Compressive Strength of Coal Fly-ash Based Geopolymer with Integration of Graphene Nanosheets (GNs)

The compressive strength of coal fly ash-based geopolymers has been improved by integrating the graphene nanosheets (GNs) as additive. Proximate analysis and crystal structure were also investigated using atomic absorption spectroscopy (AAS) and X-ray diffractometer. The geopolymer composites were created by mixing the solid fly ash and sand (weight ratio of 1:3) with 10 M NaOH and sodium silicate (Na₂SiO₃) solutions (weight ratio of 1: 2.5), where the liquid to solid weight ratio reached an economical composition of 1: 4. Low-cost GNs with various concentrations of 5 – 20 mg/ml was then added to the mixtures. The prepared mixtures were poured into mortar molds and allowed to stand for few hours at room temperature before heat treatment (curing) in the oven at various temperatures of 40°C, 60°C, and 80°C for 24 hours. Investigation results showed that the average compressive strength of geopolymer increased about 113.8 % or more than double compared to geopolymer without the addition of GNs. The highest compressive strength (29.5 MPa) was shown by a sample with GNs of 20 mg/mL and a curing temperature of 8°C. Meanwhile, geopolymer without GNs showed the lowest compressive strength in all curing temperatures. Proximate analysis showed that fly ash used in this work was the high calcium of type-C fly ash with the CaO content of 11.18%. XRD analysis results indicated that the GNs had integrated well in the geopolymer matrix. The presence of graphene-like structure was also detected, but it was not agglomerated with GNs. Good compressive strength and inexpensive production processes make this geopolymer very prospective for further development.