Effect of crusher fine gradation on the compressive strength and physical properties of cement-based brick specimens

Crusher fines generated during aggregate production are commonly stockpiled or discarded despite their potential use as a sustainable construction material. This study investigates the effect of crusher-fine gradation on the compressive strength of cement-based bricks. Beyond compressive strength, the water absorption and oven-dry bulk density of the hardened specimens were also evaluated to provide a broader assessment of their physical properties. The raw crusher fines were collected from the Al-Ad Mountains area south of Jeddah, Saudi Arabia and were characterized in terms of particle size distribution, moisture content, specific gravity, mineralogical composition, and microstructure. These fine materials were classified into coarse, medium, and fine fractions, and seven mix designs were prepared while keeping the cement content constant and varying the internal gradation of the crusher fines under practical mixing conditions. The specimens were cast and tested for compressive strength after 7 and 28 days of water curing. In addition, XRD and SEM were performed to interpret the observed mechanical behavior. Water absorption and oven-dry bulk density were also measured for all mixtures, and the strength differences were verified by one-way ANOVA with Tukey HSD post-hoc testing. The results showed a non-linear relationship between fine fraction content and compressive strength. Strength increased progressively with increasing fine content up to an optimum of 45%, at which the highest average compressive strengths of 13.14 MPa and 16.55 MPa were achieved at 7 and 28 days, respectively, compared with 4.76 MPa and 14.41 MPa for the control mix. Beyond this level, strength decreased noticeably, reaching about 9.3–9.5 MPa at fine contents of 60–90%. The findings demonstrate that moderate fines contents enhance particle packing, matrix densification, and hydration development, whereas excessive fines increase water demand and lead to less favorable internal structure and reduced strength.