Issue 36

P. Jinchang et alii, Frattura ed Integrità Strutturale, 36 (2016) 130-138; DOI: 10.3221/IGF-ESIS.36.13 135 It can be seen from Tab. 3 that, as curing period prolonged, unhydrated cement phase (C 3 S, C 2 S, C 3 A, C 4 AF) gradually reduced, amorphous phase of hydration products (C-S-H gel, etc) increased, Ca(OH) 2 generated from hydration decreased, and CaCO 3 had no obvious change; moreover, in early period (before 7 d), amorphous phase of double-doped test specimen was less than single-doped test specimen, suggesting some nano-CaCO 3 involves in hydration of cement and plays a supplement role in cement based composite material. In addition, we can know from the table that, when curing exceeded 28 d, variation tendency of hydration product content slowed down; in early period (before 7 d), addition of nanomaterials had an obvious promotion effect on hydration of UHPCC. The accelerated hydration of cement is attributable to high reaction activity of nano-SiO 2 . Nano-CaCO 3 acting as a supplement role increases the density of UHPCC and also improves its mechanical performance. MIP Result and Analysis Preparation of test sample: cement block which has finished test of strength of concrete was crashed and put into absolute ethyl alcohol for stopping hydration. Before test, the broken concrete was moved into a vacuum drier (50°C). 24 h later, it was taken out and packed with closed bag. We made test on pore structure of four test specimens with different curing age. Tab. 4 shows data of pore structure of cement paste test block. Fig. 5 demonstrates MIP results of four materials after 28 d-standard maintenance. Sample NSCO NSC1 NSC3 NSC5 Average pore size(nm) 38.1 25.1 27.3 30.9 Critical poresize (nm) 427 171 223 249 Total pore space (ml/g) 0.2226 0.2043 0.2137 0.2184 Table 4 : Statistics of pore structure of test block. ) ) ( ( log 1    nm g L D d dV  (a) Distribution curve for pore size. (b) Distribution curve for porosity. Figure 5 : MIP analysis results of UHPC. It can be seen from Fig. 5(a) that, pore volume of test specimen added with nano-SiO 2 only and maintained for 28 d was the largest when pore size was between 0.003 and 0.03 μm, and the peak value appeared when pore size was 11 or 4 nm; for double-doped test specimen, distribution curve of pore size deviated to the right slightly and the peak pore size was relatively smaller. Fig. 5(b) shows distribution curve for porosity of four test specimens. It can be seen from the figure that, adding two nanomaterials had an obvious influence on lowering porosity of composite material, about 2%. Thus it is concluded that, reasonable selection and optimization of UHPCC, hydration promotion effect of nano-SiO 2 and supplement effect of nano-CaCO 3 can greatly improve density of UHPCC, reduce microscale and microscopic scale defects, and thus enhance performance of material.