Understanding reactivity of calcined marine clay as a supplementary cementitious material through structural transformation of clay minerals

Compared to the fairly pure kaolinitic clay, dredged marine clay is a mixture of kaolinite, other 2:1 clay minerals (e.g. illite), and impurities like quartz. Upon thermal activation, the calcined marine clay emerges as a low-grade clay-type supplementary cementitious material (SCM). However, a thorough understanding about the underlying mechanism and key factors governing its reactivity evolution is necessary before its widespread application. In this work, the reactivity, physical properties, and mineralogical evolution of marine clay upon calcination between 650 °C and 900 °C were investigated, with particular emphasis on linking reactivity to the structural transformation of aluminosilicates in the clay. The reactivity arises from dehydroxylation and amorphization of kaolinite and 2:1 clay minerals with increasing proportion of disordered Al (4- and 5-fold coordination). After complete dehydroxylation (>650 °C), the structural disordering of the aluminosilicate continues to enhance with increasing amorphous content and more polymerised Q⁴ framework with Al substitution. However, condensed silica networks (Si-O-Si) forms at higher temperature may hamper the reactivity. Additionally, the specific surface area of marine clay decreases significantly above 650 °C, becoming relatively low at high temperature (>750 °C) as a result of structural rearrangement of the clay minerals and interparticle sintering. The highest reactivity is achieved at 750 °C. By exhibiting comparable compressive strength to reference Portland cement mortar at 28 days at 30 % replacement level, the calcined marine clay shows potential as a sustainable SCM alternative in low-carbon concrete production.