Article

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

01/2008; DOI:10.3989/mc.2008.v58.i291.101
Source: OAI

ABSTRACT 18 pages, 6 figures, 2 tables. [ES] Las cenizas volantes activadas alcalinamente constituyen la base de una nueva generación de cementos con muy interesantes propiedades mecánicas, adherentes y durables (a veces incluso mejores que las de los cementos Portland tradicionales). Adicionalmente el desarrollo de estos cementos podría contribuir a mitigar las emisiones de CO2 a la atmósfera, ya que el material base de los mismos puede estar formado por subproductos industriales. En la presente investigación se realizó un estudio para determinar la influencia de la composición de los materiales iniciales (ratios SiO2/Al2O3 y Na2O/Al2O3) en las propiedades mecánicas y en la naturaleza y composición de los productos de reacción. Los resultados obtenidos indican que no existe una relación lineal de dichas ratios con las resistencias mecánicas, sino que existen unos valores óptimos, por encima y debajo de los cuales las resistencias mecánicas disminuyen. En el caso concreto de las ratios estudiadas en el presente trabajo estos valores serian: SiO2/Al2O3= 4,0 y Na2O/Al2O3= 1,0 (relaciones molares) [EN] 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 CO2 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 (SiO2/Al2O3 and Na2O/Al2O3 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: SiO2/Al2O3= 4.0 and Na2O/Al2O3= 1.0 (molar ratios). Proyecto BIA2006- 28530-E; y contrato I3P-PC2004L co-financiado por la Unión europea. Peer reviewed

0 0
 · 
2 Bookmarks
 · 
142 Views
  • [show abstract] [hide abstract]
    ABSTRACT: Few attempts have been made to understand the effect of the structural and surface properties of source materials on geopolymerization. In the present work, kaolinite, albite, and fly ash have been chosen for a case study to answer this question. X-ray diffraction, X-ray fluoresence, X-ray photoelectron spectroscopy, 27Al and 29Si magic-angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, and scanning electron microscopy (SEM) have been used to characterize the source materials and geopolymers. The fly ash that has an amorphous structure and possesses the lowest binding energies in its structure shows the highest reactivity during geopolymerization. Apart from the reactivity, the desired Si/Al molar ratio in the gel phases and the leaching of the source materials are observed to significantly affect the geopolymerization. The geopolymerization of mixtures of two or three source materials demonstrates the benefits derived from the interaction between the source materials. Such interactions could optimize the gel phase composition and result in high strength geopolymers. It is expected that the geopolymers containing Ca and K and having Si/Al molar ratios of about 2 in the gel phase could have a high compressive strength.
    Industrial & Engineering Chemistry Research - IND ENG CHEM RES. 03/2003; 42(8).
  • [show abstract] [hide abstract]
    ABSTRACT: Reaction products formed during alkaline activation, with 8M NaOH solutions, of fly ashes have been characterized with 29Si and 27Al magic-angle spinning nuclear magnetic resonance (MASNMR) spectroscopy. In particular, the influence of curing conditions (time and temperature of reaction) has been analyzed. NMR results show the formation of amorphous tecto-silicates in which the amount of aluminum decreases in the two consecutive formed phases. The Si/Al ratio of zeolite precursor obtained at 85°C changes from 0.95 to 1.86 when curing time is increased from 5 h to 1 week. The evolution of the mechanical properties of prepared cements has also been discussed in terms of the phases formed and texture and morphology of samples.
    Journal of the American Ceramic Society 07/2008; 87(6):1141 - 1145. · 2.11 Impact Factor
  • [show abstract] [hide abstract]
    ABSTRACT: This paper shows the effect of the nature of some alkaline activators in the microstructural development of thermal-alkali activated f/y ash systems. The alkaline compounds employed in this investigation were: NaOH, KOH, Na<sub>2</sub>C0<sub>3</sub>, K<sub>2</sub>C0<sub>3</sub>, sodium silicate and potassium silicate. Results confirm that the main reaction product of the activation process (throughout the studied systems) is the amorphous alkaline aluminosilicate gel with a three-dimensional structure already observed in earlier research. It has been proved that the type of anion and cation involved in the activation reaction of the ashes not only affects the microstructural development of the systems but the Si/Al ratio of that prezeolitic gel too. For example, in the presence of soluble silicate ions the content of Si in the final structure is notably increased (Si/Al =2.7-3.0), however carbonate ions play a different role since the formation of Sodium or Potassium carbonate/bicarbonate acidifies the system and consequently the reaction rate is considerably slowed. Finally it is evident that; when all experimental conditions are equal, sodium has a greater capacity than potassium to accelerate the setting and hardening reactions of fly ash and also to stimulate the growth of certain zeolitic crystals (reaction by-products). In general it can be affirmed that OH<sup>-</sup> ion acts as a reaction catalyst; and the alkaline metal (M<sup>+</sup>) acts as a structure-forming element. Este trabajo muestra el efecto de la naturaleza del activador alcalino en el desarrollo microestructural de sistemas de ceniza volante, activados térmica y alcalinamente. Los componentes alcalinos empleados en esta investigación fueron: NaOH, KOH, Na<sub>2</sub>C0<sub>3</sub>, K<sub>2</sub>C0<sub>3</sub>, silicato sódico y silicato potásico. Los resultados obtenidos confirman que el principal producto de reacción del proceso de activación (a través de los sistemas estudiados) es un gel de aluminosilicato alcalino amorfo con estructura tridimensional ya observada en trabajos previos. Se ha demostrado que el tipo de anión y catión involucrado en la reacción de activación de las cenizas no sólo afecta al desarrollo microstructural de los sistemas sino también a la relación Si/Al del gel prezeolítico. Por ejemplo, en presencia de silicatos solubles el contenido de Si en la estructura final aumenta notablemente (Si/Al = 2,7-3,0). Sin embargo la presencia de iones carbonato juega un papel diferente: la formación de carbonatos/ bicarbonatos de sodio o potasio acidifica el sistema y, por consiguiente, la velocidad de reacción es considerablemente menor. Finalmente, es evidente que, cuando todas las condiciones experimentales son iguales, el sodio tiene una capacidad mayor que el potasio para acelerar las reacciones de fraguado y endureciendo de las cenizas volantes y para estimular el crecimiento de zeolitas cristalinas (productos secundarios). En general puede afirmarse que los iones OH<sup>-</sup> actúan como un catalizador de la reacción, mientras que el metal alcalino (M<sup>+</sup>) actúa como un elemento formador de la estructura.
    Materiales de Construccion. 01/2006;

Full-text

View
2 Downloads
Available from