Effect of Dosage and Seawater Mixing on the Hydration and Phase Evolution of OPC-CSA Paste

Using fiber-reinforced polymer as reinforcement has opened unprecedented opportunities for the use of seawater in concrete production, particularly in arid regions, island nations, and water-stressed communities, providing potential economic and environmental benefits. This study investigated the effect of CSA dosage and seawater mixing on the hydration kinetics, setting time, phase assemblage, and compressive strength development of binary ordinary Portland cement and calcium sulfoaluminate cement paste using complementary experimental techniques, including isothermal calorimetry, Vicat needle technique, X-ray diffraction, thermodynamic modeling, and compressive strength testing. Increased CSA dosage in a deionized water (DI) mixed system enhanced the heat of hydration, increased initial ettringite formation, and accelerated renewed aluminate reaction, resulting in faster setting time. However, the heat intensity of the main (alite) hydration period was reduced, resulting in slightly reduced early-age compressive strength. On the other hand, seawater mixing slightly reduced initial ettringite formation, shortened the dormant period and promoted alite reaction. Consequently, the initial setting time of plain 100% OPC system (M1) was reduced while that of OPC-CSA systems (M2 and M3) slightly increased. Seawater also reduced the dissolution of ye’elemite, increased and stabilized ettringite, and delayed monosulfate formation due to the preferred chloride binding by AFm phases, resulting in the preferred formation of Friedel’s salt. Seawater mixing increased the early-age compressive strength of M1, M2, and M3 systems due to the promotion of alite reaction. However, the compressive strength reduced slightly at 28 days. The results of this study demonstrate the possibility of tailoring the early-age properties of OPC-CSA systems with seawater.