Durability Study of Low-Alkalinity Sulfoaluminate Cement Seawater–Sea Sand Concrete in a Marine Environment

Replacement of steel bar with fiber-reinforced polymer (FRP) bar in the preparation of seawater–sea sand concrete (SSC) can solve the steel corrosion problem and save natural resources. However, the strong alkaline in SSC degrades the performance of FRP. Reducing the alkalinity of SSC can provide a satisfactory service environment for FRPs, which is essential for improving the durability of FRP-SSC structures. In this study, adjusting the water-to-binder ratio (W/B), replacing ordinary portland cement with low-alkalinity sulfoaluminate cement, and adding mineral admixtures—including phosphogypsum (PG), fly ash (FA), and silica fume (SF)—were applied to reduce the alkalinity of SSC. The long-term axial compressive strength, pH, and swelling properties of the low-alkalinity sulfoaluminate cement SSC (L-SACSSC) immersed in seawater were investigated. The results show that a smaller W/B and the addition of mineral admixtures reduced the alkalinity of L-SACSSC. PG showed the most apparent effect since it promoted the hydration of L-SACSSC and consumed unhydrated calcium sulfoaluminate (C4A3S¯), but the rapid generation of large amounts of ettringite (AFt) induced swelling and cracking. The PG content was recommended to be controlled at 12% by weight, with an SSC compressive strength of 59.5 MPa and a pH of 10.7. The SO42− and Ca2+ in seawater accelerated the reaction of unhydrated C4A3S¯ in single-doped FA and SF L-SACSSC to form AFt, resulting in denser concrete internally. The alkalinity and compressive strength of the system varied slightly because of the high degree of hydration and low content of unhydrated C4A3S¯ of PG-doped L-SACSSC.