Durability of hemp fibre-reinforced fly ash-based geopolymers

The construction industry is a significant contributor to global CO2 emissions, largely due to the production of Portland cement. To address this environmental challenge, geopolymers – cement-free, inorganic, environmentally friendly materials formed by the chemical reaction between aluminosilicate-rich solid precursors and alkaline activators made from industrial by-products, have emerged as promising low-carbon alternatives. This study focuses on geopolymer composites used in climate areas where they are exposed to alternating wet and dry conditions and subjected to flexural stresses that demand improved energy absorption capacity. The objective of this PhD work was to examine and improve the durability of shorthemp fibre-reinforced fly-ash based geopolymer mortars, with a focus on improving their energy absorption capacity. In order to improve durability of hemp fibre-reinforced geopolymers, two methods are conducted separately: i) on composite level – composite treatment via accelerated carbonation to reduce a matrix alkalinity and reduce fibre degradation; ii) on fibre level – fibres NaOH- and hornification pre-treatments to reduce fibre/matrix debonding. The study shows that accelerated carbonation leads to a densification of the fibre–matrix interface, while simultaneously inducing embrittlement within the composite structure. This effect leads to increased stiffness, reduced deformation capacity, and is believed to increase fibre/matrix bonding. The carbonation process improves physical characteristics of the geopolymers, resulting in their higher density and lower water absorption. Additionally, it also enhances their compressive and flexural strengths. However, these gains accompany reduced energy absorption under flexural loading— resulting in a 20% decrease for the less compact, fly ash-based geopolymer mortar and a 17% reduction for its more compact fly ash/slag-based geopolymer counterpart. Pre-treatment of fibres using NaOH and hornification methods effectively clean fibre surfaces and reduces fibre water absorption capacity. This leads to improved dispersion within the geopolymer matrix and increased fibre/matrix interfacial bonding. These modifications resulted in notable enhancements in physical properties of hemp fibre-reinforced, fly ash-based geopolymer mortars—including increased density, reduced porosity, and lower water absorption. Regarding mechanical properties, the pre-treated fibre reinforced geopolymers exhibit higher compressive and flexural strengths, both before and after accelerated ageing through 10 wet/dry cycles. In terms of the energy absorption capacity, the NaOH-fibre pre-treatment leads to a consistent increase of up to 19%, both prior to and after accelerated ageing (consisting of 10 wet/dry cycles), whereas the hornification-fibre pretreatment leads to an improvement of 11% and 15%, prior to and after the same accelerated ageing, respectively. Additionally, fibre hornification leads to the geopolymer’s 9% higher energy absorption capacity after 1.5 years of controlled laboratory ageing. When comparing the two tested methods for improving the durability of hemp fibre reinforced geopolymer mortars, the fibres pre-treatments method (including NaOH- and hornification fibres pre-treatments) shows higher potential than composite treatment via accelerated carbonation. Of the two fibre pre-treatments methods, NaOH fibre pre-treatment results in higher improvements in geopolymer’s energy absorption capacity, consistently surpassing the performance of hornification-treated fibres, both prior to and after accelerated ageing. To further improve the durability of the natural fibre-reinforced geopolymers, future research should focus on developing hybrid strategies that simultaneously address fibredegradation in alkaline environments and reduce fibre/matrix debonding, using sustainable and cost-effective methods. These approaches should aim to further enhance energy absorption andoverall composite durability. Moreover, composite design must carefully consider additional parameters such as matrix compactness, activator alkalinity, chemical composition of the solidprecursor, and curing regime, as these factors critically influence porosity, pore wateralkalinity, and fibre/matrix bonding.