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Journal of Thermal Analysis and Calorimetry
Authors: Isabel Galan, Carmen Andrade, and Marta Castellote

Abstract

Cement paste carbonation, i.e., the reaction between CO2 and the hydrated cement phases, mainly calcium hydroxide or portlandite, can lead to a pH decrease, which in turn can give rise to steel corrosion in reinforced concrete. At the same time, the carbonation reaction contributes to combine CO2 and fix it as calcium carbonate. It is a crucial phenomenon from the point of view of structure durability and also for cement-based materials sustainability. Cement paste specimens with two w/c ratios and eight types of cements were submitted to different environmental conditions for 4 years and the evolution of calcium carbonate formed or carbon dioxide bound was followed by TG performed in inert atmosphere. The amounts of calcium hydroxide, evaporable and C–S–H gel water were also measured. The CO2 bound follows the same trend in all samples and environments: at the beginning there is a sharp increase followed by a very slow stretch and reaching a maximum after less than 2 years in most cases. The calcium hydroxide amounts evolve very differently in each environment. While outside it is almost consumed after 1 year, inside there is a decrease in the first year, but an increase in the next 3 years. The behavior of the C–S–H water in both environments is similar to that of the portlandite inside. The evaporable water diminishes in all cases to 1 %. From the data obtained by TG, the quantification of the C–S–H gel as well as the calculation of the Ca/Si ratio and the hydration of the gel formed by different type of binders has been possible.

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Abstract

The hydration process of the cements induces the formation of different kinds of hydration products. The main products of hydration are C–S–H gel and portlandite [Ca(OH)2]. The C–S–H gel is an amorphous compound that is discomposed progressivity with the temperature until approximately 1,000 °C, while the portlandite is discomposed between 450 and 550 °C. Also, calcium carbonate can be formed as a consequence of the portlandite carbonation. All of these processes can be analysed and quantified by simultaneous differential thermal analysis and thermogravimetric analysis. And by X-ray diffraction it is possible to identify the crystalline phases. Some authors have corroborated that the portlandite can be rehydrated, after dehydration processes due to thermal exposition of the cement paste. But all of these experiments have been made with young cement pastes or at temperatures lower than 650 °C. In this work the behaviour of young and mature cement pastes have been studied in relation with the portlandite decomposition and the possibility of the rehydration of it in water presence. We found that young pastes and old pastes, stored at laboratory conditions, and later burned, show a certain grade of rehydration, specially the pastes burned at 650 °C (with ≈80% of reformation of portlandite) with respect to the pastes burned at 1,000 °C (between 20 and 40%). It is corroborate that the rehydration process is directly related to the formation of CaO during the burning. Also, a formation of unstable portlandite is detected in young pastes burned at 650 °C, which can be rehydrated easily. Although, the mature pastes that have been burned initially and stored under laboratory conditions cannot be rehydrated, due to the formation of stable products during the storage.

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Abstract

In field structures affected by fire, the temperature progress through the material. The progression of temperature in the concrete material can be determined by simultaneous differential thermal analysis and thermogravimetry. Also, the analysis of the behaviour of concrete in real concrete, by different techniques, permits the corroboration of the hypothesis of cover calculation. In this study, the analysis of concrete exposed to a very severe fire is studied in order to corroborate the calculus hypothesis and to determine the progression of the temperature inside the affected structure. In this study, the potentiality of the thermal instrumental techniques is studied to determine the situation of the concrete exposed to fire. These results can be used to calculate the residual strength of the concrete structural elements. Also, other auxiliary techniques are used to have some supplementary information about the situation of the concrete exposed to fire. The results are based in concrete samples from a real fire in the Windsor Building in Madrid. The Windsor Building in Madrid was project in 1974 and built between 1975 and 1979. This building was severely damaged by a serious fire on the 12th of February 2005, which lasted approximately 12 h.

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Abstract  

The most widely identified degradation process suffered by calcium aluminate cement (CAC) is the so-called conversion of hexagonal calcium aluminate hydrate to cubic form. This conversion is usually followed by an increase in porosity determined by the different densities of these hydrates and the subsequent loss of strength. Mixes of calcium aluminate cement (CAC) and silica fume (SF) or fly ash (FA) represent an interesting alternative for the stabilization of CAC hydrates, which might be attributed to a microstructure based mainly on aluminosilicates. This paper deals with the microstructure of cement pastes fabricated with mixtures CAC-SF and CAC-FA and its evolution over time. Thermal analysis (DTA/TG), X-ray diffraction (XRD) and mid-infrared spectroscopy (FTIR) have been used to assess the microstructure of these formulations.

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