Material properties of F-concrete at high temperatures

F-cement is a binder for concrete based on finely ground granulated blast furnace slag activated with an alkaline admixture. The base material consists mainly of Ca-silicates of considerably lower CaO/SiO2-ratio than ordinary Portland Cement (OPC) clinker. It has a lower water requirement than pure clinker, it liberates little or no Ca(OH)2 during hydration and the liquidising effect of superplasticizers is much stronger than on OPC clinker. This leads to a different mineralogical and structural composition of the cement paste that significantly influences the thermal behaviour of the concrete. It has been necessary to investigate the properties of F-concrete also at hightemperatures to answer the question whether concrete members and structures madewith F-concrete indicate the same fire safety level as those made with OPC-concrete. This paper reports the results of investigations of the behaviour of F-concrete at elevated temperatures and parallel investigations of OPC-concretes of comparable mix design comprising thermal stability and strength and deformation behaviour. The thermal stability has been investigated concerning deterioration reactions bythe aid of DIA, alteration of the microstructure by the aid of N2-sorption and mercury porosimetry as well as thermal expansion. The mechanical properties of the concretes at high temperatures have been studied by determining the sigma, epsilon-relationships both with steady state and transient tests (transient strains and restraint forces), the latter simulating more realistically real fire conditions. Using the concept of "normalised transient strain" it is shown that the transient strain depends linearly on the load level up to an ultimate value. All the concretes behaved nearly in the same way up to a temperature of almost 350 °C. Owing to a different mineralogical and structural composition (especially low Ca(OH)2-content), the F-concrete indicates in many respects a more advantageous high temperature behaviour than OPC-concretes. Strong increase in transient strain and loss of strength occur first above 600 °C.