Performance of mortar incorporating calcined marine clays with varying kaolinite content

This study is an attempt in exploring the feasibility of low-grade kaolinite clays, generated as waste from excavation works, as supplementary cementitious materials and potential candidate for cement replacement. Clays from five sources were investigated for composition, pozzolanic reactivity and effect on mortar strength and durability. Clays were grinded and calcined at 700 degrees C to convert kaolinite into metakaolin. Thereafter, 30% by wt. of cement was replaced with a combination of calcined clay and limestone in a 2:1 wt ratio. The pozzolanic nature of the clays was studied through thermo-gravimetric analysis (TGA), isothermal calorimetry (ITC) and X-ray diffraction (XRD). The effect on corrosion resistance was examined using electrical resistivity and rapid chloride penetration tests (RCPT). Results show that marine clays in Singapore in general have low to moderate kaolinite content (20-40%). In the ITC tests, blended mixes displayed accelerated hydration and 20-30% higher heat of hydration per gram of cement at 7th day compared to the reference mix without any substitution. The bound water per gram of cement calculated from the TGA results showed a noticeable increase in blended mixes, indicating additional hydration products in those samples. Though a 10-20% decrease in strength was observed in blended mortars compared to reference samples at 28-days, their corrosion resistance showed remarkable improvement. The charge passed during RCPT reduced by 20-60% while the electrical resistivity increased by 50-200% depending on the clay and its properties. Overall, the results were encouraging to suggest the use of low-grade marine clays as viable alternative to replace cement thereby producing a sustainable and low carbon concrete. (C) 2020 Elsevier Ltd. All rights reserved.