Studies On Thermal Expansion Properties And Thermal Fatigue Of Cement-based Materials

A cement-based composites may be regarded as a highly heterogeneous material system with multiple components that have different thermal expansion properties. The thermal expansion divergence of components are generated in the cement-based composites.When the temperature changes, the thermal induced deterioration into the cement-based materials will happen due to this effect, such as the thermal induced stress and micro-or macro-scale cracking, even the structure damage. In this paper, the thermal expansion divergence of components was measured, and both thermal expansion differences were explored. The microstructure of the cement-based composites was observed after some thermal cycles.With in depth research in the thermal expansion properties and mechanism of the hardened cement paste, thermal expansion incompatibility of components was adjusted.It comes to conclusion that, thermal expansion coefficient of hardened cement paste is over two times than that of aggregate, which proved that the thermal expansion divergence of components happens subjected to the temperature change. After thermal cycles, the thermal induced microcrack appeared and propagated along interfacial transition zone between hardened cement paste and aggregate, and the width of microcrack was enlarged with the increase of thermal cycles in microscopic scales. The compressive strength of cement-based materials decreases with the increase of thermal cycles in macroscopic view.The thermal dilation rates (TDR) and coefficient of thermal expansion (CTE) of hardened cement pastes (HCP) decreases with the increase of the water-cement ratio. Replacing Portland cement with mineral admixtures (fly ash, ground granulated blastfurnace slag, silica fume and metakaolin) were found to lower the TDR and CTE of HCP, the extent of this lowering effect increases with the increasing replacement proportions of mineral admixtures in the pastes.Carbon fiber and polypropylene fiber were also found to lower the TDR and CTE of HCP, the lowering effect were similar. The polymer latex could also limit thermal expansion properties of HCP, the restriction effect differ from the types and properties of the polymer latex.The CTE of HCP decreases with the increase of the water-cement ratio for the higher water-cement ratio lead to the more pore in HCP, which supply buffer space for the dilation of materials. The restriction of the mineral admixtures to the thermal expansion properties of HCP are mainly due to the change of the porosity and the amount of portlandite (CH) in the pastes, among which, CH content is more important than porosity as a factor affecting the CTE of the cement paste. It is revealed that the fibers could lower the TDR and CTE of HCP because of the lower CTE of fibers, furthermore the fibers and cement paste are binding together. The types and amount of the polymer latex could lower the TDR and CTE in different extent, this is because the polymer film is formed by polymer latex on the surface of the C-S-H gel and unhydrated cement particles, so the CTE of HCP is lowered and the toughness of HCP is reinforced.