Alexandre Silva de Vargas

Cinzas volantes álcali-ativadas com solução combinada de NaOH e Ca(OH)2

The main objective of this work was to evaluate over time the development of the compressive strength of alkali-activated fly ash mortars.xa0 A combined solution of NaOH and Ca(OH)2 was used as alkaline activator. The curing of the samples was carried out in two distinct ways. In the first one, samples were kept at 70°C during the first 12 hours after mixing and at 22°C afterwards, until the age of testing.xa0 Although compressive strengths of about 20 MPa were achieved during the first 24 hours, the strength started to decrease after 7 days. Considering this behaviour, a second method of curing was adopted. The mix proportions were the same as before, however the mortar samples were kept in an oven at 70°C until the age of testing. In this case, the compressive strength values decreased more significantly and in a shorter period of time, as compared to the results obtained for samples cured using the first method. Complementary studies were performed in order to better understand the observed reduction in compressive strength. Microstructural analyses of the fly ash, before and after the alkali-activation, have been carried out using Scanning Electron Microscopy (SEM) and Energy-Dispersive Spectroscopy (EDS). It was possible to identify three different morphologies in the alkali-activated pastes: one composed by regions with dense aspect; other showing partially solubilised particles of fly ash; and a third showing the formation of products with a needle-like shape. The information provided by the SEM/EDS experiments was not enough to enable the complete understanding of the mechanism of the deleterious reaction that took place among the alkaline activators and the fly ash. Therefore, future studies should address this question by using additional analysis tools in order to clarify this deleterious mechanism.

Study of Geopolymeric Binders of Fly Ash/Metakaolin Mixtures Cured at Room Temperature

Geopolymerization is a chemical process in which aluminosilicate materials are precursors to obtain binders that have a low environmental impact. Fly ash has been used as a precursor for the development of these binders. However, thermal curing is needed to accelerate the polycondensation of aluminosilicate, which limits the application of this new binder in the construction industry. Thus, the objective of this study was to evaluate the feasibility to obtain such binders with good mechanical properties when cured at room temperature. The precursor material consisted of different mixtures of fly ash and metakaolin that were activated using combined sodium hydroxide and sodium silicate alkaline solutions. The effect on the compressive strength of different proportions of the alkaline solutions was studied. Compressive strengths of about 40 MPa were achieved at 91 days for the samples containing 70% fly ash and 30% metakaolin, activated using an alkaline solution of 50% sodium hydroxide and 50% sodium silicate. X-ray diffraction analysis showed the formation of natrite in geopolymeric samples, as well as the presence of crystalline compounds, such as quartz, mullite and hematite, in fly ash and metakaoline. Scanning electron microscopy analysis showed that in geopolymeric mixtures with higher compressive strength dissolution of fly ash and metakaolin particles occurred almost completely and that aluminosilicate dense gel has been formed extensively.

Resíduos: a matéria-prima da indústria do futuro

Um dos grandes problemas ambientais da atualidade e o elevado volume de residuos solidos gerados pela industria para suprir a demanda por bens de consumo da populacao mundial. Neste sentido, a apresentacao deste artigo tem como objetivo apresentar cases praticos onde ocorreu a efetiva participacao dos autores na utilizacao de residuos industriais como materia-prima na producao de novos produtos, alinhados ao Eco-design. Explicita ainda a importância dos ensaios preliminares para a caracterizacao dos residuos, o detalhamento dos processos produtivos necessarios a sua incorporacao e a validacao dos produtos finais em relacao a sua aplicacao final.

Geopolimeros com base em cinzas volantes activadas com naoh y Ca(OH)2: caracterização microestrutural por rmn-mas 29Si y 27Al

In fly-ash alkali-activation technology, depending onxa0the combination of activators, deleterious reactions mayxa0result in a drop in resistance with age. This Studiy aimedxa0to evaluate the compressive strength of alkali-activated flyxa0ash from combined solutions of NaOH and Ca(OH)2. Threexa0molar ratio CaO/SiO2 (C/S) were studied: 0.05 (M5), 0.15xa0(M15) e 0.25 (M25). Resistances order of 21 MPa (M15) andxa019 MPa (M25) were reached at the age of 7 days. However,xa0after 28 and 91 days, the resistances were 8 MPa and 7 MPa, respectively. The M5 samples showed rising behavior of resistance with age. Thus, the drop in resistance has conditioned the C/S matrices. In the 29Si NMR spectra of the M25 sample Si has decreased species Q4 (3Al) and Q4 (2Al), those responsible for the mechanical properties of activated samples.

