Issue 36

P. Jinchang et alii, Frattura ed Integrità Strutturale, 36 (2016) 130-138; DOI: 10.3221/IGF-ESIS.36.13 131 constantly explored by concrete material researchers in practice. Ye Q et al. [6, 7] found that, pozzolanic activity of nano- SiO 2 was much stronger than ganister sand and adding 1% ~ 3% nano-SiO 2 could remarkably enhance compressive, rupture and splitting strength of concrete and thus improve microstructure of concrete. Bigley C et al. [8] discovered that, nano-SiO 2 could improve segregation resistance of self-compacting concrete. Chen RS et al. [9] pointed out that, concrete paste mixing with nano-SiO 2 was featured by weaker flowability and shorter setting time; and cement block obtained had high strength in early stage. Nanomaterial can improve microstructure and enhance mechanical performance of material in a certain extent if being applied in ordinary concrete or high-performance concrete. But preparing UHPC using nanomaterials has not been researched yet. Considering the extremely low water-binder ratio of UHPCC, applying nanomaterials with large surface and severe agglomeration into UHPCC will encounter with dispersing and forming difficulty. Thus based on preliminary work, we systematically studied action mechanism of nano-SiO 2 and nano-CaCO 3 adding into UHPCC, aiming to lay a scientific foundation for improvement of UHPC and its promotion.. M ATERIALS AND METHOD Raw Materials aw materials used included P·II 52.5R Portland cement (density: 3.1 g/cm 3 ; chemical components: Tab. 1), ultra- fine fly ash (level I from Nanjing thermal power plant; density: 2.1 g/cm 3 ; specific surface area: 400 m 2 /kg; chemical components: Tab. 1), nano-SiO 2 (Hangzhou Veking New Material Co., Ltd.; superficially porous; average grain diameter: 20 nm; content of SiO 2 : over 99%), nano-CaCO 3 (Zhoushan Mingri Nano Material Co., Ltd.; average grain diameter: 30 nm; content of CaCO 3 : over 99.9%), fine aggregate (ordinary yellow ground with maximum grain size of 2.5 mm; fineness modulus: 2.26; continuous grading; bulk density: 1.4 g/cm 3 ; apparent density: 2.4 g/cm 3 ) and polycarboxylic high performance water-reducing agent (BASF Aktiengesellschaft; water-reducing rate: over 40%). A previous study [10] suggests that, the best mixing proportion of nano-SiO 2 in UHPCC was 3%. In this study, mixing proportion of nano-SiO 2 was fixed at 3% and mixing proportion of nano-CaCO 3 was adjustable, as shown in Tab. 2. Raw material SiO 2 Al 2 O 3 Fe 2 O 3 CaO MgO SO 3 K 2 O N 2 O LOI Cement 20.40 4.70 3.38 64.7 0.87 1.89 0.49 0.33 3.24 Coal ash 53.98 28.84 6.49 4.77 1.31 1.16 1.61 1.03 0.72 Table 1 : Components of cement and coal ash (wt%). Test specimen Cement Coal ash Nano-SiO 2 Nano-CaCO 3 Water-binder ratio Additive NSC0 52 35 3 0 0.2 2 NSC1 51 35 3 1 0.2 2 NSC3 49 35 3 3 0.2 2 NSC5 47 35 3 5 0.2 2 Table 2 : Mix proportion of different components (wt%). Test Method - Moulding Technique UHPCC was prepared using wet mixing technology, i.e., evenly mixing raw materials (coal ash, cement, fine aggregate) in forming process. Detailed procedures are as follows: (1)Stirring ① Add cement mortar into the mixture of quartz sand and silicon ash mixed according to certain mix proportion and then stir for 5 min; ② Then add cement, coal ash, quartz powder and nanomaterial and stir for 5 min; ③ Add half quantity of water containing water reducing agent and stir for 3 min; ④ Add the remaining water and stir for 6 min; ⑤ Pour the mixture into a triple mould (40 mm × 40 mm × 160 mm) and then vibrate the mould on vibrating table with a frequency of 50Hz. Manufacturing procedure of concrete mortar matrix is shown in Fig. 1. R