Ángel Palomo

Alkali-activated fly ashes: A cement for the future

The alkali activation of waste materials (especially those coming from industrial and mining activities) has become an important area of research in many laboratories because it is possible to use these materials to synthesize inexpensive and ecologically sound cementlike construction materials. In the present paper, the mechanism of activation of a fly ash (no other solid material was used) with highly alkaline solutions is described. These solutions, made with NaOH, KOH, water glass, etc., have the common characteristic of having a very high OH 2 concentration. The product of the reaction is an amorphous aluminosilicate gel having a structure similar to that of zeolitic precursors. Temperature and time of curing of specimens together with the solution/fly ash ratio are some of the variables that were studied. These variables have been shown to notably influence the development of the mechanical strength of the final product. Mechanical strengths with values in the 60 MPa range were obtained after curing the fly ash at 85 8 C for only 5 h.

Characterisation of fly ashes. Potential reactivity as alkaline cements

A representative group of Spanish fly ashes has been characterised in order to determine its capacity for being alkali activated and give place to a material with cementitious properties. The characterisation studies have been carried out through chemical analysis, laser granulometry, Blaine, BET, particle size distribution, XRD and 29Si MAS NMR. Compressive mechanical strength test was used to determine the reactivity of the fly ashes as alkaline binders. The results obtained have demonstrated that all investigated fly ashes are suitable to be alkali activated. Additionally it has also been demonstrated that the key factors of their potential reactivity are: the reactive silica content, the vitreous phase content and the particle size distribution.

Chemical stability of cementitious materials based on metakaolin

The alkali activation of metakaolin is a way of producing high strength cementitious materials. The processing of these materials has been the subject of numerous investigations. The present paper describes the results of a research project initiated to study the stability of these materials when exposed to aggressive solutions. Prisms of mortar made of sand and alkali-activated metakaolin were immersed in deionized water, ASTM sea water, sodium sulfate solution (4.4% wt), and sulfuric acid solution (0.001 M). The prisms were removed from the solutions at 7, 28, 56, 90, 180, and 270 days. Their microstructure was characterized and their physical, mechanical, and microstructural properties were measured. It was observed that the nature of the aggressive solution had little negative effect on the evolution of microstructure and the strength of these materials. It was also found that the 90-day and older samples experienced a slight increase in their flexural strengths with time. This tendency was most pronounced in those samples cured in sodium sulfate solutions. This behavior may be related to the change in microstructure of the cementitious matrix of the mortars cured longer than 90 days. Some of the amorphous material present had crystallized to a zeolite-like material belonging to the faujasite family of zeolites.

Engineering Properties of Alkali-Activated Fly Ash Concrete

In this paper, results are reported from experimental research carried out on certain engineering properties of a new (portland cement-free) concrete made with alkali-activated fly ash. Lab tests were conducted to determine its bending and compression mechanical strength, modulus of elasticity, bond strength, and shrinkage. The results show that mortar and concrete made with portland cement-free activated fly ash develop a high mechanical strength in short periods of time, have a moderate modulus of elasticity, bond better to reinforcing steel, and shrink much less than ordinary portland cement concrete.

Calorimetric study of alkaline activation of calcium hydroxide-metakaolin solid mixtures

The products originated in the alkaline activation reaction of metakaolin (MK) in calcium hydroxide presence vary as a function of the concentration of the alkaline hydroxide that activates the solid mix. In order to study these reaction products, a series of MK+calcium hydroxide mixes were activated in 1:1 proportion, with different sodium hydroxide concentrations: 5, 10, 12, 15 and 18 M at 45°C during 24 h. The activation steps were examined through isothermal conduction calorimetry and the reaction products characterisation was carried out by means of chemical analysis and instrumental techniques (XRD, FTIR and nuclear magnetic resonance (NMR)). When the activator concentration is 5 M or less, MK activation in the conditions previously described occurs only to a minor extent and the main reaction product is CSH gel. If activator concentration is 10 M or higher, MK dissolution is very fast and the main reaction product is an aluminosilicate with high mechanical performance. In all cases, the formation of CSH gel as the secondary reaction product is also observed.

