Elucidating the reaction of seashell powder within fly ash cement: A focus on hydration products

Abstract

The use of shellfish waste as a raw material in cement and concrete production is recognized as a feasible approach. Similar to limestone, seashell powder (SP), which is primarily made up of calcium carbonate, has been identified as a promising supplementary cementitious material (SCM). However, the current understanding of SP's chemical interactions within aluminum-rich cement systems remains limited, often resulting in its classification as simply an inert filler. Therefore, this research seeks to clarify the contribution of SP to fly ash (FA) cement by investigating the hydration products. The investigation reveals that SP facilitated hydration albeit to a lesser extent compared to limestone. In cements incorporating SP, the AFm phases, characterized by carboaluminate formations, became predominant, and the calcium carbonate content diminished over time, indicating reactive engagement of SP. The addition of 5 % SP enhanced the polymerization of the C-(A)-S-H structure in FA cement. Moreover, the substitution of cement with SP elevated the effective water to cement ratio, leading to a reduction in the Ca/Si ratio of C-(A)-S-H, while the Al/Si ratio remained virtually unchanged. Analysis identified the presence of monocarboaluminate (Ca/Al/C atomic ratio of 4: 1.97: 4.39) and hemicarboaluminate (Ca/Al/C atomic ratio of 4: 2.25: 2.40) in FA cements with 5 % and 10 % SP additions, respectively. Nonetheless, incorporating SP negatively impacted the compressive strength of FA cement, underscoring the importance of using SP in moderation. Specifically, the 28-day compressive strengths of FA cements with 5 % and 10 % SP additions measured at 42.9 MPa and 40.2 MPa, respectively. Moreover, SP-FA cement offers significant environmental advantages. This research innovatively elucidates the reaction mechanism of SP within fly ash cement, offering vital insights into SP's impact on the hydration process and advancing the recycling of shellfish waste.