The Effect of Mechanical Grinding Fineness on the Pozzolanic Activity and Hydration Mechanism of Coal Bottom Ash as Supplementary Cementitious Materials

This study investigates the use of mechanical grinding to activate coal bottom ash (CBA) as a low-carbon supplementary cementitious material. Two CBA powders with different fineness levels (75 μm and 45 μm, denoted as SCBA and GCBA) were used to replace 10–50% of cement in mortar specimens. Performance was evaluated through ISO-standard strength tests and the activity index, while micro-analytical techniques characterized the hydration mechanism. The results show that this grinding treatment significantly enhanced pozzolanic activity; at 28 days, the compressive strength of the mixture with 30% GCBA replacement reached 30.6 MPa, which was 44% higher than that of the corresponding SCBA mixture (21.2 MPa). Microstructural analysis confirmed the consumption of portlandite (CH) and the predominant formation of interwoven C-S-H gels and ettringite (AFt), along with residual quartz, calcite, and mullite. These products refine the pore structure and densify the interfacial transition zone. Economic and environmental analysis reveals that CBA substitution reduces raw material costs by 120 CNY/ton and carbon emissions by approximately 261.3 kg CO2/t. Based on the balance of mechanical integrity and environmental benefits, mechanical grinding of CBA to 45 μm at a 30% cement replacement level is proposed as a promising approach for producing low-carbon cementitious materials and for future application in green concrete.