Clay mineralogy influence on cementitious phase development in calcined gravel wash fines cement blends

Gravel wash fines, a by-product of aggregate processing that contains mixed clay mineral phases, have demonstrated potential as an effective supplementary cementitious material upon calcination. Compositional analysis revealed that replacing >40% ordinary Portland cement (OPC) was hindered by a lack of portlandite (calcium hydroxide, CH), either due to cement dilution or being fully consumed by pozzolanic reactions, limiting the effectiveness of the calcined mixed clays as SCMs. Through the addition of CH at 0–8 wt% binder content, three calcined gravel wash fines (CGF), each having different dominant clay mineralogy, were shown to replace 40 wt% OPC in CGF-OPC blends with significantly increased calcium (aluminate) silicate hydrate and ettringite phase formation, as confirmed by infrared spectroscopy, thermogravimetry, electron microscopy and chemical analysis. Both kaolinitic/smectitic CGF and illitic CGF blends showed 28-day compressive strength improvements of up to 20% with 4 wt% CH added, while the strength of the smectitic CGF blend was enhanced by 42% at 28 days and 30% at 56 days with 8 wt% CH. Differences in strength development of the CGF-cement blends were attributed to differences in clay mineralogy: smectitic CGF A, a 2:1 clay, formed high-density C-S-H phases due to its higher silica availability, whereas CGF B (kaolinite 1:1–smectite 2:1) and CGF C (illite 2:1–kaolinite 1:1) incorporated both alumina and silica, leading to formation of low density C-(A)-S-H. These findings highlight that targeted CH supplementation can be an effective strategy for mixed mineral calcined clay SCMs for high cement replacement level, lower embodied carbon cementitious systems. • External CH added to 40 wt% mixed clay CGF mortars to offset portlandite deficit. • Smectite-rich CGF-A formed high density C-S-H cement product with added CH. • Kaolin-smectitic CGF-B and illite-kaolinitic CGF-C both formed C-(A)-S-H. • CGF-A gained 40% strength at 8% CH; CGF-B and C gained 20% with 4% added CH. • OPC replacement beyond 40% feasible without strength loss using external CH .