Long term engineering properties of fly ash geopolymer concrete

Geopolymer concrete produced using 100% fly ash as the main binder is a sustainable construction material capable replacing Portland Cement (PC) concretes. At present PC production generates 5-7% of anthropogenic CO2 emissions world-wide, but it has been estimated that this can be reduced by 26-45% by replacing PC with geopolymer concrete. In order to be confident of the real world application of geopolymer concrete and the environmental benefits it can provide, in depth knowledge is required to ensure that the fly ash used will provide acceptable long term performance from the concrete produced. A series of geopolymer concrete specimens were prepared using four different fly ashes and tested for compressive strength, flexural strength, splitting tensile strength and elasticity modulus up to one year. Microstructural development between 28 and 365 days in different geopolymers were assessed using state of the art techniques. The 365-day compressive strength, flexural strength, splitting tensile strength and elasticity modulus of four fly ash geopolymer concretes were ranged 28-88MPa, 3.9-6.3MPa, 1.8-4.7MPa and 10-29GPa, respectively. Compressive strength variation is attributed to degree of reactivity of the fly ash, which is primarily governed by the quantity of finer particles in the reactive amorphous phase and sodium-aluminosilicate (N-A-S-H) gel formation. The CaO in fly ash also reacts with alkali to produce calcium-aluminosilicate (C-A-S-H) gel which provides additional strength to the concrete. The tensile strength of geopolymer concrete is governed by the gel-aggregate bond strength. Micro-cracks formed in the concrete due to high temperature curing can negatively affect the gel-aggregate bond strength. Moreover, larger crack widths lead to discontinuity and hence the formation of a less dense microstructure. This results in lower density and elasticity modulus for geopolymer concrete.