Alternative fine aggregates are often assessed using compressive strength at a single reference age, which may conceal an early maximum followed by later strength loss. This preliminary screening study compared mortars containing river sand (RS), standard sand (StS), desert sand (DS), soil sand (SS), coal gangue sand (CGS), and metamorphic rock sand (MRS) under one nominal mixture design. For each aggregate, one mortar batch was prepared and nine 70.7 mm cubes were cast, with three specimens tested at 3, 7, and 28 d. RS, StS, DS, and SS continued to gain strength. Within the single CGS batch, strength decreased from 15.56 ± 0.31 MPa at 7 d to 11.68 ± 0.15 MPa at 28 d; within the single MRS batch, it decreased from 21.63 ± 0.48 MPa to 15.75 ± 0.20 MPa. The corresponding losses were 24.95% and 27.18%, and exploratory within-batch Tukey tests yielded p < 0.001. These statistics describe specimen-level variation within the tested batches and do not establish batch-to-batch reproducibility. Representative 28 d SEM fields, raw-aggregate EDS, and qualitative XRD provide contextual observations but lack the temporal and spatial resolution needed to reconstruct a defect-formation process between 7 and 28 d. Because aggregate moisture state, absorption, flow, air content, and compaction were not independently controlled, the results identify a screening signal that requires independent-batch validation rather than a general material mechanism.