Study on the performance of fly ash-based geopolymer fillers in composite linings of seawater conveyance tunnel

The rapid construction of nuclear power plants has increased seawater demand, with double-layer lining being the preferred choice for seawater conveyance tunnels. However, due to the effect of chloride salts, seawater seeps out from the joints of the inner steel layer and corrodes the steel bars in the outer lining. Adding a layer of filling material in the interlayer can block the seepage of seawater. The filling material is composed of fly ash, GGBS, water, and sodium hydroxide. Key properties including setting time, strength, permeability and durability were tested, along with NMR, XRD, SEM and FTIR analyses. Results indicate that higher sodium hydroxide and GGBS contents shorten setting time and increase UCS. When the GGBS content increases, the hydraulic conductivity first drops sharply and then stabilizes. The hydraulic conductivity of the filling material in the dry-wet cycle increases rapidly at first and then tends to stabilize. The microscopic test results show that the increase in the GGBS content leads to a denser pore structure and ultimately a decrease in the hydraulic conductivity. This study provides a reference for the application of GGBS-fly ash geopolymers as filling materials in seawater conveyance tunnels and reveals their anti-seepage mechanism.