Effect of Curing Conditions on the Compressive Strength of Geopolymer Concrete made from Oil Shale Ash

Abstract

Concrete production is one of the largest industrial sources of global CO2 emissions, mainly due to the Portland cement as a binder. In light of environmental concerns, particularly within the construction sector, researchers are seeking alternative binders that can reduce the carbon footprint of concrete. Current study contributes to this effort by examining three concrete mixes and the variation of their properties depending on binder type and curing conditions. A Portland composite cement-based concrete was compared with two geopolymer concretes produced from ground and unground oil shale ash. The aim was to assess oil shale ash as a cement substitute and determine mechanical and processing characteristics of such mixes. Three series of mixes were prepared: Portland composite cement concrete and two geopolymers based on oil shale ash. Each series included nine 100 × 100 × 100 mm cubes, divided among three curing regimes: underwater, ambient laboratory and elevated temperature at 80 °C. Specimens were demoulded after 24 h and cured for 28 days before testing. After 28 days, compressive strength was tested, and pH measured. Results revealed that the binder type had the greatest influence on strength. Ground shale ash concrete reached 12.9 MPa, unground shale ash reached 5.1 MPa, while Portland composite concrete achieved 32.9 MPa. Although geopolymer concretes remained inferior in strength, these values demonstrate their potential as lower-carbon alternatives. In conclusion, ground oil shale ash activated with alkaline solutions shows promise as low-carbon alternative binder. With further optimization, such geopolymer concrete could support more sustainable concrete production.