Strong porous ceramic made from soda lime glass by an energy efficient process

Abstract

We report the development of a novel ceramic, made from recycled soda lime glass, which has an unusual microstructure and an interesting combination of properties. It is made by a process which is simple and robust; the glass is powdered, blended with a minor proportion of Ca(OH)2, compacted in a die and cured in saturated steam at ∼100 ◦C. Sophisticated equipment is not needed, and energy requirements are low. We discuss potential applications, and propose the name casamic for this novel calcium, silica ceramic. Green glass bottles were crushed, dry-milled and dry-screened at 150 μm. The particle size distribution (psd) of the product was measured using a Malvern Mastersizer to be 94 mass% <100 μm, 33% <10 μm, 1% <1 μm. The surface area was 0.73 m2 g−1 (BET nitrogen). In a typical synthesis, 100 parts by mass of glass powder was thoroughly mixed with 7–10 parts of commercial grade Ca(OH)2, using a mortar and pastle. Ten parts of water were mixed in at high shear, and the damp powder sifted at 1.4 mm. Portions were placed in cylindrical dies and pressed uniaxially over the range 0.1–200 MPa. The disc of compacted powder was removed from the die and cured in saturated steam within the temperature range 80◦–135 ◦C. After curing, the ceramic (casamic) was allowed to equilibrate in the atmosphere for at least 2 days, before testing. A similar curing procedure has been used [1] to investigate mixtures of Ca(OH)2 and natural pozzolanas. Bending strengths were measured by the “ball-on-ring” method, using a model CK 10 instrument from Engineering Systems, UK. A ceramic disc, 32 mm diameter and 3–10 mm thick, was supported on a ball race, 26 mm diameter, and a 5 mm diameter steel ball was pressed with increasing force into the center of the disc until it fractured. Bending strengths were calculated from the force of fracture using the formula of de With [2], and assuming a value of 0.3 for the Poisson ratio. Figs 1 and 2 show that bending strength increases with compaction pressure, curing temperature and curing time. Other experiments, not reported here, showed that bending strength is relatively insensitive to the type of soda lime glass, the psd of the glass powder, or the relative proportions of glass, Ca(OH)2 and water. It was convenient to adopt a standard procedure of compacting at 50 or 200 MPa and curing at 115 ◦C for 3 h. This resulted in bending strengths of 15–19 MPa and 23–30 MPa for the two compaction pressures respectively. It is instructive to compare these values with the bending strengths of related construction materials (Table I), which were measured using discs of similar TABLE I Bending strengths of casamic and related materials. Earthenware discs were cut from commercial wall tiles. Samples of porcelain were prepared in the laboratory from commercial formulations