Improved fire resistance by using slag cements

Concrete is a composite material that mainly consists of mineral aggregates bound by the matrix of hardened cement paste. Composition and microstructure of hardened cement paste have important influences on the properties of concrete exposed to high temperatures. An extensive experimental study was carried out to analyse the post-heating characteristics of concretes subjected to temperatures up to 800 A degrees C. Major parameters of our study were the slag content of cement (0, 16, 25, 41 or 66 m%) and the value of maximum annealing temperature. Our results indicated that (1) the number and size of surface cracks as well as compressive strength decreased by the increasing slag content of cements due to elevated temperature; (2) the most intensive surface cracking was observed by using Portland cement without addition of slag. The increasing slag content of cement increased the relative post-heating compressive strength. Tendencies of surface cracking and reduction of compressive strength were in agreement, i.e. the more surface cracks, the more strength reduction.