Recycling of fly ash-slag Geopolymer concrete as aggregate in Portland cement mixes

Abstract

Geopolymers and alkali activated materials are deemed as the sustainable alternative to Portland cement-based binders. Despite extensive reference on the cradle-to-gate environmental benefits, end-of-life impact of geopolymers remains largely overlooked. The present thesis aims to address this by investigating the recyclability of fly ash-slag based geopolymer concretes as aggregates in Portland cement mixes. The relationship of matrix composition and recyclability was investigated by fabrication and testing of geopolymers to be recycled (Source Concretes), chemical and physical properties of crushed geopolymers (Recycled Aggregates-RA) and their effect on Portland cement mixes (Fresh properties and cement hydration, Mechanical Properties &Microstructure, Durability & Long-term Properties).

The Source Concretes comprised 3 fly ash-slag geopolymers with varying percentages of sodium and silica (S-Standard, HA-High Alkali, HS-High Silica) and one Portland cement mix (PC). Porosity of RA was mainly affected by crushing and the distinct carbonation mechanism of geopolymers, but not by mechanical and physical properties of Source Concretes. Ion Exchange Chromatography indicated leaching of Na⁺ above 150mg/l and K⁺, SO₄^-3and Ca⁺² below 100mg/l after 24 hours, depending on RA type.

Two series of concretes, slump categories S1 and S3, with 20% recycled aggregates were fabricated. Aggregate replacement by volume was found to provide more consistent mixes. The kinetics of cement hydration were not altered, but according to Vicat needle test presence of Na⁺ and Ca⁺² on leachate delayed setting by 2-3 hours. Overall the strength reductions exhibited from S3 concretes were within expected limits. Concretes with HA and PC RA exhibited similar strength development across series by having significantly low early strength. It appeared that leached elements Na, K and S were uniformly incorporated in the hydrated paste and SEM imaging revealed variety of C-S-H morphologies and hydrates at 7 days. No clear correlation between an RA type and specific hydrates or a distinct effect on strength was found though. The evidence strongly suggested that ITZ between the old geopolymer and the new Portland cement paste was the governing factor for concrete strength. No element migration took place at later stages through the ITZ. Porosity of concretes was not affected by RA. Concrete with 20% crushed HA geopolymer and glass aggregate did not show signs of alkali silica reaction after 15 months, when evaluated against a positive control mix and the concrete prism method threshold.

The findings of this thesis demonstrated recyclability of fly ash-slag based geopolymer concrete. The effects of recycled crushed geopolymers were comparable to those of Portland cement recycled aggregates. Further research on long term aspects and field testing is required.

Date of Award	22 Jul 2020
Original language	English
Awarding Institution	University of Bath
Supervisor	Juliana Calabria-Holley (Supervisor), Andrew Heath (Supervisor) & Kevin Paine (Supervisor)