Fire resistance of concrete made with Portland and slag blended cements

Abstract

The fire resistance of concrete has become a major design concern due to various high profile fire incidents such as the collapse of the Twin Towers (September 11), USA and several tunnel fires around the world. Concrete although described as incombustible, undergoes physical and chemical transformations when exposed to elevated temperatures, as in a fire event. Above 400oC, one of the main hydrates of ordinary Portland cement (OPC) paste, calcium hydroxide (CaOH2) dehydrates into calcium oxide (CaO) causing the OPC paste to shrink and crack. After cooling and in the presence of air moisture, the CaO rehydrates into CaOH2 causing the OPC paste to expand, crack and completely disintegrate. However, the long-term effects of the CaO rehydration on the mechanical properties of OPC pastes are unknown and therefore, are investigated by the present thesis. In addition, the effects of elevated temperatures and CaOH2 dehydration/CaO rehydration on the microstructure and mechanical properties of concrete are still unclear. This issue has been of much debate and due to the conflicting nature of the available literature, is not fully understood. Therefore, this thesis investigates these effects on the microstructure and mechanical properties of OPC concrete. Furthermore, despite the continuous growing popularity of ground granulated blast furnace slag (GGBFS or ‘slag’) as a partial replacement of OPC in concrete, limited research has focused on how this replacement influences the microstructure and mechanical properties of paste and concrete exposed to elevated temperatures. Therefore, this thesis also addresses this issue. The study shows that partial replacement of OPC with slag resulted in a significant and beneficial reduction of the amount of CaOH2. An increase in the proportion of slag in the cement paste led to an improvement in the mechanical properties following exposure to temperatures beyond 400oC. The long-term effects of CaO rehydration on the mechanical properties of OPC and OPC/slag pastes exposed to 800oC were investigated using differential thermogravimetric analysis (DTG). Test results showed that CaO rehydration continued to take place throughout the period of one year, leading to a progressive deterioration of the OPC paste. After one year, the OPC paste completely disintegrated to a powder. In contrast, OPC/slag pastes were not affected by the progressive CaO rehydration as mechanical properties remained unchanged after one year. The study of the role of paste hydrates, rather than CaOH2, in the deterioration of mechanical properties of OPC and OPC/slag pastes was performed by nuclear magnetic resonance (NMR), X-ray diffraction (XRD), infrared spectroscopy (IR) and Synchrotron NEXAFS. Test results showed differences in the resulting calcium silicate hydrate (C-S-H gel) and aluminate phases of OPC and OPC/slag pastes after exposure to elevated temperatures. This indicates that the silicate and aluminate phases play a role in the higher degree of deterioration observed for OPC pastes when compared to OPC/slag pastes. The study of the effects of elevated temperatures on the mechanical properties of concrete revealed that OPC concrete heated to 800oC followed by exposure to air moisture presented strength loss of 65% while OPC pastes presented total strength loss and complete disintegration. This shows that the dehydration of CaOH2 and rehydration of CaO is significantly less detrimental for OPC concrete than it is for OPC paste. Techniques such as sorptivity tests and nitrogen adsorption were used to determine the differences in the CaO rehydration of paste and concrete. The rate of water absorption determines the growth rate of CaOH2 crystals during CaO rehydration and ultimately the type of CaOH2 crystals formed. Different rates of water absorption result in different CaOH2 crystal formation. This leads to differences in levels of deterioration, which not always result in total disintegration of the constraining body. In this study, the rate of water absorption of OPC paste and concrete was found to significantly differ. The test results revealed that the extent of the deterioration is not only related to the CaO rehydration occurrence, but most importantly, it is related to the rate at which rehydration occurs, i.e., the rate of water absorption. The rate of water absorption is the determining factor controlling the extent of deterioration caused by CaO rehydration.