Durability of natural fibre reinforced geopolymer

: (Alternative Format Thesis)

Abstract

Portland cement/fibre composites (including asbestos as a fibre material) have been used in construction applications for decades; however, due to various concerns about these materials, alternatives are being actively researched. There is an increasing amount of research on geopolymer/fibre composites, and these novel materials could be used as alternatives to Portland cement-based composites in a variety of applications. Geopolymers are manufactured by reacting an aluminosilicate precursor with an activator and can be divided into two broad groups of alkaline-activated and the less frequently referenced acid-activated geopolymers. In this research, metakaolin is used as a geopolymer precursor, and the activators are composed of sodium hydroxide and sodium silicate for the alkali-activated geopolymer, and phosphoric acid for the acid-activated geopolymer. Like Portland cement, geopolymers exhibit good behaviour in compression but have low strength and display brittle behaviour in flexure, which induces tensile stresses.

This study presents a comprehensive evaluation of hemp fibre-reinforced geopolymer, focusing on mechanical performance, microstructural evolution, and environmental durability across diverse binder systems and exposure conditions. Thermal and chemical analyses revealed that mixing hemp fibres with geopolymers affected reaction kinetics in ways which were linked to binder chemistry.

Acidic environments induced severe fibre degradation via hydrolysis, while alkaline matrices caused partial dissolution of hemicellulose and pectin, moderately affecting tensile strength but preserving elastic modulus. Despite these degradative mechanisms, hemp fibres consistently enhanced flexural strength and ductility.

Environmental durability was assessed under freeze-thaw and wet-dry cycles, as well as varying relative humidity. Hemp fibres improved crack confinement and structural integrity under cyclic environmental conditions, while the addition of sand (up to 300% by mass of metakaolin) reduced porosity and further enhanced flexural performance. Fibre-free (plain) geopolymers degraded rapidly under wet-dry cycling, whereas fibre-reinforced composites maintained or improved strength.

Humidity sensitivity was pronounced in acid-activated systems, particularly those with high phosphoric acid activator concentrations, which exhibited near-zero strength under high humidity conditions. Lower acid concentrations offered greater resilience to high-humidity environments. Fungal growth on some samples raised concerns about biological degradation and indoor air quality.

Overall, hemp fibre reinforcement offers promising mechanical and durability benefits to geopolymer materials, with performance depending on the careful optimisation of binder composition, fibre content, and sand content. These findings support the potential of hemp-reinforced composites for sustainable construction applications, but further research is needed before these materials can be used with confidence.

Date of Award	20 May 2026
Original language	English
Awarding Institution	University of Bath
Supervisor	Antony Darby (Supervisor), Andrew Heath (Supervisor) & Xinyuan Ke (Supervisor)