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Autor(a/res): Alex Neves Junior, Mariana Santos Lemos, Romildo Dias Toledo Filho, Eduardo de Moraes Rego Fairbairn e Jo Dweck.
Resumo: This work complements a quantitative thermogravimetric study of the first 24 h of hydration of a high initial strength and sulphate resistant Portland cement (HS SR PC) using non-conventional differential thermal analysis (NCDTA) and Vicat needle method. Different water/cement (W/C) ratios from 0.35 to 0.85 were used to evaluate the most indicated operating conditions to maximize calcium hydroxide production for further use in CO2 capture. Thermogravimetric analysis data performed at 4 and 24 h of hydration were also compared to the NCDTA and Vicat data for each kind of paste, to analyze the influence of the W/C ratio on the simultaneous hydration and setting process. The increase of the W/C ratio increases the induction time retards the solidification and setting processes but increases the hydration degree as the W/C ratio is increased from 0.45. At 24 h, products prepared with 0.35 W/C ratio present a little higher hydration degree than those prepared with W/C = 0.45, because of the highest level of temperature in the reacting mixture in the former case, during the first 8 h. There is a practical limit of W/C = 0.66 to prepare the pastes, due to a limit of the miscibility between HS SR PC and water, above which, the excess of water forms a separated phase that does not interfere in the hydration process.
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Autor(a/res): João de Almeida Melo Filho, Flávio de Andrade Silva e Romildo Dias Toledo Filho.
Resumo: The kinetics of vegetable (sisal) fiber degradation and the mechanisms responsible for deterioration of continuous sisal fiber cement composites are presented in this paper. Two matrices were used: one with 50% partial cement replacement by metakaolin (PC–MK) and a reference matrix having as binder only Portland cement (PC). The durability performance of the composite systems is examined and the mechanisms for the significant delay in the fiber degradation when the total amount of calcium hydroxide is reduced from the matrix discussed. The composites were subjected to 5, 10, 15, 20 and 25 cycles of wetting and drying and then tested under a four point bending load configuration in order to determine the flexural behavior and cracking mechanisms with progressive aging. Furthermore, composites stored under controlled lab conditions were tested under bending load at ages ranging from 28 days to 5 years. Fibers extracted from the aged composites were subjected to thermal analysis, Fourier transform infrared spectroscopy and microscopical observations in order to evaluate the changes in chemical composition and microstructure. Two fiber degradation mechanisms were observed in the PC composites: fiber mineralization due to the precipitation of calcium hydroxide in the fiber cell and surface and degradation of cellulose, hemicellulose and lignin due to the adsorption of calcium and hydroxyl ions. The degradation process occurs rapidly and after 10 cycles of wetting/drying a quite expressive modification in the flexural behavior is observed. The residual mechanical parameters after 25 cycles were the same as those observed in the unreinforced matrix. For the PC–MK composite fiber mineralization was not observed due to the low content of CH in the matrix.
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Autor(a/res): Alex Neves Junior, Romildo Dias Toledo Filho, Eduardo de Moraes Rego Fairbairn e Jo Dweck.
Resume: During the formation of pastes, mortar and concretes have been used to capture CO2. This work presents a methodology to estimate the carbon dioxide (CO2) sequestered by high strength and sulfate-resistant Portland cement pastes during their early stages of hydration, by Thermogravimetry and Derivative Thermogravimetry. Water to cement ratio equal to 0.50 and 0.70 were evaluated and the captured CO2 amount was determined through TG/DTG curve data on initial cement mass basis, obtained during accelerated carbonation from the fluid state and accelerated carbonation after a first hydration process. The experiments were performed in a controlled chamber, maintaining the CO2 content at 20 vol % and the temperature at 25 °C, at different relative humidity (RH) (60 and 80 %) ambient. The procedure allows one to estimate the amount of CO2 sequestered by the initial cement mass of a given volume of paste, as well as to evaluate the RH and W/C ratio influence on the amount of hydrated formed products, mainly on the Ca(OH)2, important for CO2 fixation.
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