Strength development and microstructure of recycled gypsum-soda residue-GGBS based geopolymer

The utilization of waste materials in road construction is one type of sustainable infrastructure practice, which reduces the dependence on natural resources. The waste, including recycled gypsum powder (from construction and demolition waste), soda residue (from the Na2CO3 industry), and ground granulated blast-furnace slag (GGBS), are favorable supplementary cementitious materials. This paper aims to evaluate the recycled potential of gypsum waste and wet-basis soda residue combined with GGBS to prepare geopolymer for subgrade applications. In this regard, the geopolymer mixture containing 0%, 5%, 10% recycled gypsum and 60%, 70%, 80% soda residue by dry weight will be prepared as partly substitute for GGBS. The mechanical performance will be measured using unconfined compression strength (UCS) and studied strength mechanism via scanning electron microscopy (SEM), X-ray diffraction (XRD), and thermogravimetric analysis (TGA). Test results reveal that the strength of the RG-SR-GGBS based geopolymer decreased with increasing SR dosage and increased with increasing curing age. Excessive dosing of 10% recycled gypsum as a partial substitute for GGBS was detrimental to the strength of geopolymer, while a 5% dose was beneficial to increase the UCS value. The geopolymer of group SR60-RG5-S35 achieved a maximum UCS value of 9310 kPa at the curing age of 28 days. Ettringite (AFt) is a stable hydration product only if there is a sufficient supply of sulphate from recycled gypsum. Many rod-like AFt crystals form cavities in the geopolymer thus reducing the UCS value, while the right amount of AFt fills the capillary pores and increases the structural density and strength. GGBS was activated by the alkalinity of soda residue to generate dense calcium silicate hydrate (C-S-H gel), which is the main reason for the strength development.