Browsing by Author "Puertas, F."
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Item C-A-S-H gels formed in alkali-activated slag cement pastes. Structure and effect on cement properties and durability(2014) Puertas, F.; Palacios, M.; Manzano, H.; Dolado, J. S.; Rico, A.; Rodriguez, J.; Tecnalia Research & InnovationThe development of alternatives to traditional Portland cement produced with more eco-efficient processes (lower energy consumption and CO2 gas emissions) is an item on climate change and innovation agendas. Alkaline cements and concretes are an effective alternative to traditional cements. The structure of the C-S-H gel in Portland cements consists mostly of 14-nm tobermorite (with a chain length of five) and jennite (2-link chain). The mechanical properties of C-S-H gels can be explained in terms of the three types of packing found in these gels: low density (LD), high density (HD) and ultra-high density (UHD). The main reaction product in alkali-activated slag (AAS) cements is a C-A-S-H gel, which adopts different structures depending on the nature of the alkaline activator. When the activator is a NaOH solution (4 % Na2O by slag weight), the C-A-S-H gel formed has an intermediate structure between 14-nm tobermorite with a chain length of five links and 11-nm tobermorite with 14 links. When the activator is a waterglass solution (4 % Na2O by slag weight), traits characteristic of 14-nm tobermorite with 11-link chains and 11-nm tobermorite with a chain length of 14 co-exist in the structure of the C-A-S-H gel formed. This densely packed structure (with three HD states) yields excellent mechanical properties. Like the C-A-S-H gels obtained in NaOH (4% Na2O)-activated AAS paste, the C-A-S-H gels forming in AAS gels activated with waterglass have no UHD states. The structure and composition of these C-A-S-H gels determine strength development in AAS mortars and concretes as well as their resistance to aggressive chemicals.Item A model for the C-A-S-H gel formed in alkali-activated slag cements(2011-10-15) Puertas, F.; Palacios, M.; Manzano, H.; Dolado, J. S.; Rico, A.; Rodríguez, J.; Tecnalia Research & InnovationFor first time, an experimental and computational study has been conducted to define a structural model for the C-A-S-H gel forming in alkali-activated slag (AAS) pastes that would account for the mechanical properties of these materials. The study involved a comparison with the C-S-H gel forming in a Portland cement paste. The structure of the C-A-S-H gels in AAS pastes depends on the nature of the alkali activator. When the activator is a NaOH, the structure of the C-S-H gel falls in between tobermorite 1.4. nm with a mean chain length of five, and tobermorite 1.1. nm with a mean length of 14. When waterglass is the activator the structure of the C-A-S-H gel is indicative of the co-existence of tobermorite 1.4. nm with a chain length of 11 and tobermorite 1.1. nm with a chain length of 14. This very densely packed structure gives rise to excellent mechanical properties.Item Synergy of T1-C3S and β-C2S Hydration Reactions(2011-04) Hernández, M. S.; Goñi, S.; Puertas, F.; Guerrero, A.; Palacios, M.; Dolado, J. S.; Tecnalia Research & InnovationThe hydration processes of T1-C3S and β-C2S and their mixtures 70-30 and 30-70 (% in weight) have been followed by conduction calorimetric studies. The synergy of the T1-C3S and β-C 2S hydration reactions, across their mixtures in proportions of 70-30 and 30-70, has been studied through the quantification of the portlandite carried out by thermogravimetric analysis. The hydration reactions and the quantitative evolution of all the components with the time have been studied during a period of 90 days at the room temperature of the laboratory. The microstructure was studied by scanning electron microscopy, the type of silicate tetrahedron, and mean chain length of the C-S-H gel by 29Si magic angle spinning nuclear magnetic resonance, and the porosity and pore-size distribution by mercury intrusion porosimetry. The main results showed that the hydraulic activity of the β-C2S strongly increased in the presence of T1-C3S, mainly at early stages, avoiding its inactive induction period. As a consequence of the increase of the hydraulic activity of the β-C2S, the pore microstructure is refined and the surface area is diminished.Item Textural and mechanical characterization of C-S-H gels from hydration of synthetic T1-C3S, β-C2S and their blends(2011) Goñi, S.; Guerrero, A.; Puertas, F.; Hernández, M. S.; Palacios, M.; Dolado, J. S.; Zhu, W.; Howind, T.; Tecnalia Research & InnovationThe textural and mechanical characterization of C-S-H gels formed from the hydration of pure T1-C3S, β-C2S and their blends are studied by Nitrogen sorption and nanoindentation experiments. The surface area and nanoporosity of C-S-H gels formed from the hydration of β-C2S and the 30-70 (T1-C3S and β-C2S mixture) are higher than those from hydration of T1-C3S, and 70-30, with the difference decreasing with hydration age. Such changes are well supported by findings of nanoindentation study, which shows the greater relative volume of C-S-H phases with lower densities in the β-C2S and the 30-70 pastes. With the increase in hydration age, the relative volume of C-S-H phases with higher densities increased at the expenses of those with lower density. Important quantitative correlations were found among these textural characteristics and the mean chain length, determined from 29Si magic-angle-spinning (MAS) NMR, of the C-S-H gels.