Effect of delayed addition of calcined clay on the rheology and performance of limestone-calcined clay cement (LC3) systems

Abstract

To mitigate bleeding and segregation caused by excessive superplasticizer, this study proposes a modified mixing strategy for limestone–calcined clay cement (LC³), in which calcined clay is added with delay (DCAS). The effects of DCAS on flowability, rheology, superplasticizer (PCE) adsorption, hydration kinetics, and mechanical properties were investigated. At a PCE dosage of 0.20%, DCAS increased the flow value by 42.2% and reduced the yield stress by approximately 47.9%. Although the total PCE uptake remained unchanged, the adsorption was redistributed—DCAS minimized over‑adsorption on calcined clay and promoted a more uniform coating of all solid phases, thereby enhancing particle dispersion and water‑film thickness. Isothermal calorimetry revealed a reduced initial heat‑release rate, a prolonged induction period, and an intensified deceleration‑stage reaction, with total heat evolution approaching that of the conventional sequence after about 55h. Accordingly, DCAS extended setting times. Compressive strength exhibited a consistent development pattern: lower values at 1 day, comparable results at 3 days, and slightly higher strength at 28 days. This trend is attributed to reduced early-age hydration followed by the enhanced pozzolanic reaction of amorphous aluminosilicate phases, which promotes the formation of C–(A)–S–H and aluminate hydrates at later ages.