Challenges in production and placement of alkali activated concrete for in-situ applications

Abstract

Despite the significant decarbonisation potential of alkali-activated concrete (AAC), its adoption for in-situ structural applications remains limited due to challenges related to workability retention, ambient curing, batching-plant compatibility, and safe handling of highly alkaline activators. This study has presented a comprehensive demonstration of the successful scale-up, production, placement, and structural application of a two-part AAC system in a conventional ready-mix concrete (RMC) plant. An optimised AAC mix incorporating locally available ground granulated blast furnace slag (GGBFS) and fly ash in a 70:30 ratio has been developed to achieve structural-grade performance under ambient curing conditions. The AAC has exhibited compressive strength, flexural strength, and drying shrinkage comparable to OPC-based M35 concrete, while achieving significantly higher early-age strength development. A critical operational barrier - heat generation during alkaline activator preparation (about 70 - 80 °C) has been addressed through targeted batching-plant modifications, including a water-circulation-based cooling system that has reduced activator temperatures to 35–40 °C, enabling safe handling and continuous production. Compatibility of a modified sulphonated naphthalene formaldehyde (SNF)-based admixture has ensured stable rheology and workability retention, with a slump of approximately 135 mm after 60 min, facilitating pumping and placement using a boom placer. Full-scale field casting has confirmed dense, homogeneous structural elements with sharp surfaces and no evidence of efflorescence or leaching. The results have demonstrated that, with minor plant modifications and robust safety protocols, two-part AAC can be reliably produced and placed in an RMC environment, providing a technically viable pathway for sustainable in-situ structural concrete construction.