, thaumastie can form but it will decompose in the solution.

Abstract: In this paper the effect of chlorides on the thaumasite form of sulfate attack in limestone cement concrete is studied. Concrete specimens made from ordinary Portland cement and two Portland limestone cements (limestone content 15% and 35% respectively) were prepared. After 28 days of curing the specimens were immersed in six solutions of various sulfate and chloride content and stored at 5oC. Visual assessment of the specimens, mass measurements and compressive strength tests took place for a period of 24 months. XRD method was used to identify thaumasite in the deteriorated parts of the specimens. All measurements showed that Portland cement concrete exhibits a lower degree of deterioration than Portland limestone cement concrete. Specimen disintegration was more severe, the higher the limestone contents of the cements and the higher the sulfate content of the corrosive storage solutions. Chlorides play an inhibitory role, delaying the deterioration of the concrete specimens. XRD analysis showed the presence of thaumasite at the deteriorated parts of the specimens after nine months of curing.