Calcination and Mechanical Activation of Australian Upcycled Waste Clays for Low-Carbon Concrete

Abstract

This study examines natural waste clays (in Australia) as supplementary cementitious materials (SCMs) for concrete. Seven mixed-layer waste clays were analyzed, investigating calcination and low-energy mechanical grinding as activation techniques. The influence of calcination temperature and the duration of calcination/mechanical activation on the chemical and physical characteristics of clays was investigated. Also, the reactivity of activated clays in blended cement systems was studied. All of the calcined clays achieved a comparable gain in strength to the control mix. Results highlight the optimal parameters for enhancing clay reactivity and complying with local Australian standards. The study revealed that in the absence of detailed clay characterization, a 750°C calcination temperature and a 1-h calcination duration can be used for Australian mixed-layer clays to achieve Grade 1 pozzolans with a strength activity index (SAI) ≥ 0.85 . Additionally, high-intensity grinding using a ring mill is a plausible alternative activation method to produce highly reactive clays (Grade 2 pozzolans with SAI > 0.75 ) consisting of lesser kaolinite content ( ≤ 5 % ). Increasing the calcination temperature above 600°C reduced the specific surface area, negatively impacting reactivity, even if amorphous content increased. The specific surface area and the total clay mineral content in calcined clay are proportional to the SAI. High specific surface area ( > 15    m 2 · g − 1 ) clays with low total mineral content ( ∼ 25 % ) can provide similar strength to a clay with high clay mineral content ( ∼ 50 % ) and low specific surface area ( < 15    m 2 · g − 1 ). In mechanically activated clays, the particle fineness is proportional to SAI.