Showing papers by "Francisca Puertas published in 2015"

Alkali activated slag cements using waste glass as alternative activators. Rheological behaviour.

Abstract: The purpose of this study is to investigate news activators in the preparation of alkali-activated materials (AAMs) alternative to Portland cements by reusing waste glass. Alkali-activated blast furnace slag (AAS) constitutes an alternative to Portland cement due to high energy and environmental pollution associated with industrial Portland cement. Moreover, alkali activated materials offer a series of higher properties than ordinary Portland cement (OPC), such as better strength and durability behaviour. However, the rheology of these materials has been much less intensely researched. The present study aimed to study the effect of waste glass as activator and as replacement of blast furnace slag on the rheological behaviour of AAS pastes, with a comparison between the rheological parameters and fluidity of these pastes to the same parameters in standard cements (CEM I and CEM III/B). The findings show that AAS paste behaviour of rheology when the activator was a commercial waterglass solution or NaOH/Na2CO3 with waste glass was similar, fit the Herschel-Bulkley model. The formation of primary C-S-H gel in both cases were confirmed. However, the rheological behaviour in standard cements fit the Bingham model. The use of the waste glass may be feasible from a rheological point of view in pastes can be used. © 2015 Sociedad Espanola de Ceramica y Vidrio. Published by Elsevier Espana, S.L.U. This is an open access article under the CC BY-NC-ND license

Durability of Alkali-Activated Slag Concretes Prepared Using Waste Glass as Alternative Activator

Abstract: Concrete is by far the most used building material in the world, but is facing a large environmental challenge due to its cement content. The production of portland cement is responsible for 5 to 8% of worldwide CO₂ emissions. Waste and supplementary cementing materials (SCMs) such as blast-furnace slag can be used as partial or total substitute for portland cement to avoid or to reduce this negative effect. This work explores the study of durability and the mechanical behavior of concretes using urban and industrial waste glasses as a potential alkaline activator for slag (AAS). The development of strengths and microstructure in the concretes activated with waste glass were also comparable to the parameters observed in AAS concretes prepared with conventional activators. Durability tests, including chloride penetration resistance, freezing-and-thawing resistance, carbonation, resistivity, and porosity, were conducted and the effect of the different parameters such as the activator type is discussed.

Reuse of urban and industrial waste glass as a novel activator for alkali-activated slag cement pastes: a case study

Abstract: With increasing environmental pressure to reduce solid waste and to recycle as much as possible, the cement and concrete industries have adopted a number of methods to achieve this goal. Alkaline cements and concretes exhibit highest strength and longest durability when activated with a solution of alkaline silicate hydrates (waterglass). To obtain these alkaline silicates, however, an aqueous solution of the proper proportion of carbonate and silica salts must be heated to a temperature of around 1300 °C. This chapter studies the possibility of synthesizing a new activatorlike waterglass from waste glass reuse through processes of solubility, and explores the feasibility of using this solution as a potential alkaline activator for blast furnace slag (AAS) cement pastes.

Decalcification of alkali-activated slag pastes. Effect of the chemical composition of the slag

Abstract: Portland cement decalcification and its effects on paste microstructure and mechanical strength have been widely studied. Decalcification in alkali activated slag (AAS) pastes is still not fully understood, however. The present study therefore explored the process in AAS cement pastes, accelerated by submerging specimens in concentrated ammonium nitrate solutions (NH4NO3) for 3–21 days to induce leaching. Two AAS pastes were prepared with slag of different origins (Spanish and Colombian) and chemical compositions. OPC pastes were used as a reference. The findings showed that decalcification has a more adverse impact on OPC than AAS pastes strength. BSEM/EDX and 29Si MAS NMR data nonetheless confirmed that Ca leaches out of C–A–S–H gels (formed in AAS pastes) to an extent that depends on the nature of the prime material. OPC pastes were shown to generate more silica gel with a very low Ca content (Q3 and Q4 units). Moreover, the higher the percentage of such units, the lower was mechanical strength. Decalcification in slag with lower MgO and higher Al2O3 contents leads to the formation of smaller amounts of silica gel. The resulting gel was more compact and stable due to more intense chain cross-linking a possible tri-dimensional structure.

Performance of composites with metakaolin-blended cements

Abstract: Nowadays the blended cements acquire the merit of high significance due to the thermal, energetic and ecological demands for ordinary Portland cement (PC) production. Metakaolin as a partial substitute of PC represents important pozzolana contributing to production of effective cement composites with high quality. Pozzolanic reaction of metakaolinite with PC in the presence of water is main reason of this statement. Comparison of three types of metakaolin sand (fineness below 60 μm) with different metakaolinite content (31–40 mass%) is presented in this study. The substitution of PC with metakaolin sand of the maximal metakaolinite content (40 mass%) leads to the highest compressive strengths of relevant composites. This is valid for composites with the highest substitution of PC by metakaolin sand in specimens (20 and 40 mass%). The best effectiveness of pozzolanic reaction is given especially by the highest consumption of portlandite which represents composite with the maximal metakaolinite content in metakaolin sand (40 mass%) and the higher substitution level of PC by metakaolin sand in specimens. This fact is connected with the improvement of pore structure parameters resulting in the pore structure refinement as well as permeability decreases. Both 29Si MAS NMR and 27Al MAS NMR spectra of metakaolin sands and respective composites confirm the most intense pozzolanic reaction in the case of metakaolin sand with the highest metakaolinite content (40 mass%). The results are properly supplemented by scanning electron microscopy (SEM) identifying the formed typical phases. The study has shown that metakaolin sand with reduced metakaolinite contents is also applicable as a pozzolanic addition to PC in the on-coming building practice.