Enhancing CO2 Mineralization and Mechanical Performance of Fly Ash-Based Alkali-Activated Mortar Using Calcined Seashell Ash

This study investigated the use of calcined seashell ash (CSA) as a Ca-rich modifier to enhance the mechanical performance and CO2 mineralization of high-calcium fly ash-based alkali-activated mortar. CSA was incorporated at 0–20% replacement levels, while liquid-to-binder (L/B) ratios of 0.35, 0.40, and 0.45 were investigated. Fresh and mechanical properties were evaluated through spread flow, initial setting time, compressive strength, and flexural strength tests. Mineral carbonation was assessed using accelerated carbonation depth, X-ray diffraction (XRD), and thermogravimetric/derivative thermogravimetric (TG/DTG) analyses. Increasing CSA content generally reduced flowability and shortened initial setting time, while 15% CSA produced the highest mean 28-day mechanical strengths. At this level, the 28-day compressive strength increased by 11.5–38.8%, depending on the L/B ratio, while the 28-day flexural strength increased by approximately 102–107% relative to the corresponding mixtures without CSA. Carbonation behavior was strongly influenced by the L/B ratio. At 20% CSA replacement, the carbonation-depth enhancement factor (CEFx) relative to the corresponding mixture without CSA increased from only 1.2% at L/B = 0.35 to 13.2% and 16.6% at L/B = 0.40 and 0.45, respectively. XRD revealed substantially increased calcite-related diffraction features following accelerated carbonation, particularly in the CSA-containing mixtures, while TG/DTG showed a stronger carbonation-associated thermal response for the mixture containing 20% CSA at L/B = 0.45 than for the corresponding mixture without CSA. Together with the carbonation-depth results, these complementary observations consistently indicate that CSA enhanced CO2 mineralization through increased carbonate formation under accelerated CO2 exposure. However, the absolute amount of chemically bound CO2 was not directly quantified in the present study. Overall, CSA can function as both a partial fly ash replacement and a Ca-rich modifier, with its mineral carbonation response governed by the balance between the availability of Ca-bearing phases and CO2 accessibility within the hardened matrix.