Influence of high slag content on the basic mechanical properties and carbonation of concrete: Dissertation

The microstructural and basic mechanical properties, carbonation and permeability of slag concretes made from neutral (CaO/SiO2) and low alumina (Al2O3) blast furnace slags were investigated. Blended cement concrete and alkali activated slag concrete were studied by comparison with PC concrete. The main and general scope of this research is to improve the understanding of the behaviour of slag concretes. The special objectives of the experimental and theoretical study were as follows: - to examine and model the special features of the hardening properties of blast furnace slag - to examine the reasons for the cracking tendency in alkali activated slag concrete and to evaluate the effect of microcracks on the permeability and basic mechanical properties of concrete - to investigate the influence of blast furnace slag on the carbonation and permeability of concrete in different strength grades. The results indicated that in low strength grades the effect of binder type on carbonation is rather small. Possibly the open pore connections due to high water-binder ratios reduce the significance of binder type. In normal strength grades a significant difference prevails between GS and PC concrete, although the difference decreases with time. Possibly the low rate of hardening of slag enables the rapid drying of surface layers initially. In high strength grades the differences in carbonation depths in GS and PC concretes are rather small. Alkali activated slag concrete shows a number of narrow microcracks mainly in the paste aggregate interfaces. Apart from the cracks, the basic paste in alkali-activated slag concrete is considerably denser than that in the corresponding PC concrete. The microcracks in alkali activated slag concrete affect to a certain extent the carbonation and the water permeability of concrete. The hardening products of alkali activated slag are grainlike, but fibrous, needle like and crystalline products are virtually absent. Certainly, the strain capacity is very low in this kind of paste where the degree of intergrowth of the phases is low and crystalline bridges are almost absent. It is suggested that rnicrocracks are formed in the structure of concrete in the early stages of hydration due to the volume changes combined with hardening and drying. As the strength increases, the initiation and propagation of cracks are prevented. It seems that the strength of the dense basic paste is high enough to compensate for the effect of microcracks and to prevent the propagation of cracks under relatively high loading states. As the prevailing cracks begin to propagate, failure occurs rapidly. The phenomenon known as quasi ductility related to crack propagation in PC concrete seems to be less noticeable in alkali activated slag concrete than in PC concrete. The inhomogeneity of alkali-activated slag concrete resulting from the microcracks increases the size effect compared with PC concrete. High strength concrete with good workability properties in the fresh state can be produced from cement activated slag. The use of silica fume as a component brings a marked improvement to the properties of fresh concrete. The cementitious efficiency of blast furnace slag compared to that of rapid hardening PC depends on the water binder ratio of concrete. With low water binder ratios the cementitious efficiency of slag, as evaluated on the basis of 28-day compressive strength at normal temperature, is close to that of rapid hardening PC. With high water binder ratios the compressive strength of slag concrete at 28 days is markedly lower than that of the corresponding PC concrete. It seems that the packing and proximity of cementitious and other fine graded particles is of particular importance with regard to strength development, possibly because of the low degree of intergrowth of the hydrated slag phases.