Preparation and properties of nano-carbon black modified ultra-high-performance concrete

Ultra-high-performance concrete (UHPC) having ultra-high strength and excellent durability holds tremendous application potential in civil engineering. However, UHPC generally requires high temperature and steam curing, which not only increases the cost of UHPC, but also consumes a lot of energy. This paper presents a parametric experimental study to ascertain the basic mix proportion of UHPC cured in normal condition (wet curing and then air curing) by optimizing the gradation of sand, water-binder ratio, vibrating method, steel fiber and superplasticizer content. Afterward, nano carbon black (NCB) was used to improve the properties of this designed UHPC, and the microstructure was characterized by using mercury intrusion porosimetry (MIP). According to the test results, silica sand had the maximum packing density when mixed with 56 portions of 26–40 mesh sand, 104 portions of 40–70 mesh sand and 40 portions of 70–110 mesh sand. In addition, by using silica sand with maximum packing density, 0.18 of water/binder ratio, 1.2 of cement-sand ratio, 1.6 vol% of steel fiber and 1.0 wt% of superplasticizer, concrete had the 28-days compressive and flexural strengths respectively larger than 120 MPa and 25 MPa just after 7-days wet curing and then 21-days air curing, which meets the requirements of UHPC according to GB/T 31387–2015 (Chinese Standard). Moreover, the test results shows that the adding of 0.01%− 0.5% of NCB increased the 28-days compressive and flexural strengths of UHPC by 8.3–18.1% and 13.3–23.1%, respectively. The workability, water absorption, and drying shrinkage of UHPC were significantly improved by incorporating proper NCB content. MIP test results showed that the addition of NCB was capable of refining the pore size distribution and decreasing the porosity of UHPC.