Research On The Thermal Expansion Performance Of Cement-Based Materials

While the cement-based materials have been widely used in construction and building, their properties can be greatly altered when the structure have exposed to high temperature for overlong periods. The knowledge on the thermal expansion performance of a cement-based material and the underlying mechanism are thus critical in the prediction of its behavior after burning, and also of great help in the design of structures encountered high temperature or in the repair of damaged structures. However, research on thermal expansion performance of cement-based material have long been hindered by insufficient data and shortage of convincing evidences due to inaccessibility to appropriate testing instruments.In the dissertation project, an integrative methodology was applied to study the performance and mechanism of the thermal expansion of harden cement-based materials for the first time with the help of equipments NETZSCH D/L 402EP and TG-DTA, XRD and SEM. The objectives and consequently the experiment comprises six parts: 1) to determine thermal expansion rate for hardened cement paste and aggregate, and to establish mathematical models for computing thermal expansion coefficient; 2) to compare thermal expansion rate and compressive strength for cement-based materials that are mixed with different aggregates; 3) to determine the effect of mineral additives on thermal expansion performance of cement-based materials; 4) to determine the influence of occluded air on thermal expansion performance of cement-based materials; 5) to determine the effects of calcium sulfoaluminate hydrates on thermal expansion of Portland cement-based material; and 6) to compare hardened cement paste made with Portland cement, calcium aluminate cement and sulphoaluminate cement with respect to their thermal expansion performance.In the first experiment, studies focused on the thermal expansion properties of hardened cement paste and aggregates. A special molding method was first applied in the test of thermal expansion coefficients for cement-based materials in three heating treatments - (I) directly heated from 20°C to 600°C, (II) 3 thermal cycles ranging from 20°C to 180°C, then heated from 20°C to 600°C and (III) 3 thermal cycles ranging from 20°C to 400°C, then heated from 20°C to 600°C. In all treatments, the rate of thermal expansion increased with an increasing temperature during the initial stage and then decreased as the temperature continued to rise. A conclusion that the temperature at which the shrinkage started was postponed due to the treatment of preliminary heat cycles in treatment II and III was arrived for the first time. The temperature of maximum thermal expansion in treatment I was 70°C lower than that in treatment II, and 260°C lower than that in treatment III. On the other hand, the thermal expansion rate of aggregates increased with a rising temperature, with an obvious peak occurring range from 570°C to 600°C. The DSC/TG and XRD images revealed that the difference in thermal stress between hardened cement paste and aggregate was a major factor contributing to the decrease in concrete strength. A function derivative method and a direct differential method were first used to develop mathematical models for the relationships between temperature and thermal expansion rates for hardened cement paste and for aggregates respectively.In the second experiment, the influences of quartz sand, limestone and granite at different cement/sand ratios was studied. The thermal expansion rate of concrete increased much quickly as the dosage of aggregate increased, and the increase rate varied with the nature of aggregate in use. The effects of hardened cement paste strength and stress difference in interface zone on concrete strength, was studied by TG/DTA and XRD.In the third experiment, the influence of mineral addtives on the thermal expansions was first studied by adding variable dosage of silica fume, fly ash and blast-furnace slag. At high temperature range the shrinkage rates of cement-based material amended by mineral addtive were greater than those of the reference cement. The order of thermal shrinkage was silica fume > blast-furnace slag > fly ash. While the thermal shrinkage rate was insignificantly affected by the dosage of silica fume, it increased with an increasing content of blast-furnace slag, but decreased with an increasing content of fly ash. The mechanism has been studied through TG-DTA,XRD and SEM.In the fourth experiment, the effect of air on thermal expansion of cement-based materials was first studied through adding air entraining agent, its mechanism was studied by means of MIP and XRD. The curve of thermal expansion rate for hardened cement paste that had been amended with air entraining agent was similar to that for the reference. The occurrence of air phase in hardened cement paste can reduce the expansion rate when heated from room temperature to 150℃. Above 150℃, air phase provides a way for water escape, so the shrinkage become more evident in harden cement paste. Thermal shrinkage rate increased with an increasing dosage of air entraining agent.In the fifth part of the experiment, more calcium sulfoaluminate hydrates were observed in cement-based material amended with C3A and dehydrate gypsum. While the thermal expansion curves were alike, the shrinkage rate of mixing samples was greater than that of the reference. It indicates that the addition of calcium sulfoaluminate hydrates can increase the shrinkage rate of cement-based materials at high temperature (>150°C) and thus pose an adverse effect on their heat stability.In the last part of the experiment, the thermal expansions of hardened cement paste of three types of cement (Portland cement, calcium aluminate cement and sulphoaluminate cement) were compared. The thermal expansion rates for all three cements increased at first, then decreased as temperature continued to rise, and finally became stable at certain temperature. In decreasing order, the thermal shrinkage rate was Portland cement > sulphoaluminate cement > calcium acuminate cement. The heat stability, thermal shrinkage and shrinkage rate at high temperature varied with cement composition. Relationships between thermal expansions of the three hardened cement paste and their hydration substance have been discovered using TG/DTA, XRD and SEM. The results provde guidance that can be used in the application of three types of cement pastes at high temperature.