Sustainable solution for soil cement stabilisation reinforced with fibres – monotonic and cyclic behaviour

The sustainability-performance paradox in soil improvement is addressed in this study by comparing geopolymer and Portland cement binders reinforced with polypropylene (PP) and sisal fibres. A multi-scale experimental programme was conducted, integrating monotonic and cyclic Unconfined Compressive Strength (UCS) tests, ultrasonic pulse velocity (UPV) measurements, and SEM/EDS microstructural analyses. It was found that geopolymer stabilisation yields a significantly denser matrix, dominated by sodium aluminosilicate hydrate (N-A-S-H) gels, resulting in a stiffness increase of up to 270% compared to cement. Superior chemical compatibility was exhibited by sisal fibres within the alkaline geopolymer matrix, with a 55.2% increase in strength being achieved. However, a critical performance gap was identified under cyclic loading: despite the high monotonic strength, brittle failure and rapid plastic strain accumulation were observed in geopolymer-stabilized soils within the first 2,500 loading cycles. Conversely, superior resilience to cyclic degradation was demonstrated by cement-stabilized samples through a strain-hardening mechanism, sustaining 5,000 loading cycles at deviatoric stress levels up to 75%. Optimal PP fibre contents were identified as 0.55% for cement (19.8% strength gain) and 1.10–1.65% for geopolymer composites (48.6% and 20,5% strength gain, respectively) to mitigate brittleness. These findings provide a strategic framework for the selection of binders and reinforcements based on the prevailing loading regime, monotonic vs. cyclic, and recommended geopolymers for high-load monotonic applications, such as foundations, and Portland cement for cyclic loading conditions like transportation subgrades.