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.