ISSN 0004-881X

Comparative assessment of medium-term properties and performance of fly ash and metakaolin Geopolymer systems

by T.J. Brown*, K.K. Sagoe–Crentsil, A.H. Taylor
Sustainable Materials & Processes
CSIRO Manufacturing & Infrastructure Technology
PO Box 56, Highett, Victoria 3190, Australia

Abstract

This paper evaluates the medium-term performance of inorganic polymer (geopolymer) concretes derived from both fly ash and metakaolin precursor materials. The strength development and mineralogical changes of this class of concretes were evaluated over a three-year period, using an equivalent conventional Grade N40 (40 MPa) ordinary Portland cement (OPC) concrete as the reference product. The initial strength gain of the metakaolin samples was high, as the concrete reached 57 MPa at seven days compared to 11 MPa for the fly ash samples. However, the overall strength gain for fly ash samples at three years was about 64% higher than that of OPC concretes, and 8% higher than the corresponding metakaolin concrete. Also, binders used in all concretes fell well below the 0.1% expansion threshold in standard accelerated ASR tests.

After three years of curing, the early-age reaction products of inorganic polymer samples appeared to be mainly amorphous solid solutions of sodium potassium aluminium silicate structures. However, the X-ray diffraction (XRD) results suggested that the resulting compounds were gradually becoming more ordered with time. Clear similarities in respect of the position of the X-ray amorphous band—characteristic of the gel phase—could be established for concretes made with metakaolin and fly ash; although marginal differences were evident in the evolution of isolated crystalline peaks between the two formulations. These results indicate medium-term phase stability, suggesting that inorganic polymers may yet be viable candidate binders in specialized concrete applications, with predictable medium-term performance characteristics.

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