Identification and characterization of tephra for geopolymer precursor

Abstract

Abstract Volcanic ash and pumice are products of volcanic eruptions characterized by a high amount of amorphous silica and alumina. They can potentially serve as a source of raw materials for the production of geopolymers. Geopolymers are more environmentally friendly compared to conventional cement. However, there is a requirement for aluminosilicate for creating geopolymers. This study aims to identify the mineralogical composition and chemical properties of the volcanic ashes and pumices that are potentially used as geopolymers precursor materials. Tephra and pumice samples were collected from several volcanoes in Indonesia. The samples include two tephras from recently erupted volcanoes in Java and one pumice material from West Sumatra. Tephras were collected one week after the eruptions of Mt. Tangkuban Perahu (26th July 2019). Another source of tephra was collected from Mt. Kelud, which erupted on 13th February 2014. Pumice samples of Mt. Maninjau were collected at Sungai Limau district, 30 km from the Maninjau caldera, a product of the Holocene (52K) volcanic eruption of Mt. Maninjau. Mineralogy of the tephra and pumice samples was identified with XRF and XRD. Chemical properties characterized include pH, exchangeable cations, cation exchange capacity (CEC), potential-P, and available-P. Results showed that crystalline feldspar minerals and amorphous volcanic glass dominated volcanic ash and pumice. The SiO2 content is higher in pumice, followed by tephras from Mt. Tangkuban Perahu and Mt. Kelud (74.97%-60.28%). But the Al2O3 content is higher in Mt. Kelud compared to pumice and Mt. Tangkuban Perahu (23.57%-4.03%). The pH is very acidic to slightly acidic (2.35–6.1). Mg is higher than Ca, Na, and K. The potential-P and available-P were considered very low. Among the samples, Mt. Kelud volcanic ash has the highest available P compared to Tangkuban Perahu volcanic ash and pumice. The SiO2/Al2O3 ratio are 4.35, 8.00, and 26.38 for samples of Mt. Kelud, Mt. Maninjau pumice, and Mt. Tangkuban Perahu, respectively. The optimum ratio of geopolymer is between 2.5–5; thus, volcanic ash Mt. Kelud is the most suitable option. Nevertheless, the exchangeable basic cations of volcanic ash and pumice indicated that they are more suitable for plant nutrition.