Mechanical and Microstructure Characterizations of Geopolymer Mortars Using Industrial Waste Materials

Sustainable utilizations of industrial waste materials to construction field have been given considerable worldwide attention. Geopolymer is a novel binding material produced from the reaction of aluminosilicate solid materials with an alkaline solution. In this study, Class C fly ash and bentonite as precursor, together with alkaline solution and steel fibers from waste tires are used to synthesize Geopolymer cement. The influence of bentonite content, sodium hydroxide concentration and content of steel fibers recycled from waste tires, on the short term properties of the geopolymer mortar is studied. The experimental results show that geopolymer mortar using fly ash and NaOH of 9 M possesses the highest compressive strength of 31.6MPa and 35.9 MPa at 3 and 7 days, respectively. As the content of bentonite increases from 25 to 50%, the compressive strength of the geopolymer mortar decreases from 12 to 0.2 MPa and the failure behavior changes from brittle and split model to split + shear failure. Besides, the increase of steel fiber content from 2 to 6 wt%, the 7-day compressive strength of the geopolymer mortar is found to decrease from 24.2 to 1.2 MPa. However, the bridge effect of steel fibers is found in the geopolymer mortar, helps to prevent the crack propagation in the mortar and shear off. The interfacial transition zone (ITZ) is identified using the Scanning Electron Microscope equipped with an Energy Dispersive X-ray (SEM-EDX), features the thickness of ~ 25 μm for the geopolymer mortar containing 100% fly ash, and 10–12 μm for the mortar containing 50% fly ash, respectively. The EDX results additionally demonstrate the low concentration of major elements in the ITZ, implying the low formation of the gels. This leads to weak bonding and cracking in the ITZ.