AVALIAÇÃO DO DESEMPENHO DE ARGAMASSAS DE ASSENTAMENTO MODIFICADAS POLIMERICAMENTE PARA ALVENARIA ESTRUTURAL

Structural masonry requires technological advances to equated the actual state of development constructions that usually operates with the conventional reinforced concrete structures. Evaluate different technological alternatives it is a necessity in order to improve the performance and mechanical properties of masonry. This study aims to use polymer-modified bedding mortars for structural masonry to increase the mechanical strength and ductility. For this purpose a standard mortar is used with addition of 0%, 5% and 10% of polyvinyl acetate (PVA) and copolymer styrene-butadiene rubber (SBR). Several tests to characterize the mortars, that include, in fresh condition (quantity of incorporated air) and hardened condition (dynamic modulus of elasticity, flexural strength, compressive strength and mercury intrusion porosimetry), were performed. It was also performed compressive strength tests in prisms ceramic blocks with the different types of mortars and adherence tests between block and mortars. The results show that the mortar with 10% PVA recorded the best mechanical performance. This mortar has an increased adherence to substrate and obtained the compressive strength higher than the value obtained with 0% polymer materials. nKeywords: structural masonry, bedding mortar, polymer.

Evaluation of the Mechanical and Environmental Behavior of Alkali-Activated Mortars Containing PU/EVA-Based Waste

The objective of this study is to evaluate the use of polyurethane (PU) coated textile wastes processed with ethylene-vinyl acetate (EVA) as lightweight aggregates in fly ash-based and alkali-activated metakaolin mortars, targeting the development of non-structural elements for civil construction. The PU/EVA waste was processed in three different proportions: 30/70, 50/50 and 70/30. Reference mortars were prepared using a 1:2 (fly ash + metakaolin: conventional construction sand) ratio, by mass. The PU/EVA-waste-containing mortars were prepared via partial replacement of natural sand with the waste in percentages of 10%, 20%, 30% and 40%, relative to the volume of the sand. The activators used in all mortars were NaOH and Na2SiO3. The mortars were cured at 80 °C for 21 hours and subsequently removed from the molds and maintained at room temperature until testing was conducted for compressive strength (at ages of 7, 28 and 91 days), leaching (7 days) and solubilization (7 days). Based on the results for compressive strength, the optimum PU/EVA content was 50% for a sand-to-waste replacement percentage of 40%. At an age of 28 days these mortars reached strengths greater than 2.5 MPa which, from a mechanical standpoint, allows for the production of non-structural elements for civil construction like sealing blocks, side walls or finish mortars. From an environmental perspective, no hazardous substances were detected in the leaching and solubilized extracts in any of the PU/EVA waste containing mortars.

Evaluation of the Influence of PU/EVA Waste-Based Lightweight Aggregates on the Physical Properties of Alkali-Activated Mortars

The objective of this study is to evaluate the effect of the relationship of waste polyurethane (PU) and Ethylene-Vinyl Acetate (EVA), as a replacement for conventional construction sand, on the physical properties of water absorption, bulk density and open porosity of fly ash-based and alkali-activated metakaolin mortars. By means of processing involving extrusion and milling, PU/EVA-based aggregates were prepared in three proportions: 30/70, 50/50 and 70/30. The reference alkali-activated mortars were prepared using a 1:2 (fly ash + metakaolin : conventional construction sand) ratio, by weight. The waste-containing mortars were prepared via partial replacement of construction sand with the PU/EVA waste in percentages of 10%, 20%, 30% and 40%, relative to the volume of sand. The activators used in all mortars were NaOH and Na2SiO3. The mortars were cured at 80°C for 21 hours. Based on the procedures of ASTM C 642, the water absorption, open porosity and bulk density of the alkali-activated mortars were determined at the ages of 7, 28 and 91 days. The open porosity and water absorption values increased as the amount of sand replaced with PU/EVA waste increased. The absorption values were found to be in the range from 8.0% to 17.00%. The values obtained for open porosity varied from 20.00% to 29.00%. With regard to bulk density, the behavior was reversed: increasing the amount of sand replaced by waste, decreased the bulk density value. The values ranged between 2.10 and 2.48 kg/dm3. Therefore, based on these results, mortars containing up to 40% PU/EVA have potential for use in the manufacture of non-load-bearing elements for civil construction.