Changes in color and thermal properties of fly ash cement mortar after heat treatment

Fly ash (FA) is a powdery substance discharged by power plants after burning coal or other fossil raw materials. The storage and reuse of FA have been under debate since the release of FA into the atmosphere could be detrimental to human health. Fortunately, FA has been mostly collected by plants, and these large amounts of collected FA have led to its reuse, which not only reduces pollution, but FA can be also used in lieu of other materials, such as cement, for environmentally friendly building materials. However, the properties of building materials will undergo deterioration at high temperatures. Therefore, in this study, the influence of heat treatment at high temperatures on FA cement mortar is examined in relation to the color and thermal conductivity changes. Color changes are an apparent indicator, for example, of the variation in moisture content. Thermal conductivity (λ) is an important parameter for evaluating the heat transport properties of materials. Mass loss and density also affect thermal conductivity. In this study, 15 FA cement mortar samples are made from mixtures that contain 3 different FA replacement percentages: 10%, 20% and 40%, for a total of 45 samples. The samples undergo heat treatment at temperatures that range from room temperature to 800 °C before testing is carried out. A color meter and a thermal conductivity tester are used to obtain the color parameters L∗, a∗, b∗ and thermal conductivity, respectively. The samples are also sprayed with a phenolphthalein reagent after treatment to qualitatively observe the changes in pH. It is found that a temperature range of 400-600 °C is critical in which the pH of the mortar decreases due to carbonization and the decomposition of calcium hydroxide, which lead reduced lifespan of the FA cement mortar. The thermal conductivity also decreases at higher temperatures, and is linearly correlated with the density ratio. The experimental results show that the FA percentage has a considerable influence and therefore, there could be an optimum FA percentage.