Impact of metakaolin content and fineness on the behavior of calcined clay blended cements admixed with HPEG PCE superplasticizer

In this study, three calcined clays (CCs) possessing different metakaolin contents (23; 51 and 86 wt%) were investigated with respect to their pozzolanic reactivity, water demand and workability in the presence of an HPEG PCE superplasticizer. Furthermore, mixed CC samples were prepared by blending a specific CC of low metakaolin content (23 wt%) with pure metakaolin to achieve metakaolin contents of 30, 40 and 51 wt% in the final CC. Results on pozzolanic reactivity from the R3 test evidenced that increased metakaolin contents result in higher pozzolanic reactivity and improve 1d compressive strength. Whereas, higher metakaolin contents increased water demand significantly. Moreover, PCE dosages of up to 12 times were recorded. Furthermore, it was found that also fineness of the CC influences workability. Fineness mainly affects the water demand, while the metakaolin content plays a dominant role regarding PCE dosage. A mechanistic investigation revealed that in cement pore solution calcined clays initially exhibit a negative surface charge, but then sorb huge quantities of Ca2+ which enables PCE to adsorb onto the surfaces of CCs, as was evidenced by zeta potential measurements and from adsorption isotherms.