Design of zeolite based geopolymer mortars with fly ash and limestone additives by Taguchi method

Today, one of the most significant and critical factors is a sustainable environmental approach, and practically all industrial sectors encourage the manufacture of environmentally friendly products. Due to their low carbon dioxide (CO2) emission, strong mechanical, and durability properties, geopolymers, which are regarded as the third generation cement after lime and Portland cement, are considered an environmentally friendly product that can be used as an alternative to Portland cement. Geopolymers have environmental and economic benefits in addition to reducing CO2 emissions, such as recycling and reusing industrial byproducts like fly ash and blast furnace slag. In this study, samples were prepared in order to determine the properties of geopolymer mortars containing natural zeolite (DZ), C-type fly ash (UK) and powder limestone (LS), sodium hydroxide (NH), and sodium silicate (NS). By using the Taguchi optimization method, the optimum mixing values of zeolite-based geopolymer mortar components, including the binder content, activator/binder ratio, NS/NH ratio, and sodium hydroxide molarity were determined and these values were used in all mixtures. After finding the optimum mixing values of zeolite-based geopolymer mortars, different amounts of fly ash, limestone, and fly ash-limestone were used as additives in the place of zeolite. Geopolymer mortar samples were first cured at 80 oC in the oven for 24 hours and then kept at room temperature until testing days. After the curing process, physical properties of the geopolymer mortar samples like specific gravity, unit weight, water absorption, void ratio, sorptivity, compressive and flexural strengths of 7, 28, and 90 days, freezing-thaw, wetting-drying, sulfate, and acid resistances were determined. The microstructural properties of the geopolymer mortars were investigated by using SEM and EDS analyses. As a result of the mechanical tests applied on the produced geopolymer mortars, the highest values in the flexural and compressive strengths at the 7th, 28th, and 90th days of all mixtures were obtained in geopolymer mortars containing natural zeolite, fly ash, and powder limestone. Zeolite-based geopolymer mortars with fly ash-limestone additives containing 20% fly ash and 20% powder limestone showed the highest compressive strength (31.8 MPa) and flexural strength (7.5 MPa). When the physical properties of geopolymer mortars were examined, it was observed that the samples with the best workability value were zeolite-based geopolymer mortars with fly ash additives. In zeolite based geopolymer mortars, the use of powder limestone as an additive slightly increased the water absorption and void ratio, while the use of fly ash as an additive decreased the water absorption and void ratio. In addition, it was observed that the sorptivity values of mortars containing powder limestone increased, while samples containing fly ash provided a significant benefit to geopolymer mortars by reducing the sorptivity values. From the result of the durability tests applied on the produced geopolymer mortars, compressive strength values decreased after 50 freeze-thaw cycles. After freeze-thaw cycles, geopolymer mortars containing 100% natural zeolite showed the highest compressive strength loss (24.1%), while mortars containing 30% fly ash and 30% powder limestone showed the lowest compressive strength loss (7.3%). In addition, after applying 12 wetting-drying cycles, geopolymer mortars containing 100% natural zeolite showed the highest compressive strength loss (62.7%), while the mortars containing 30% fly ash and 30% powder limestone showed the lowest compressive strength loss (33.9%). However, the compressive strength showed an increase after the zeolite based geopolymer mortars with fly ash and powder limestone as additives were exposed to the effect of 5% magnesium sulfate solution for 2 months. Mortars containing 20% fly ash and 20% powder limestone provided the highest increase in compressive strength (16%) after the effect of sulfate. Finally, it was observed that the compressive strength of the geopolymer mortars decreased after being exposed to a 2.5% sulfuric acid solution for 6 weeks. Mortars containing 100% natural zeolite showed the highest compressive strength loss (50.6%) after the acid effect, while mortars containing 30% fly ash and 30% powdered limestone showed the lowest compressive strength loss (18%). SEM images also confirmed the contribution of fly ash and powder limestone. From the microstructural point of view, it was observed that the characterization of geopolymer mortars generally was good, and geopolimerization bonding with dense matrix compactness formed. The elemental analysis of geopolymer mortars using EDS measurements indicated the existence of a higher Si/Al ratio with respect to the original natural zeolite, fly ash, and powder limestone.