Effects of Calcination Protocol on Performance of Phosphorus Mining‐Derived Clays as Supplementary Cementitious Material

Abstract

Upcycling industrial by‐products as supplementary cementitious materials (SCMs) represents a viable strategy for advancing sustainable cement production. This study investigates the valorization of non‐kaolinitic clay derived from phosphorous mining by systematically evaluating the effects of calcination peak temperature (550–850°C), dwell time (1–8 h), and specimen geometry on the properties and reactivity of calcined clays. For the first time, heat‐transfer effects during calcination are explicitly examined by comparing pellet and powder geometries subjected to identical thermal protocols. The results show that increasing calcination temperature and dwell time does not necessarily enhance reactivity. Moreover, excessive thermal input can induce partial recrystallization and reduce calcined clay reactivity. Optimal reactivity was achieved under moderate conditions (650°C for 2 h), where controlled heat transfer through the pellet geometry enabled uniform dehydroxylation while maximizing crystalline‐to‐amorphous phase transitions. Pellets exhibited substantially greater reactivity, ∼50% higher than powders and approximately four times that of the raw clays. When incorporated into cementitious systems, the calcined clay pastes achieved 7‐day compressive strengths ranging from 17.8 to 34.6 MPa, showing a strong positive correlation with their measured reactivity. At 28 days, all calcined clay mixtures exhibited good potential as SCMs, achieving strength activity index (SAI) values greater than 0.75. The framework provides an energy‐efficient pathway for optimizing calcination parameters by balancing thermal input, sample geometry, and reactivity to produce high‐performance calcined clays for sustainable cement applications.