Browsing by Author "Guedes, Mafalda"
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- Physical, chemical and mineralogical properties of fine recycled aggregates made from concrete wastePublication . Evangelista, Luis; Guedes, Mafalda; De Brito, Jorge; Ferro, Alberto C.; Pereira, M. F.This paper assesses the physical, chemical and mineralogical characteristics of fine recycled aggregates obtained from crushed concrete waste, comparing them with two types of natural fine aggregates from different origins. A commercial concrete was jaw crushed, and the effect of different aperture sizes on the particle size distribution of the resulting aggregates was evaluated. The density and water absorption of the recycled aggregates was determined and a model for predicting water absorption over time is proposed. Both natural and recycled aggregates were characterized regarding bulk density and fines content. Recycled aggregates were additionally characterized by XRD, SEM/EDS and DTA/TG of individual size fractions. The results show that natural and recycled fine aggregates have very different characteristics. This should be considered in potential applications, both in terms of the limits for replacing amounts and of the rules and design criteria of the manufactured products. (C) 2015 Elsevier Ltd. All rights reserved.
- Reaction path in the mechanosynthesis of calcium phosphates using a biogenic calcium sourcePublication . Ferro, Alberto C.; Seixas, Tomás; Guedes, MafaldaCalcium phosphates were produced by mechanosynthesis, using eggshell, H3PO4 and 6-83 wt%-H2O. Mixtures were milled at 600 rpm during 0.5-12 h. Resulting calcium phosphates occur within specific milling domains; H2O concentration determines onset and stability of monophasic hydroxyapatite. Within the tested conditions, brushite is the first detected calcium phosphate precipitated from solution. In 6, 40 and 56 wt%-H2O, monetite intermediates brushite's transformation to hydroxyapatite; in 71 and 83 wt%-H2O transformation is direct. Hydroxyapatite formation is favoured at the H2O extremes tested, 6 and 83 wt%. CO2 build-up in the confined jar and nucleation-and-growth events during drying are possible obstacles upon control of morphology and composition of synthesised particles. The potential of chicken eggshell as direct biogenic source and the ability of high-energy milling as corresponding processing route to produce calcium phosphates was demonstrated. This signals a route for reliable production of brushite, monetite and hydroxyapatite. A preliminary milling map was built, allowing to obtain desired final product under specific milling conditions.