<![CDATA[Background: The utilization of recycled coarse aggregate in polymer concrete is a prospective approach to support sustainable construction, but the presence of old mortar on RCA tends to increase material porosity and heterogeneity. At the same time, epoxy-based polymer concrete is highly sensitive to internal voids because micro-defects can reduce stress transfer efficiency and accelerate damage. Objective: This study aims to evaluate the effect of the compression casting technique on the physical and mechanical properties of 100% RCA-based epoxy polymer concrete, focusing on compressive strength, water absorption, and specific gravity. Methods: The base mixture used a semi-dry consistency with a binder matrix of Bisphenol-A epoxy resin (Bakelite® EPR 174) and cycloaliphatic amine hardener (Bakelite® EPH 555), along with Type C fly ash as a mineral filler. 50 × 50 × 50 mm cube specimens were made at four initial pressure levels, namely 0.00, 3.60, 4.80, and 6.00 MPa, each with three replications. Compressive strength testing was conducted referring to ASTM C579 Method B, water absorption based on ASTM C413 Method B, and specific gravity using the hydrostatic method based on ASTM D792. Results: The results showed that compression casting significantly enhanced material densification. The mean compressive strength increased from 33.754 MPa in specimens without compaction to 57.426 MPa at 3.60 MPa and reached an optimum value of 60.190 MPa at 4.80 MPa, before decreasing to 54.009 MPa at 6.00 MPa; outlier screening using the Grubbs method indicated no data were eliminated. Simultaneously, the mean water absorption decreased from 1.563% to 0.387%, 0.233%, and 0.216%, while the mean specific gravity increased from 2.162 g/cm³ to 2.248, 2.272, and 2.309 g/cm³ for the 3.60, 4.80, and 6.00 MPa levels, respectively. These findings confirm that compression casting effectively improves the internal structure of RCA-based epoxy polymer concrete by reducing connected voids and increasing packing density. However, excessive pressure tended to decrease the mean compressive strength due to potential local non-uniformity and densification heterogeneity. Conclusion: Thus, a pressure of 4.80 MPa can be identified as the optimum compaction level to maximize compressive strength, whereas 6.00 MPa is more effective in improving physical properties such as specific gravity and absorption reduction.]]>
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