08-11-2012, 12:54 PM
Alkali-activated fly ashes A cement for the future
[attachment=38551]
Abstract
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
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.
Introduction
The alkali activation of waste materials is a chemical
process that allows the user to transform glassy structures
(partially or totally amorphous and/or metastable) into very
compact well-cemented composites. Nowadays, the knowledge
concerning the mechanisms controlling the alkali activation
process is considerably advanced; however, there are
still many things to investigate.
The first aspect to be emphasized in the general process
of alkali activation is related to the variability of the kinetics
and the governing mechanisms used to describe the mentioned
process. Actually, two different models of alkali activation
could be established, both corresponding to two very
different conditions of the starting situation. A very wellknown
example of the first model is represented by the activation
of blast furnace slags with a mild alkaline solution.
With respect to this case.
Discussion and conclusions
The alkali activation reaction of fly ashes takes place
through an exothermic process of dissolution during which
the breakdown of the covalent bonds Si-O-Si and Al-O-Al
in the glass occurs and ions (silicon and aluminium) pass
into the solution. The products generated due to the destruction
of the fly ash structure start to accumulate for a period
of time (induction period), during which heat release hardly
exists. Finally, a condensation of the structure is produced
(strongly exothermal step), which involves the appearance
of a cementitious material with a poorly ordered structure
but having a high mechanical strength.
The activation reaction is notably influenced by the parameters
studied in the present investigation. For example,
temperature for example is a reaction accelerator. Its effect
is so intense that the reaction steps overlap each other and
calorimetry cannot detect these steps separately. In general
terms, if all the factors remain constant, the temperature increase
tends to result in a gain of mechanical strength.
[attachment=38551]
Abstract
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
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.
Introduction
The alkali activation of waste materials is a chemical
process that allows the user to transform glassy structures
(partially or totally amorphous and/or metastable) into very
compact well-cemented composites. Nowadays, the knowledge
concerning the mechanisms controlling the alkali activation
process is considerably advanced; however, there are
still many things to investigate.
The first aspect to be emphasized in the general process
of alkali activation is related to the variability of the kinetics
and the governing mechanisms used to describe the mentioned
process. Actually, two different models of alkali activation
could be established, both corresponding to two very
different conditions of the starting situation. A very wellknown
example of the first model is represented by the activation
of blast furnace slags with a mild alkaline solution.
With respect to this case.
Discussion and conclusions
The alkali activation reaction of fly ashes takes place
through an exothermic process of dissolution during which
the breakdown of the covalent bonds Si-O-Si and Al-O-Al
in the glass occurs and ions (silicon and aluminium) pass
into the solution. The products generated due to the destruction
of the fly ash structure start to accumulate for a period
of time (induction period), during which heat release hardly
exists. Finally, a condensation of the structure is produced
(strongly exothermal step), which involves the appearance
of a cementitious material with a poorly ordered structure
but having a high mechanical strength.
The activation reaction is notably influenced by the parameters
studied in the present investigation. For example,
temperature for example is a reaction accelerator. Its effect
is so intense that the reaction steps overlap each other and
calorimetry cannot detect these steps separately. In general
terms, if all the factors remain constant, the temperature increase
tends to result in a gain of mechanical strength.