Reutilización de residuos vítreos urbanos industriales en la fabricación de cementos alcalinos: Activación, comportamiento y durabilidad

Abstract

This study was conducted against the backdrop of the need for alternative binders, i.e., other than Portland cement, made from industrial by-products such as alkali-activated blast furnace slag or fly ash. The mortars and concretes made from such binders are less environmentally harmful and entail lower energy demands than the respective Portland-based products, while exhibiting higher mechanical strength and durability. Solutions containing NaOH, Na2CO3 or sodium silicate hydrates (waterglass) are the alkalis most commonly used for these purposes. Of the three, sodium silicate hydrate generates alternative systems with the highest mechanical strength and durability. Waterglass manufacture, however, is CO2 emissionintensive and calls for high fusion temperatures. The question that should be posed, then, is whether recent research on alternative binders has delivered truly eco-efficient materials globally considered, or whether further study is required to develop activators other than the ones conventionally used with a view to minimising the environmental impact of the manufacture of sodium silicates. The ultimate aim of the research conducted on the occasion of this PhD. thesis was to find a way to improve the economic and environmental balance of alkaline cements by partially or wholly replacing traditional alkaline activators such as waterglass with urban and industrial waste glass. As an amorphous material with a chemical composition consisting essentially of SiO2 and Na2O, such waste may potentially form part of the waterglass family of alkaline activators. This primary aim was pursued through a series of partial objectives which sought to determine the conditions in which blast furnace slag and fly ash would be most effectively activated with the alternative waste glass solution. The first such objective was to ascertain waste glass solubility in highly alkaline media. That information was then used to establish the optimal conditions for dissolving SiO2 out of the glass and the resulting solution was applied to activate blast furnace slag and fly ash. The conditions defined were: mixed glass; particle size, <45 microns; solvent, for blast furnace slag, 50/50 molar NaOH/Na2CO3 solution (5 % NaO2 by slag mass) and for fly ash 10-M NaOH solution; magnetic stirring time, 6 hours; temperature, 80 ± 2 °C. These results were corroborated by statistical analysis. The aforementioned optimal conditions for waste glass solubility were applied to determine the feasibility of using such solutions as activators in the preparation of alternative pastes, mortars and concretes. In the study of the performance of these materials, their mechanical strength was found to be similar to the strength of materials activated with a commercial sodium silicate. Furthermore, characterisation of the main hydration products, C-A-S-H gel in the blast furnace slag system and N-A-S-H gel in fly ash, showed that the gels forming in the systems activated with the alternative solution were very similar to the ones present in the sodium silicate hydrate-activated materials. Lastly, durability studies based on exposure to aggressive media such as chlorides, carbonation and freeze-thaw showed that alkali-activated slag concretes performed as well as or better than conventional Portland cement concrete. The concretes activated with waste glass solutions delivered results comparable to those for conventionally activated materials, and proved to be even more resistant to freeze-thaw stress