Hydration, pore structure, and related moisture properties of fly ash blended cement-based materials : Experimental methods and laboratory measurements

Abstract: Cement-based materials, such as mortar and concrete, are the most employed construction materials in the world. Cement is an important component; it reacts with water to form a glue, called cement paste, which causes the material to harden. Today, cement production accounts for a significant portion of the world’s annual carbon dioxide emissions. Replacing parts of cement with other materials that have cement-like properties, such as fly ash, can help reduce the climatic impact.Fly ash is a residual product from coal-fired power plants that has previously landed on landfills. Replacing part of ordinary cement with fly ash changes the chemical composition and reactivity of the material. Knowledge of how these changes affect material properties is necessary to fabricate long-term durable structures with fly ash blended cement-based materials. This applies not least to the pore structure and related moisture properties because most processes that degrade cement-based materials are both penetrating and moisture dependent. The aim of this study was to contribute new data and knowledge in this area.Unlike ordinary cement, fly ash has little ability to react directly with water. For fly ash to react with water, it depends on the reaction of cement with water. This causes cement-based materials with fly ash to cure more slowly during in the first days. However, this study shows that materials with fly ash have a lower degree of reaction and are more porous well-beyond the first days of curing. The latter part can be partly explained by the fact that the fly ash reaction binds less water than the cement reaction. Instead, fly ash uses part of the products formed by cement for its reaction.Although the volume of pores increases with increasing fly ash replacement, this study shows that the material’s ability to transport water in the vapour and liquid phase decreases. The porous structure in fly ash blended cement-based materials seems to be more heterogeneous and less connective than that in materials with only ordinary cement. This is of considerable practical importance because the ability to transport moisture controls the drying of the material; for example, the time required before moisture-sensitive flooring materials can be applied to a concrete surface.Furthermore, the present study show that fly ash makes the curing and material properties more temperature sensitive. The reaction of fly ash is considerably delayed at low temperature. The effects are similar to those previously documented for ordinary cement during the first days but differ over longer time. For ordinary cement, low temperature leads to the formation of a more homogenous porous structure that allows the reactions to proceed for a longer amount of time. After a long time, materials with ordinary cement cured at low temperature seem to have the most well-developed properties. These long-term effects of low temperature are not observed as clearly for fly ash blended materials. The results highlight that it is especially important to protect structures with fly ash blended cement-based materials from low temperatures.Finally, the laboratory work in this study involved some method development. A new measurement and evaluation routine was employed, to show that heat development can be measured from the reactions between cement (with and without fly ash) and water for up to a year after mixing. Previous studies have argued that this is not possible after the first few weeks of reaction. The results create new opportunities for researchers and the industry to study long-term reactions in cement-based materials. In this study, we also presented a new method for determining the amount of binder (e.g., cement and fly ash) in small samples of cement-based material. The method makes it possible to compare the measurement results for small samples of mortar or concrete obtained from a larger volume of material (i.e., with unknown compositions).