Alkali-activated cementitious materials: Alternative matrices for the immobilisation of hazardous wastes: Part II. Stabilisation of chromium and lead

Abstract This research focuses on the study of the stabilisation/solidification capacity of a cementing matrix, which has been made using alkali activation of fly ash, in the presence of toxic elements chromium and lead. Such capacity has been compared with that of Portland cement. Leaching tests carried out proved that this new matrix is able to stabilise and solidify lead in a very efficient way (analysed lead concentrations from leaching are in parts per billion). However, it does not present itself as efficient concerning chromium fixation since this element strongly disturbs the alkali-activation mechanism of the ash.

Influence of the starting kaolin on alkali-activated materials based on metakaolin. Study of the reaction parameters by isothermal conduction calorimetry

The properties of the product obtained through alkaline activation of metakaolin are directly influenced by the characteristics of the starting kaolin. For the study of this influence, a complete characterization of two spanish kaolins used to synthesised the material has been carried out. A JAF conduction calorimeter was used to follow metakaolin reaction with NaOH solutions. Calorimetric data were obtained isothermally at 45°C with variable solution/solid ratio and NaOH solution concentrations varying from 12 to 18 M.

Alkali-activated cementitous materials: Alternative matrices for the immobilisation of hazardous wastes: Part I. Stabilisation of boron

Boron is a nonmetal element that is present in nature in many kinds of minerals and in a lot of industrial products of public use. The importance of this element lies in the fact that boron compounds are present in some water streams from nuclear power plants (Pressure Water Reactor [PWR]) and that the boron-soluble salts modify Portland cement hydration retarding setting and hardening, and negatively affecting its durability characteristics. Thus, the main objective of this research was to study the efficiency against boron of the solidification systems based on alkali-activated fly ashes. Results show that boron does not significantly alter the hardening process of the new matrices. The presence of boron hardly modifies mechanical strengths of activated fly ashes and additionally, boron leaching tests indicate that this stabilisation/solidification system is more effective than traditional ones.

Railway sleepers made of alkali activated fly ash concrete

Actualmente se estima que las vias de ferrocarril a nivel mundial contienen aproximadamente tres billones de durmientes, de los cuales 400 millones son de hormigon. Conjuntamente, sobre el 50% de la demanda mundial por durmientes (estimada en 20 millones anualmente) fue por durmientes de hormigon; ademas, entre el 2% y 5% de los durmientes de hormigon son utilizados en reemplazar elementos danados. La durabilidad del hormigon, sin embargo, depende de muchos factores. En este contexto se debe hacer notar que el proceso de prefabricacion de elementos de hormigon industrial en planta difiere de la fabricacion in-situ, lo que les da a los primeros caracteristicas que los hacen destacar en terminos de durabilidad. En general, se sabe que el proceso de activacion de cenizas volantes permite obtener un material con caracteristicas cementantes similares a aquellas del cemento Portland. En realidad, las cenizas volantes activadas con alcalis es un procedimiento singular en el cual el residuo resultante de la combustion de carbon en plantas termoelectricas es mezclado con disoluciones alcalinas que curadas bajo ciertas temperaturas generan materiales resistentes. A diferencia del hormigon tradicional, este nuevo tipo de hormigon puede alcanzar altas resistencias en un periodo muy corto de tiempo (1 dia) ademas de desarrollar una durabilidad excelente

Factors affecting early compressive strength of alkali activated fly ash (OPC-free) concrete

Este trabajo presenta los resultados de una investigacion experimental llevada a cabo para evaluar las principales caracteristicas de un nuevo tipo de hormigon fabricado solamente con ceniza volante activada alcalinamente (AAFA); es decir, sin cemento Portland comercial (OPC). Los resultados de los ensayos realizados para determinar las propiedades especificas del hormigon fresco y el desarrollo de resistencias mecanicas mostraron que la mayoria de los factores que afectan al proceso de fabricacion y a las propiedades finales de los hormigones de cemento Portland (relacion agua/cemento, condiciones de curado, etc.) tambien afectan a la preparacion y calidad final de estos nuevos materiales. Tambien fueron estudiados otros parametros especificos de los hormigones de AAFA (la naturaleza y concentracion del alcali presente en el sistema) para determinar su papel en el proceso de fraguado y endurecimiento.