Hydration Characteristics of Tricalcium Aluminate in the Presence of Nano-Silica

Tricalcium aluminate (C(3)A) is the most reactive component of the Portland cement and its hydration has an important impact on the workability and early strength of concrete. Recently, nanomaterials such as nano-silica (nano-SiO(2)) have attracted much attention in cement-based materials because of its pozzolanic reactivity and the pore-filling effect. However, its influence on the hydration of C(3)A needs to be well understood. In this study, the hydration kinetics of C(3)A mixed with different percentages of nano-SiO(2) were studied and compared with pure C(3)A. The hydration products were examined by different characterization techniques including XRD, XPS, and NMR spectroscopy and isothermal calorimetry analyses. The XRD results showed that the addition of nano-SiO(2) promoted the conversion of the intermediate product C(4)AH(13). The isothermal calorimetry results showed that the addition of nano-SiO(2) significantly reduced the hydration exotherm rate of C(3)A from 0.34 to less than 0.1 mW/g. With the presence of nano-SiO(2), the peaks for Q(1) were observed in (29)Si MAS-NMR measurements, and the content of Q(1) increased from 6.74% to 30.6% when the nano-SiO(2) content increased from 2 wt.% to 8 wt.%, whereas the proportion of Q(4) gradually decreased from 89.1% to 63.6%. These results indicated a pozzolanic reaction provoked by the nano-SiO(2) combined with aluminate structures generating C-A-S-H gel.