Influence of TiO₂ and ZnO photocatalyst onto the physical, compressive strength and self-cleaning properties of fly ash-based geopolymer paste

Nowadays, concepts of self-cleaning have received great attention in construction building materials. Self-cleaning with the presence of photocatalyst was applied in building materials to overcome the problem of surface building tend to become dirty after exposure for a long time in highly polluted areas. In order to pursue the concept of green environment blending materials, a new binding material towards green material which is geopolymer with addition photocatalyst was developed. Although self-cleaning has been widely applied in conventional cement paste, in general their applications in geopolymer are still in early stage of development that requires further study. Besides sustainability in terms of appearances was maintained, the influence of photocatalyst towards self cleaning ability of geopolymer paste that offer good physical and mechanical properties has been focused on this research. The geopolymer paste was first prepared by dry mixing of the fly ash with various wt% of zinc oxide (ZnO) and titanium dioxide (TiO₂) nanoparticles. These nanoparticles will act as a photocatalyst to induce the self-cleaning behavior of the geopolymer paste. Then, dry-mixing mixtures were mixed with alkaline activator, which is a combination of sodium hydroxide (NaOH) solution with 12M concentration and sodium silicate (Na₂SiO₃) solution. Ratio of alkaline activator, Na₂SiO₃ to NaOH is 2.5. The samples were cured at room temperature for 28 days (early age) and 640 days (later-age). From the chemical composition analysis, the result indicates fly ash as a raw material used in this research contain low amounts of calcium oxide (less than 10 wt%), which categorized under Class F (low calcium oxide content) type. Phase analysis revealed that the addition of ZnO and TiO2 nanoparticles did not change the chemical phase of the geopolymer paste, but did influenced the mechanical properties of the geopolymer paste, where the compressive strength decreased as the wt% of the ZnO and TiO₂ nanoparticles increased. The evidence from microstructure analysis showed microcracks in the geopolymer matrix which caused by agglomeration of ZnO nanoparticles, leads towards low strength. However, for sample TiO₂-geopolymer paste, it can be noticed, the strength was enhanced from 56 MPa to 86 MPa when small amounts of TiO₂ photocatalyst was added from 2.5 wt% to 5.0 wt%. The addition of ZnO and TiO₂ nanoparticles also affect the physical properties (hardening time, porosity and water absorption) of the geopolymer paste. By increasing the addition of ZnO and TiO₂ nanoparticles, prolongation period of setting time was increased that leads longer hardening time. The porosity result demonstrates that sample with higher porosity has higher water absorption percentage. The self-cleaning abilities of the geopolymer paste were evaluated through the photocatalytic activity test and surface degradation test under sunlight and ultraviolet (UV) light (lamp). The sample containing photocatalyst shows degradation of methylene blue where the colour changed from blue to colourless from 30 to 150 minutes under exposure sunlight and UV light. From UV-Vis analysis, the result indicates that, the degradation rate was increased by increasing the wt% of photocatalyst for coated and uncoated sample under both conditions (sunlight and UV light). For surface degradation test, the results show that methylene blue are fully degraded after 150.