Raphaele Lira Meireles Castro Malheiro

Carbonation Resistance of High Volume Fly Ash Concrete

The cement industry is responsible for a large part of the global environmental problems: is the largest consumer of natural resources; the most responsible for the emission of greenhouse gases, including about 1.8 Gt of CO2; and requires huge amounts of energy, corresponding to between 12 and 15% of industrial energy use. The cement is also not used in the most appropriate manner, since 40% of the consumption of concrete is due to the renovation and repair of buildings, making concrete structures inefficient because its durability is relatively low. However, in the future, concrete can and should evolve in order to improve its eco-efficiency, with a smaller amount of cement in its composition, replacing it with high quantities of mineral additions, particularly fly ash. Nevertheless, current technology may not allow this type of concrete to be very efficient, because its long-term durability may be compromised. In fact, with increasing dosage of pozzolanic mineral additions, alkali paste components are consumed in the reaction leaving it vulnerable to concrete carbonation which may compromise the passivation layer needed for steel rebar protection against corrosion. This article explores a promising approach to mitigate this problem, which consists in the careful addition of hydrated lime in the concrete composition, highlighting the synergy of its components, significantly enhancing its carbonation resistance. It is proposed, therefore, to manufacture a concrete with high volume of fly ash, low cement content and high service life period: an efficient and sustainable concrete. In this context, an experimental campaign was developed with the aim of characterization of pastes behavior with high fly ash content, in particular with respect to its durability. The results will be presented and properly analyzed.

Carbonation Front Progress in Mortars Containing Fly Ash Considering the Presence of Chloride Ions

The incorporation of fly ash (FA) in cementitious matrices have been frequently used in order to make the matrix more resistant to the action of chlorides. On the other hand, it is known that Ca (OH)2 existing in the matrix is partially consumed by the pozzolanic reactions, which facilitates the advancement of carbonation. Given that the combined action between carbonation and chloride penetration is a fact little known, we speculate about the behaviour of the matrix in this context. This study investigates the influence of the presence of chlorides on the carbonation in mortars with FA. Samples with 0% and 40% replacement of cement CEM I 42.5 R for FA were molded with water/binder 0.56 and 0.52 respectively. After 90 days of curing the specimens were subjected to cycles of immersion/drying for 56 days. Half of the samples was subjected to the following cycle: two days in a solution containing NaCl (concentration equal to 3.5 %); 12 days in the carbonation chamber (4% of CO2). The other half was: two days in water; 12 days in the carbonation chamber. Then, the development of carbonation was evaluated. The results indicate that the presence of chlorides influences the carbonation. The specimens submitted to the exclusive action of CO2 showed a greater depth of carbonation compared to that presented by the specimens subjected to combined action. This may be related to changes in properties of the matrix which may lead to further refinement of the pores and related to the presence of the salt that can lead to partial filling of the pores and the increase in moisture content .

Estudio experimental de la penetración de cloruro en componentes con doble capa de hormigón y mortero de recubrimiento

This work studied the influence of rendering mortar layer on chloride transport into concrete structures. Concrete specimens rendered with three different mortar mixtures and two rendering thickness were used. Five of the six faces of the specimens were coated with epoxy resin to simulate unidirectional flux. The specimens were subjected to weekly wetting and drying cycles in a sodium chloride solution for 49 days. At the end, total chloride profiles were obtained. Results show that rendering mortars influence chloride transport into concrete and this is more accentuated for less porous mortars and with higher cement content. There is also a chloride accumulation close to the interface mortar-concrete region, which is explained by the differences on chloride transport ability between mortar and concrete. Although mortars are more porous than concrete, they can represent an additional protection against chloride penetration into concrete.

Methodology for analysis of the reactivity of coal fly ash using selective dissolution by hydrofluoric acid

The use of some additions as cement replacement has played an important role in the amount of clinker reduction. Due to its huge worldwide availability one of the most important and recognised replace agent is the fly ash, but is also estimated that only 30 to 40% of fly ash is used, although there are some challenges to be overcome, such as the reactive potential quantification. Chemical analysis is typically determined by the oxides content and does not take into account the vitreous and crystalline phase ratio. Therefore, this paper describes in detail one selective dissolution method, based in the hydrofluoric acid attack, to the quantification of the vitreous phase (reactive) and crystalline phases (not reactive). Fly ash from Portuguese Pego thermoelectric power plant was submitted to different attacks. To confirm the method and its effects, chemical and physical analysis were performed, such as STA, XRF, XRD with Rietveld refinement, SEM, EDS and laser diffraction granulometry. The results can be used to quantify the potential reactivity of this type of fly ash. The best results were achieved with 1% hydrofluoric acid attack during 6 hours.