Water retention properties of porous geopolymers for use in cooling applications

A series of geopolymers were prepared with varying ratios of sodium silicate, metakaolinite, NaOH and H₂O and their porous properties, water retention and mechanical properties were determined, to develop materials for counteracting heat island effects. Samples were prepared with the molar ratios SiO₂:Al₂O₃:Na₂O:H₂O of 3.66:1:x:y, where x = 0.92-1.08 and y = 14.2-19.5. The porous and mechanical properties of the geopolymers showed a good correlation with the H₂O/Al₂O₃ ratio (y); an increase in y produced an increase in the pore volume (from 0.26 to 0.46 ml/g), the pore size (from 15 to 390 nm) and the water absorption (from 27.2 to 51.1%). The same increase in y decreased the bulk density (from 1.29 to 0.99 g/cm³), the bending strength (from 14.2 to <5 MPa) and the water retention. Thus, the H₂O/Al₂O₃ ratio is the most important factor for controlling the porous properties of these materials, since geopolymers with higher H₂O/Al₂O₃ ratios are more porous and have higher water absorption rates, making them suitable as materials for surface cooling by water evaporation. Geopolymers with lower H₂O/Al₂O₃ ratios are more suitable for water retention applications, and have the advantage of higher mechanical strength.