Engineering performance and expansion mechanism of MgO expansion agent in ultra-high performance concrete (UHPC)

Ultra-high performance concrete (UHPC), despite its excellent strength and toughness, has greater shrinkage. The effects of different reactivity (R-MgO, M -MgO and S-MgO) and content (0-12 wt%) of MgO expansive agent on the restrained expansion and mechanical properties of UHPC were investigated and the relationship among restrained expansion, hydration process, and pore structure was established. It was found that with 4-6 wt% R-MgO, the restrained expansion of UHPC in water for 14 days is (200-300) x 10-6, and the restrained shrinkage of UHPC cured in air for another 14 days is (100-180) x 10-6, which can meet the requirements of the national standard GB/T 50,119-2013 for compensation shrinkage without compromising the early me-chanical properties. The rate of restrained expansion of UHPC is controlled by the generation rate of Mg(OH)2, while the magnitude of restrained expansion of UHPC is controlled by the amount of Mg(OH)2. Under the same MgO amount, the restrained expansion of UHPC containing R-MgO was highest during 1-14 d. The restrained expansion of UHPC containing M -MgO exceeded that containing R-MgO during 14-28 d, while the restrained expansion of UHPC containing S-MgO reached the highest during 28-60 d. The dispersion coefficient was proposed and the mathe-matical model of UHPC at 14 d age regarding the relationship among restrained expansion, hy-dration process, and pore structure, epsilon re=(1.2 m x g(phi i) x Delta P-ft)/E, was developed.