High-strength self-compacting alkali-activated concrete produced with fly ash and steel slag: rheological behavior and mixing rheology comparisons with a Portland cement concrete

The construction industry has recently seen a growing demand for sustainable materials. Alkali-activated binders (AAB) have emerged as a viable alternative to Portland cement-based materials. This study investigates the influences of the composition and mixing methods on the rheological and mechanical properties of an alkali-activated concrete (AAC) based on fly ash (FA) and steel slag (SS), compared to a reference Portland cement concrete (PCC) with equivalent volume fractions of aggregate and paste. Two mixing methods were examined: a free fall mixer and a planetary mixer that also functions as a rheometer. In the fresh state, the performance of concretes was assessed, focusing on rheological parameters such as mixing energy, maximum torque, and equivalent apparent viscosity indicator. In the hardened state, compressive strength tests were conducted. Pseudoplastic rheological model effectively described AAC behavior, while the Bingham model better characterized PCC. AAC demonstrated high passing ability and extended flow time, with flow behavior significantly influenced by the mixing process. Rheological analysis revealed that AAC required five times more mixing energy and exhibited greater equivalent apparent viscosity indicator compared to PCC. Additionally, AAC achieved higher compressive strength than PCC, which presented values from 34 to 43 MPa (PCC) depending on curing conditions. Thermal curing increased compressive strength by nearly 60 % at 28 days for AAC, from 48.6 MPa to 76.8 MPa. Furthermore, the mixing procedure influenced the fresh and hardened properties of both AAC and PCC, though PCC exhibited only minor variations. Mixing methods with higher energy input led to improved compressive strength.