Rheological advancement of activated clay concrete: comprehensive review

The prospective application of activated clays in civil engineering is a primary driver for implementing supplementary binders in concrete to reduce global carbon emissions. To achieve this goal, the study focuses on the synergistic rheological characteristics of activated clay blended cementitious systems in accordance with factors such as clay mineral type, content, physical properties of clay and activation mechanisms. The review identified that a 20% substitution of binder increased the yield stress in high grade kaolinites (75%–90% kaolinite), due to dominant effect of the clay minerals with respect to specific surface area. Furthermore, activated kaolinite at 45% substitution elevated the yield stress by 75% in concrete. To achieve rheological characteristics for mortar and paste, the review suggests a replacement level of calcined kaolinite in the range of 20%–25% replacement, when activated at 750 °C. Substituting 0.5%–1% montmorillonite with cement resulted in a rheopectic flow and an overall increase in thixotropy of 50%–75%. Intense milling hindered flow efficiency due to the formation of an interparticle network over a period of 35–40 min. In addition, the overall thixotropy of the activated clay-cement systems were found to be a cumulative effect of colloidal forces due to flocculation and particle hydration. The review also identifies future pathways related to the rheology of mechanochemically activated clays, especially illite and montmorillonite, in mortar and concrete systems.