Effect of Waste Granite Stone Powder on the Mechanical and Microstructural Properties of Cement Mortars

The construction industry is under increasing pressure to reduce cement consumption and associated CO2 emissions while managing the growing generation of industrial by-products. Granite stone powder (GSP), produced in large quantities during aggregate crushing operations, is commonly treated as waste despite its potential application in cementitious systems. This study evaluates the feasibility of using GSP as a supplementary cementitious material (SCM) in cement mortars, benchmarked against fly ash (FA). Cement mortars were prepared with 0%, 10%, 15%, 20%, and 25% replacement of Ordinary Portland Cement (OPC) using GSP and, for comparison, identical replacement levels of FA. Fresh behaviour, physical properties, mechanical performance, and microstructural characteristics were evaluated using flow tests, isothermal calorimetry, SEM, and XRF. FA and GSP exhibited distinct effects on mortar performance. FA improved workability at higher replacement levels, with flow diameter increasing by 2% above the control at 25% replacement, whereas GSP progressively reduced flowability by approximately 33% at 25% replacement due to its angular particle shape and higher surface area. Hydration analysis showed that both SCMs reduced early hydration intensity compared to the control; however, GSP consistently exhibited higher peak heat-flow values than FA, indicating stronger early-age hydration supported by physical filler and nucleation effects. At 28 days, the 10% GSP mixture achieved 30 MPa, retaining about 94% of the control strength (32 MPa), while FA mixtures showed strength reductions exceeding 23% at comparable replacement levels. Granite stone powder is most effective at low replacement levels (10%), where it promotes early hydration, improves matrix densification, and preserves compressive strength, demonstrating its suitability as a low-carbon supplementary cementitious material in cement-based construction.