Synergistic Tuning of Boron-Containing Concrete for Neutron Shielding: Mix Design, Dosage Optimization, and Mechanistic Insights

Boron-containing concrete is an important neutron-shielding material for nuclear engineering, but the addition of boron compounds may adversely affect cement hydration and mechanical properties. This study employed boric acid and boron carbide as boron-10 neutron-absorbing sources. Within the material system investigated, the comprehensive effects of boric acid, boron carbide, and their combined use on setting time, mechanical properties, and effective boron loading were compared. MAA-based mix design and the analytic hierarchy process (AHP) were then employed to screen candidate mix proportions. Based on particle packing optimization, a series of concrete mixtures were prepared to evaluate workability, setting time, mechanical strength, boron content, and hydration products. The results show that boric acid provides good boron dispersibility but strongly retards cement hydration, leading to prolonged setting time and reduced strength. Pretreatment with calcium hydroxide and the use of an early strength admixture can partly mitigate this negative effect; the final setting time of the boric acid-containing mixtures still reached as high as 34 h 11 min. Boron carbide exhibits better chemical stability and has less influence on cement hydration. When incorporated at 5–10%, boron carbide improves the balance between mechanical performance and neutron-shielding potential. In particular, the 10% boron carbide mixture achieved a 28-day compressive strength of 47.3 MPa and a splitting tensile strength of 4.21 MPa, compared with 55.5 MPa and 6.2 MPa, respectively, for the control mixture, while maintaining relatively high strength. An analytic hierarchy process was further applied to comprehensively evaluate compressive strength, splitting tensile strength, final setting time, and boron content. Furtherly, XRD results confirmed the different interaction mechanisms of boric acid and boron carbide in cementitious systems. The study provides a framework for developing boron-containing concrete with potential neutron-shielding applications.