ACI Materials Journal, November 2018

Effect of Geopolymer Aggregate on Strength andMicrostructure of Concrete

by Chamila Gunasekera, David W. Law, and Sujeeva Setunge

Newswise — Researchers in the School of Engineering at RMIT University in Melbourne conducted a study investigating the mechanical properties and microstructure of a novel manufactured geopolymer coarse aggregate that uses low calcium fly ash. Overall, the geopolymer investigated in this research shows potential as a lightweight coarse aggregate for concrete, with the additional benefit of reducing the environmental impact of fly ash from coal-fired power generation.

In the study, testing included compressive and tensile strengths of concretes made with the manufactured geopolymer coarse aggregate and a comparative natural crushed coarse aggregate. In addition, the microstructure and pore structure development of both concretes at the interfacial transition zone (ITZ) and bulk cement matrix were studied though scanning electron microscopy and X-ray-computed tomography. The data showed that the novel geopolymer coarse aggregate satisfied the requirements of the Australian Standard, AS 2758.1:2014, “Aggregates and Rock for Engineering Purposes,” and is comparable to the results from the natural aggregate.

The manufacture of a geopolymer coarse aggregate using low calcium fly ash is a recent development. The technology for the manufacture of this geopolymer coarse aggregates uses novel techniques employing high pressure and reduced temperature production methods (Fig. 1). The reaction mechanism of the geopolymer coarse aggregate is similar to that of fly ash-based geopolymer concrete.

Results of the study showed that concrete made with the geopolymer aggregate achieved mean compressive strengths up to 40 MPa (Fig. 2(a)) and flexural strengths up to 4.5 MPs (Fig. 2(b)), with similar initial and final setting times to those of natural aggregate concrete. The dry density of the geopolymer aggregate concrete was 2140 kg/m3 compared to 2645 kg/m3 for the natural aggregate concrete (Fig. 3). Thus, the geopolymer aggregate could potentially save in construction costs due to the lightweight nature of the aggregate and, overall, shows potential as a lightweight coarse aggregate for concrete.

 If viable, the use of this manufactured geopolymer aggregate could lead to improved sustainable concrete design and a greener environment. Worldwide natural aggregate production is approximately 4.5 billion tonnes with demand increasing with the expansion of construction worldwide, such that natural aggregate reserves are depleting fast. Australia alone consumes about 130 million tonnes of aggregates annually. The production of 1 tonne of natural crushed aggregate emits between 7.4 and 8.0 kg of CO2. Furthermore, quarrying of the aggregate can have a detrimental impact on natural drainage patterns and slope stability due to the removal of trees and can lead to loss of natural habitat for the local fauna.   

The research can be found in a paper titled “Effect of Geopolymer Aggregate on Strength andMicrostructure of Concrete,” published by ACI Materials Journal.

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