The Utilization of Coal Bottom Ash as a Natural Sand Replacement in Mortar Containing Fly Ash

The substantial generation of waste ashes from coal thermal power plants (CTP), particularly fly ash (FA) and bottom ash (BA), poses significant environmental challenges; meanwhile, the exhaustion of natural river sand for the construction industry is more serious. The study aims to explore the use of BA as a sand replacement and FA as a partial cement substitute in mortar production, focusing on sustainability. In this research, FA and cement served as binder materials, with FA comprising 15% of the total binder mass. BA was incorporated as a fine aggregate, replacing sand at varying levels of 0%, 25%, 50%, 75%, and 100% by weight. The study assessed the effects of BA substitution on the mortar's compressive strength (CS), ultrasonic pulse velocity (UPV), water absorption (WA), thermal conductivity (TC), and resistance to sulfate attack. Additionally, scanning electron microscopy (SEM) was utilized to analyze the mortar's microstructure. Results indicate that substituting sand with BA negatively impacted all tested properties. As BA content increased, CS, UPV, and TC of mortar samples decreased, while WA and sulfate expansion increased markedly. At 56 days, the mortar samples exhibited CS values ranging from 20.6 to 57.0 MPa, UPV values from 3395 to 4203 m/s, WA values from 5.58% to 18.70%, and TC values from 0.89 to 1.73 W/m·K. Furthermore, the control mortar demonstrated a length change of 0.0218% due to sulfate attack, whereas the length changes for specimens with 25%, 50%, 75%, and 100% BA replacement were approximately 68.8%, 96.3%, 155.0%, and 162.4% higher, respectively.