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Autor(a/res): Oscar Aurelio Mendoza Reales e Romildo Dias Toledo Filho.
Resumo: It is known that carbon nanotubes (CNT’s) modify the properties of cement based composites in fresh and hardened state; it is also known that these modifications are positive when appropriate dispersion of CNT’s within the matrix is achieved. Traditional experimental approaches used for fiber reinforced composites have been widely applied to study CNT’s-cement composites; nevertheless, high statistical dispersions and conflicting reports have been found to be a common issue due to the nanometric nature of CNT’s. This review presents a critical analysis of the most commonly used techniques to test CNT’s-cement composites, opening the discussion of the necessity of specific testing standardization for the development of the technology. Topics such as CNT’s dispersion and measurement of mechanical performance, electromagnetic properties and durability of CNT’s-cement composites are addressed.
It is concluded that the benefits of CNT’s in cement composites have been fairly identified, but the mechanisms by which nanotubes maintain dispersion within the matrix, interact with hydrating cement, and modify properties of composites are not yet fully understood. Understanding these mechanisms is considered of most importance to identify the limitations of CNT’s-cement composites and define which applications are achievable with the use of CNT’s. Better characterization of the interaction between CNT’s and hydration products at the nanoscale is required to develop more efficient composites, targeting the enhancement of multiple properties at the same time.
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Autor(a/res): Alex Neves Junior, Marcelo Mendes Viana, Sílvio de Cerqueira Mazza, Romildo Dias Toledo Filho & Jo Dweck
Resumo: A Brazilian coal power plant generates a waste composed by the fly and bottom ashes produced from coal combustion and by a spent sulfated lime generated after SO2 capture from combustion gases. This work presents a study of the early stages of the hydration of composites formed by this waste and a type II Portland cement, which will be used for CO2 capture. The cement substitution degrees in the evaluated composites were 10, 20, 30 and 40%, and the effect of the coal power unit waste on the hydration reaction was analyzed on real time by NCDTA, during the first 40 h of hydration. The results show that the higher is the substitution degree, the higher is the retarding effect on the cement hydration process. Actually, by respective thermogravimetric (TG) and derivative thermogravimetric (DTG) analysis on initial cement mass basis, this effect is caused by double exchange reactions among Ca and Mg components of the waste, during the first 4 h of hydration, which promote a much higher exothermic effect in the NCDTA curve, simultaneously to respective induction periods. The pozzolanic reactions, due to the presence of the waste silica and alumina containing amorphous phases, consume part of the original Ca(OH)2 content existent in the waste in the case of 30 and 40% substituted pastes, and also from part of the Ca(OH)2 produced in cement hydration reactions, in the case of the 10 and 20% substituted pastes.
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Autor(a/res); Alex Neves Junior, Romildo Dias Toledo Filho, Eduardo de Moraes Rego Fairbairn e Jo Dweck.
Resumo: Although the literature presents intensive studies based on monitoring cement hydration in adiabatic and semi-adiabatic environments, such as non-conventional differential thermal analysis (NCDTA) systems, studies of cement hydration in controlled climatic chambers are very rare. Using three W/C ratios (0.5, 0.6 and 0.7) and three relative humidity conditions (60, 80 and 100%) at 25 °C, the authors analyzed in real time the evolution of cement hydration reaction during the first 24 h in an environmental-controlled chamber. The main objective of this paper is to present two new developed systems of NCDTA (NNCDTA) and non-conventional TG and to show, using high-strength sulfate-resistant Portland cement pastes in a controlled chamber as application examples, how the developed systems measure on real time the thermal effects and the mass changes that occur during hydration and carbonation reactions. The captured CO2 mass can be quantified as it occurs by carbonation curves. The results are in agreement with the mechanical and structural properties of the used pastes and with their TG/DTG data, after being processed by different operational conditions. Carbonation for 24 h alters significantly the cement hydrated paste composition, resulting in final poor mechanical resistance properties. However, carbonation for 1 h, in specific conditions, enhances them when compared to a non-carbonated reference paste, due to a final higher content of silica and alumina hydrated phases and to a lower mass ratio between that of their combined water and their total mass as well.
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