SWCNT Nucleation Mechanisms

The heat flow measurements in Fig. 1 tend to argue against adsorption as the dominant mechanism behind the observed nucleation effects produced by SWCNT during OPC hydration processes. All of the nanomaterials in Fig. 1 were non-conductive, eliminating electrostatic effects as a possible nucleation mechanism in their case. Adsorption, however, would remain a possible factor in the hydration of the OPC in the presence of the nanomaterials. In fact, it is possible that the retarding effect of the nano-titania was due to adsorption of the ions in the solution on to the surface of the titania, preventing them from taking part in the hydration reactions. While one would expect different degrees of effect from the different nanomaterials if adsorption is the predominant nucleating mechanism, one would still expect to see the same general hydration behaviour in each case. The differences between the hydration behaviour of the samples sonicated with SWCNT and those sonicated with the nanomaterials instead suggests that nuclea-tion effects in the former be attributed to the electrostatic mechanisms that rely on the metallic properties of the SWCNT.

A second line of evidence against the adsorption of metallic ions as a nuclea-tion mechanism is the relative disparity between the nucleation of C-S-H and Ca(OH)2 on the SWCNT bundles. Only one example of Ca(OH)2 nucleating on SWCNT was observed (Fig. 2), with all other episodes of crack bridging occuring in C-S-H. In addition, examination of SWCNT bundles on OPC surfaces during the early stages of hydration did not show evidence of Ca(OH)2 forming around the SWCNT. Although Ca(OH)2 appeared to be less common on the surfaces of OPC during the early stages of hydration than is C-S-H [Makar and Chan 2008, Makar et al. 2007], it was readily identified when it did form. Investigation of the effect of SWCNT on the formation of Ca(OH)2, however, has shown that the same amount of Ca(OH)2 was present in samples with and without SWCNT when the same amount of hydration had occurred [Makar and Chan 2009]. The three observations together suggest the SWCNT may be preferentially nucleating C-S-H, with the Ca(OH)2 forming elsewhere in the system.

It is this relative imbalance between formation of C-S-H around SWCNT bundles and Ca(OH)2 that supports the prevalence of electrostatic effects as a nuclea-tion mechanism. Electrostatic effects would be expected to cause both Ca2+ and

Fig. 2 Crack bridging in Ca(OH)2 crystal (2% SWCNT, 7 days hydration)

Si4+ ions to nucleate on the SWCNT surfaces. On the other hand, adsorption of metallic ions would be expected to primarily nucleate Ca2+. In that case, a preferential formation of Ca(OH)2 would be the most likely outcome, not of C-S-H.

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