Understanding the role of C-S-H seeding and sodium silicate solution on the properties of LC3 derived from low-grade clays

The use of limestone calcined clay cement (LC 3 ) is expected to increase to mitigate carbon emissions associated with concrete. However, reliance on highly kaolinitic clays limits LC 3 production in regions where lower-grade clays are abundant. This study investigates the effects of C-S-H seeding and sodium silicate on LC 3 prepared with low-grade London clay containing approximately 25% kaolinite. C-S-H seeding and sodium silicate modified the early particle network through different mechanisms. Their combined use produced the fastest structural build-up, with the storage-modulus growth rate increasing by nearly one order of magnitude. C-S-H seeding primarily accelerated early clinker hydration by providing additional nucleation sites, whereas sodium silicate promoted later-age metakaolin reaction through alkaline activation, increasing the estimated 28-day metakaolin reaction degree from approximately 54% to 62%. These combined effects enhanced the formation of C-(A-)S-H, AFt and AFm-CO 3 , increasing the 1-day strength of LC 3 containing C-S-H seeding and sodium silicate to 15-25 MPa across w/b ratios of 0.35-0.45. This corresponds to up to approximately 80% of the PC reference and up to four times the strength of the reference LC 3 at the same w/b ratio. At 28 days, LC 3 mixtures containing C-S-H seeding achieved strengths comparable to PC references and met the 52.5N strength requirement.