Specimen Size and Age Effects on the Compressive Strength of FA–Alccofine 1203-Based Geopolymer Concrete

Growing environmental concerns about the impact of producing Ordinary Portland Cement (OPC) have driven the construction industry towards sustainable alternatives with reduced greenhouse gas emissions. The production of OPC is a major contributor to CO2 emissions. Geopolymer concrete (GPC), synthesized through the alkaline activation of industrial byproducts such as fly ash (FA) and ground granulated blast-furnace slag (GGBS), offers a viable low-carbon alternative. This study examines the impact of the dimension of the test specimen and age on the compressive strength (CS) of GPC produced using FA and Alccofine 1203. Sodium hydroxide (NaOH) and sodium silicate (Na2SiO3) solutions were used as alkaline activators to form an aluminosilicate gel (N-A-S-H or C-A-S-H), depending on the precursor, which is primarily responsible for the material’s strength gain. An experimental program was conducted using cube specimens of varying sizes (100 and 150 mm) and cylinder specimens of 150 mm diameter and 300 mm length, cured for 3, 7, and 28 days (D). The results indicated that GPC with Alccofine gains strength at early stages, similar to conventional concrete (CC). Additionally, smaller specimens exhibited 12% higher CS due to even stress distribution, while the curing age significantly influenced the strength gain. At 28 D, Alccofine-blended GPC achieved strengths of 45.67 MPa (150 mm cubes), surpassing CC (41.27 MPa) under similar curing conditions. These findings suggest that GPC with Alccofine 1203 is a suitable and eco-friendly choice; however, it is necessary to consider size and age effects in GPC testing protocols.