High Pressure Steam Curing of Reactive-Powder Mortars

The authors have studied the effects of autoclaving under saturated vapor at 180 degrees C on the physical and mechanical properties of reactive-powder mortars reinforced with brass coated steel fibers. The system consisted of portland cement (ASTM Type V), silica fume, natural silica sand (maximum grain size<1mm), an acrylic super-plasticizer, and brass coated steel fibers (L=13mm, theta=0.18mm); a water-to-cement of 0.255 was used to obtain a flowable system. Specimens were subjected to high pressure steam curing for 3 hours after preliminary curing at normal temperatures and for different times. Samples pre-cured at normal temperature for 24 hours and 3 days were autoclaved for up to 12 hours. Autoclaving generally produced beneficial effects on the mechanical properties, both in terms of flexural and compressive strength. High pressure steam curing for 3 hours of specimens pre-cured at ambient temperature for 3 days yielded flexural strength of 30 MPa and compressive strength of 200 MPa. The strengthening mechanisms depend only in part on the greater degree of hydration as the hydrated phases that form in the systems prepared with low water/cement are highly impermeable. The main effect appears to be the result of modifications to the microstructure that manifests itself as a reduction in porosity and hence in better mechanical properties.