Sustainable foam concrete development: Enhancing durability and performance through pine cone powder and fly ash incorporation in alkali-activated geopolymers

Pine cone powder (PCP) as a fine aggregate in geopolymers is an emerging concept that focuses on utilizing agricultural or plant-based waste materials to enhance the sustainability and performance of geopolymer composites. This study aims to produce eco-friendly alkali-activated foam concrete (AAFC) by investigating the combined impact of using fly ash (FA) and PCP as partial replacements for granulated blast-furnace slag (GBFS) and silica sand (SS), respectively. In this study, AAFC mixtures were developed with varying FA contents of 0 %, 25 %, and 50 % as partial replacements for GBFS, and PCP contents of 0 %, 25 %, 50 %, and 100 % as partial replacements for SS. A total of twelve AAFC mixtures were prepared, maintaining a constant alkaline solution-to-binder (A/B) ratio of 0.45. The mixtures were initially cured at 75°C for 24 hours, followed by ambient curing for 7, 28, and 91 days. The impact of varying PCP and FA contents on the oven-dry density, flowability, water absorption, porosity, sorptivity, thermal conductivity, compressive strength, flexural strength, high-temperature resistance, and resistance to sulfate attack (MgSO₄ and Na₂SO₄) of the AAFC mixtures was assessed. The microstructure of the mixtures was further examined using scanning electron microscopy (SEM). The results revealed that the AAFC mixture with 50 % PCP and 100 % GBFS exhibited the highest mechanical performance, attaining a compressive strength of 12.48 MPa. This represents strength improvements of 158.1 %, 265.4 %, and 256.6 % at 7, 28, and 91 days, respectively, compared to the reference mixture (F0P0). The same mixture had the lowest sorptivity, highest dry unit weight, and best high-temperature resistance. Mixtures with 25 % FA showed improved high-temperature resistance with added PCP. The thermal conductivity of the mixes varied from 0.527 to 0.662 W/mK, with the lowest value in the 100 % PCP mixture and the highest in the 50 % PCP mixture, both without FA. The mixture with 50 % FA and 0 % PCP showed the lowest strength loss after 60 days in a 5 % MgSO₄ solution.