Long term permeation properties of different fly ash geopolymer concretes

• Durability properties of geopolymer concretes improve from 28 to 365 days.
• Chloride diffusion coefficient of geopolymer concretes reduce from 28 to 365 days.
• Continuing gel production densifies microstructure and pore-structure over time.
• A high quantity of meso-pores in the gel paste increases water absorption.
• A high quantity of macro-pores leads to an increase in water and air permeability.

Geopolymer is a sustainable construction material produced by the activation of fly ash using a high concentration alkali to initiate a polymerisation reaction. A key parameter in determining the potential adoption of geopolymer concrete in the construction industry is the long term durability of the material. To determine the durability characteristics a detailed investigation of the permeation properties of four different fly ash geopolymer concretes was carried out up to one year of age. An improvement in the durability properties is observed for all geopolymer concretes with time. This is attributed to an on-going geopolymerization which results in continuing gel formation leading to a more densely packed microstructure, with an associated reduction in meso-pores and macro-pores. The packing density coupled, with the pore size distribution, were observed to determine the permeation and diffusion characteristics of the concrete. The increased in meso-pores represents the increase in the gel of the matrix and in turn this affect the increase of water absorption. On the other hand, a high quantity of macro-pores leads to an increase in the water and air permeability of geopolymer concrete. A large quantity of coarse particles in fly ash results in an uneven gel distribution which reduces pore-filling ability, while the presence of a high quantity of CaO was observed to contribute to a densely packed microstructure. Notably the initial chloride diffusion coefficients are analogous to those observed in Portland and blended cement concretes and also decrease with the age in a similar manner.