Effect of compaction pressure on the properties of hydrothermally synthesized calcium silicate hydrate from waste glass

Abstract

The growing volume of waste glass (WG) generated by the construction industry has raised environmental concerns and highlighted the need for effective recycling practices. This study investigates the hydrothermal synthesis of calcium silicate hydrate (CSH) materials from WG, with particular emphasis on the effect of compaction pressure on phase formation, microstructure, and mechanical properties. WG powder was mixed with lime, compacted at 1, 1.5, 2, 2.5, and 3 MPa before being hydrothermally treated at 180 °C for 10 h. The synthesized products were characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Scanning electron microscopy (SEM)-Energy dispersive x-ray spectroscopy (EDX), and mechanical testing. XRD analysis confirmed the formation of xonotlite and tobermorite as the dominant crystalline phases under all investigated conditions. Increasing compaction pressure had little influence on the phase composition but significantly affected crystallite growth, microstructural evolution, and mechanical performance. The specimen compacted at 2 MPa exhibited the highest diametral tensile strength, together with increased bulk density and reduced water absorption. SEM observations revealed a dense, homogeneous, interlocking network of CSH products with fewer large pores. At the same time, EDX mapping showed a uniform distribution of Ca, Si, and O throughout the matrix. The improved mechanical performance was primarily attributed to enhanced microstructural densification and interparticle bonding rather than an increased amount of crystalline phases. These findings demonstrate that a compaction pressure of 2 MPa provides the optimum conditions for producing high-performance CSH materials from WG via hydrothermal treatment.