Fly ash-based geopolymer mortar modified with silica fume, superplasticizer, and PET plastic waste

Department of Chemical Engineering, Faculty of Engineering, Universitas Diponegoro, Semarang, Indonesia

^* Corresponding author: aprilina.purbasari@che.undip.ac.id

Abstract

This study investigated the development of fly ash-based geopolymer mortar modified with silica fume, superplasticizer, and polyethylene terephthalate (PET) plastic waste. Fly ash, solid waste from coal combustion, was used as the main binder, with PET replacing 5-25% of fine aggregate. The addition of 4% silica fume increased compressive strength from 35.09 MPa to 36.67 MPa, while 1.5% naphthalene-based superplasticizer reduced it to 27.19 MPa due to its incompatibility with the fly ash-silica fume system. As for the addition of PET showed that 5% PET yielded the best performance among PET variants, achieving 29.33 MPa. SEM analysis confirmed improved compactness and reduced microcracks in PET–silica fume mixtures. FTIR spectra revealed no chemical reaction between PET and the geopolymer matrix. Thermal tests showed enhanced early thermal resistance but decreased stability at higher temperatures due to PET degradation. The best mix of 4% silica fume and 5% PET without superplasticizer achieved compressive strength of 29.33 MPa after 28 days, exceeding the ASTM C270 Type M requirement of 17.2 MPa. With a dense and homogeneous microstructure and improved thermal behavior at moderate temperatures, this mortar is suitable for structural applications such as load-bearing walls and sustainable structural uses, provided it is not exposed to prolonged direct fire.