Development of Low-Viscosity UHPC Using Fly Ash Microbeads and Modified Polycarboxylic Acid Superplasticizer

Rheological properties are essential to ultra-high performance concrete (UHPC), and it is necessary to guarantee a relatively lower viscosity to avoid fiber segregation and mechanical degradation. In this study, an innovative physical-chemical integrated approach, namely the simultaneous use of fly ash microbeads and a modified low-viscosity polycarboxylic acid superplasticizer (JN-PCE), was proposed to regulate the rheological performance of UHPC containing industrial by-products. The effect of varying microbead dosage, different superplasticizers, and their combined influence on the rheological parameters, mechanical characteristics, and microstructure evolution were systematically explored in this study. The results demonstrated that the addition of 1.5% JN-PCE led to significant improvements in the UHPC properties including a flow expansion of 775 mm, a static yield stress of 376.9 Pa, a dynamic yield stress of 188.01 Pa, a plastic viscosity of 160.87 Pa·s, and a 28-day compressive strength of 136.6 MPa. Moreover, when a combination of 10% microbeads and 1.5% JN-PCE was used, the UHPC exhibited a flow expansion of 730 mm, a static yield stress of 693.5 Pa, a dynamic yield stress of 542.90 Pa, a plastic viscosity of 202.40 Pa·s, and a 28-day compressive strength of 142.1 MPa. This study thus offers valuable insights into optimizing low-viscosity UHPC formulations using eco-friendly additives for construction applications.