Impact of delayed addition time of SNF condensate on the fire resistance and durability of SRC–SF composite cement pastes

• 1.0% SNF with 7.5 min delayed addition time work together to produce a compact structure.
• The increase the content of SNF enhances the compressive strength and bulk density of cement pastes.
• Delayed addition time, enhance the compressive strength up to 800 °C.
• Formation of compact structure and enhances the durability of the cement paste.
• Delayed addition inhibits the accessibility of penetration Cl− and SO42- into the cement paste.

This work aimed to investigate the impact of delayed addition time (DAT) of sulphonated naphthalene formaldehyde (SNF) superplasticizer on the durability and fire resistance of SRC–SF composite cement pastes immersed in both 4% MgCl2 or 4% MgSO4 solutions as well as at elevated temperatures up to 800 °C, respectively. The results indicate that, the compressive strength and bulk density of cement paste increase with DAT at 7.5 min up to 90 days in tap water. The cement pastes admixed with 1.0 mass% of SNF condensate at delayed addition time 7.5 min, which cured in both 4% MgCl2 or 4% MgSO4 for 1 year or thermally treated up to 800 °C, has the highest durability and fire resistance. This is mainly due to that, the presence of superplasticizer with delayed addition process of mixing, reduces the water demand for standard consistency, decreases the total porosity, and increases the bulk density and consequently the compressive strength of cement pastes. These factors work together to modify the microstructure producing homogeneous compact closed structure. Total sulphate and chloride contents decrease with SNF content and DAT, due to decrease the accessibility of SO42- and Cl− to penetration into the pore system to form ettringite and chloroaluminate hydrate which expands and softens the cement paste matrix at later ages.