Alkali Activation of Aluminosilicates – An Assessment of Fundamental Mechanisms

Date This project started on 01 October 2007 and ended on 01 September 2010

Status This project is Finished

Authors
Consejo Superior de Investigaciones Cientificas, SP
University of Aberdeen, UK
Objectives

As in all cementiteous systems, the chemical relationship between individual constituents (unhydrated and hydrated phases) during hydration defines the stability of phase assemblages and ultimately, the physical and mechanical properties of the product. Significant progress has been made in Core Project 1 and elsewhere in developing an understanding of phase relations in Portland cement systems and their impacts on properties. Now, Core Project 5 addresses the special case of aluminosilicate activation by highly alkaline solutions.

Aluminosilicates are widely available, naturally occurring materials. They are not cementitious in their own right but some can be activated to provide cementiteous products. These materials have attracted considerable interest in the search for sustainable cement binders because products can be demonstrated to have properties comparable to conventional cement systems; conventional cement manufacture involves the energy intensive and CO2-producing calcination of limestone.

However, if consistent performance of alkali-activated systems is to be addressed and exploited in constructional concretes, more emphasis must be placed on understanding the fundamental mechanisms which underlie the alkali activation process and how these can be optimised in real systems.

Core Project 5 seeks to investigate these basic mechanisms. Alkali-activated aluminosilicates are differentiated from Portland cement systems by their higher initial alkalinity and the absence of lime. This is already sufficient to define quite different hydration products relative to those found in Portland cements such that predictions made based on Portland cement chemistry are inappropriate.

However, the modelling approaches taken successfully in Core Project 1 are capable of translation to the activated aluminosilicate systems, and significantly, to hybrid Portland cement-alkali-activated systems, using thermodynamic data to be aquired in Core Project 5. This core project will investigate:

  • relative stability and compatibility of the different N-A-S-H gels and C-(A)-S-H via solubility studies
  • compatibility of gel phases with other cement hydration products, e.g. Ca(OH)2 (pozzolanic potential of zeolite precursors)
  • mechanisms underlying the acid-base chemistry of activation, using solubility/leaching studies under controlled conditions (varying alkali strength (pH), integrating kinetic factors)

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