A comprehensive study of the effects of re-curing on macro- and microstructure of zeolite incorporating concretes after exposure to high temperature

Re-curing partly recovers the mechanical and durability properties of fire-damaged concretes. This inexpensive method can be used to reduce the cost of rehabilitation and probably prevent the demolition of fire-damaged concrete infrastructures. This paper investigates the macro- and microstructure of zeolite containing concretes after heating and re-curing. In this research, two mixtures including OPC concrete and zeolite concrete were studied. The specimens were heated up to 800 °C and then re-cured in moist air or water for 28, 56 or 90 days. Macrostructure testing including compressive strength, water absorption, surface resistivity and gas permeability was conducted on the specimens. A set of rehydration products including portlandite, C–S–H gel, ettringite, etc. regenerated during the re-curing period, which was identified by SEM and XRD analyses. These products improved the properties of the heated concretes, especially the OPC concrete. The recovery of properties was more pronounced in the water re-cured specimens. Formation of needle-shaped crystals in the zeolite concrete, which appears to be thaumasite-ettringite solid solution, led to a decrease in compressive strength at the early ages of re-curing. However, the durability properties were improved at the early ages of re-curing due to the expansion of these needle-shaped crystals during the formation. In the zeolite concrete, strength recovery occurred after 56 days of re-curing. Although the unheated specimens of the zeolite concrete had better mechanical and durability properties compared to the OPC concrete, the properties of the zeolite concrete were inferior after heating and re-curing in moist air or water.