Abstract
The cement industry accounts for approximately 7–8% of global anthropogenic CO₂ emissions annually, driving urgent demand for low-clinker binder systems that can deliver performance comparable to conventional Portland cement. Calcined clay has emerged as one of the most promising supplementary cementitious materials due to its exceptional global abundance and independence from industrial by-product streams. However, the majority of existing research has focused on high-kaolinite clay sources, while the performance of lower-kaolinite clays, which are more widely available and less costly to process, within Portland limestone cement (PLC)-based systems remains comparatively less characterized. Furthermore, the potential of combining low-kaolinite calcined clay with reclaimed coal ash (RCA) as a complementary third binder component to improve fresh and hardened performance has not been systematically examined.
This thesis investigates the fresh, mechanical, and durability performance of PLC mortar systems incorporating a low-kaolinite content calcined clay (CC), with a kaolinite content of approximately 30%, as the primary supplementary cementitious material. The experimental program is organized into two studies. The first study evaluates binary PLC–CC mortars at CC replacement levels of 20%, 30%, 40%, and 50% by mass of binder, assessing flowability, compressive strength development, bulk electrical resistivity, and rapid chloride permeability. The second study examines ternary PLC–CC–RCA mortars at a fixed total replacement level of 30%, systematically varying the CC-to-RCA ratio across five mixture designs and evaluating flowability, compressive strength, electrical resistivity, and sorptivity.
The first study demonstrated that a CC replacement level of 30% represents the optimal balance between mechanical and durability performance, with the 30% CC mixture achieving a 28-day compressive strength approximately 6% above the plain PLC reference. Bulk electrical resistivity and chloride penetration resistance improved substantially and monotonically with both CC content and curing age, with the highest CC mixture reaching a 90-day bulk resistivity more than six times that of the reference. The second study revealed that CC and RCA play distinct roles within the ternary system. Calcined clay remained the primary driver of long-term strength and transport-related durability, while the partial substitution of CC with RCA improved fresh workability through the ball-bearing effect of RCA’s spherical particle morphology and mitigated the early-age strength penalty associated with high CC replacement levels by reducing the proportion of slow-reacting calcined clay in the binder. The 2:1 CC-to-RCA blend emerged as the most balanced ternary design, recording the lowest early-age strength penalty, a competitive 56-day strength, and the lowest capillary water absorption of all mixtures, suggesting synergistic improvements in surface pore structure that neither binary end-member system achieves alone. These findings demonstrate that widely available, lower-reactivity calcined clay sources, combined strategically with reclaimed coal ash, can form the basis of practical and sustainable PLC-based binder systems without sacrificing mechanical or durability performance relative to conventional Portland cement systems.
