Showing papers by "Ángel Palomo published in 2008"

FTIR study of the sol–gel synthesis of cementitious gels: C–S–H and N–A–S–H

Abstract: The study explored the compatibility between the main product of Portland cement hydration and the main product of the alkali activation of fly ash: C–S–H and N–A–S–H gels, respectively. Both gels were synthesized with laboratory reagents at different pH values. Blends of the two were synthesized as well, using the sol–gel procedure. All the gels were characterized with Fourier transform IR spectroscopy (FTIR). The gels synthesized with this procedure were shown to precipitate together with a silica-rich gel. In addition, the pH level was found to play a determinant role in both C–S–H and N–A–S–H gel synthesis. The C–S–H gel is the major phase formed at pH > 11 and N–A–S–H gel for pH > 12. The results relating to the joint synthesis of the two (C–S–H and N–A–S–H) gels were not conclusive. Technique used for the characterization failed to differentiate between them in the blended material.

New Cementitious Materials Based on Alkali-Activated Fly Ash: Performance at High Temperatures

Abstract: This paper reports on a comparative study of the mechanical performance at different temperatures of a commercial Portland cement, used as a control, and a new cementitious material made from an 8M-NaOH activated fly ash and containing no OPC. Two types of mechanical tests were conducted: (i) high temperature mechanical tests, to determine the strength and fracture toughness of the two materials between 25° and 600°C, and (ii) post-thermal treatment tests, to evaluate the residual strength after 1 h of exposure to different temperatures (200°, 400°, 600°, 800°, and 1000°C). In both cases, the results showed that the new cementitious material performed significantly better at high temperatures than the Portland cement control. Differential thermogravimetry (DTG)/TG, Fourier transform infrared (FTIR), and X-ray diffraction analyses were also conducted to analyze the mineralogical and microstructural variations taking place in the material as a result of high temperature exposure. The results of these tests were correlated with the mechanical behaviour observed.

Alkaline Activation of Blends of Metakaolin and Calcium Aluminate

Abstract: The present paper reports the preliminary results obtained and analyzed in the framework of a joint Spanish–Japanese project on the behavior of blends of alkali-activated metakaolin (MK) and calcium aluminate cement (CAC). In these experiments, the materials were activated with an 8M solution of NaOH, poured into molds, and subjected to brief thermal curing (2 or 20 h at 85°C). The hardened pastes were tested for mechanical strength and characterized for mineralogy and microstructure by a number of techniques (XRD, FTIR, BSEM, and MAS-NMR). The results showed that under the above conditions, the Al and Ca in the CAC were taken up into the aluminosilicate formed as the main product of the alkali activation of MK. None of the CAC hydration compounds (CAH10; C2AH8; C3AH6; AH3) normally formed were detected in any of the cases studied.

Activación alcalina de cenizas volantes. Relación entre el desarrollo mecánico resistente y la composición química de la ceniza

Abstract: Alkali-activated fly ash is the primary component of a new generation of high-strength, durable binders with excellent mechanical properties and durability (on occasion bettering traditional Portland cement performance). Moreover, development of these cements may contribute to mitigating CO 2 emissions, since the base material is an industrial by-product. The present study was conducted to determine the effect of the composition of the initial materials (SiO 2 /Al 2 O 3 and Na 2 O/Al 2 O 3 ratios) on the mechanical properties, nature and composition of the reaction products. The results obtained indicate that there is no linear relationship between these ratios and mechanical strength, but rather a series of optimal values above and below which strength declines. In the specific case of the ratios studied in the present paper, these values were: SiO 2 /Al 2 O 3 = 4.0 and Na 2 O/Al 2 O 3 = 1.0 (molar ratios).

Activación alcalina de metacaolín. Efecto de la adición de silicato soluble y de la temperatura de curado

Abstract: El objetivo del presente trabajo es determinar el efecto que la cantidad de silice soluble presente en la disolucion activadora (relacion molar SiO2/Na2O entre 0 y 0.69) y la temperatura de curado (85oC, 150oC y 200oC) ejercen sobre las propiedades fisico-mecanicas y las caracteristicas mineralogicas y microestructurales de los productos formados por activacion alcalina de metacaolin. Para ello se determinaron los valores de resistencia mecanica a compresion de los materiales objeto de estudio y se realizo un estudio mineralogico y microestructural de los mismos por DRX, SEM-EDX, 29Si RMN-MAS y porosimetria de mercurio.Los resultados obtenidos muestran que la resistencia mecanica del material guarda una estrecha relacion con la naturaleza, microestructura y composicion quimica de los productos de reaccion asi como con las condiciones de curado. En todos los casos se genera, como principal producto de reaccion, un polimero inorganico alcalino con propiedades cementantes (gel N-A-S-H) responsable, en mayor medida, de las propiedades mecanicas del material. Como productos secundarios se forman algunas zeolitas cuya proporcion y tipo (sodalita, zeolita A, faujasita, ?) dependen tanto de la naturaleza del activador como de las condiciones de curado. La presencia de silice soluble en la disolucion activadora conduce a la formacion de geles con una mayor relacion Si/Al y a la ralentizacion de la zeolitizacion, factores, ambos, con un efecto positivo sobre la resistencia mecanica. Con respecto a la temperatura de curado, existe un valor umbral, en funcion de la composicion del material, a partir del cual un incremento en dicha temperatura deja de tener efectos positivos sobre la evolucion de la resistencia mecanica.