The interface between natural siliceous aggregates and geopolymers

The reaction products as well as the formation mechanisms of alkali-activated binders, or geopolymers, have been studied intensively. However, the interface between mineral aggregates, such as sand and/or natural rocks, and geopolymers has not been studied. This paper reports the microstructure and the bonding strength (Mode I bending) of the interface between natural siliceous aggregates and fly ash-based geopolymers. It was found that when the activating solution that contained no or little soluble silicates, the compressive strengths of the geopolymeric binders, mortars and concretes were significantly weaker than those activated with high dosages of soluble silicates. The presence of soluble silicates in the initial activating solution was also effective in improving the interfacial bonding strengths between rock aggregates and geopolymeric mortars. No apparent interfacial transition zone (ITZ) could be identified near the aggregates if the systems were free from chloride contamination. Chlorides (KCl) was found to decrease,the interfacial bonding strength between the aggregates and the binders probably by causing g l crystallisation near the aggregate surfaces, which resulted in debonding. (C) 2003 Elsevier Ltd. All rights reserved.