Durability of fly ash based-geopolymers: effect of crystallization on mechanical properties

Durability of fly ash based-geopolymers: effect of crystallization on mechanical properties

This paper investigates the effects of crystallization on the mechanical stability of geopolymers, a particularly important aspect for concrete applications with a view towards durability. Three typical geopolymers are synthesized with Class F fly ash (one blended with 20% slag), and cured under 80oC hydrothermal or ambient (25°C) conditions, for 90 days. The hardened geopolymers are assessed for compressive strength and modulus at different ages, and analysed by X-ray diffractometry (XRD) and Fourier transform infrared spectroscopy (FTIR). Under hydrothermal conditions, the fly ash-based geopolymers activated with a sodium silicate solution exhibit crystallisation trend after 28 days of curing. The phase change (or local microstructural modification) leads to a 10-20% loss of compressive strength at the age of 90d compared with the maximum values achieved at 7d or 14d. However, a 30-40% loss in compression elastic modulus is observed. The geopolymers activated with NaOH solution exhibit continuous strength increase during the hydrothermal curing period, but also show a 20-30% loss in compression elastic modulus at the age of 90d. In comparison, under ambient conditions, geopolymers exhibit a continuous strength increase in the period observed, either for specimens activated with sodium silicate or for those activated with NaOH solution. The compression elastic modulus decreases by 30% only at the age of 90d, when more ordered and/or zeolitic phases are formed. Addition of 20% slag does not alter the trend in variation of compressive strength and elastic modulus under the two curing conditions. The loss in strength and modulus shows a close relationship with the zeolite formation observed by XRD and FTIR in these samples, which may be further attributed to the change of the local pore structure in geopolymers.
These findings suggest that geopolymers at ambient temperature possess high stability. However, the crystallization potential, if it is assumed that a zeolite is the final structure of geopolymer gel, may cause a loss in compressive strength and modulus to a certain extent, which should be considered in a long-term view of the performance of the materials.

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