Issue 18

G. Ferro et alii, Frattura ed Integrità Strutturale, 18 (2011) 34-44; DOI: 10.3221/IGF-ESIS.18.04 37 shaking or a combination of the two techniques, have already been tried out [18]. Moreover, as such CNTs are hydrophobic. Commercial single walled carbon nanotubes dispersed by sonication in isopropanol with ordinary Portland cement (0.02 by weight CNT/cement ratio) for four hours led to cement particles coated with bundles of carbon nanotubes after alcohol evaporation [2]. After hydration (with a 0.4 water to cement ratio (w/c) and use of 10 g/L of a superplasticizer), CNT bundles were smaller in apparent diameter and more widely distributed with respect to unhydrated cement. However, the CNTs directly affected the early hydration process, producing higher hydration rates than those experienced by control samples, as evidenced from hardness tests and SEM observations [19-21]. Common superplasticizer, Mapei Dynamon SP1, an admixture based on modified acrylic polymer for precast concrete proved also to be effective in MWCNTs dispersion into water, after 4 hours of sonication by means of an ultrasonic probe [13, 22] (Fig. 4). Figure 4 : SEM micrograph of 0.5 wt% MWCNT cement composite [17]. Recent experiments have confirmed that MWCNTs can be effectively dispersed in the mixing water by using a simple, one step method utilizing ultrasonic energy and a commercially available surfactant [19]. Shah et al. [24] have evidenced that in the samples where no dispersing technique was used, CNTs appeared poorly dispersed forming large agglomerates and bundles. On the other hand, as expected, in the samples where dispersion was achieved by applying ultrasonic energy and using a surfactant, only individual CNT were identified on the fracture surface. As a result of effective dispersion, the mechanical properties of cement matrices, studied using fracture mechanics three-point bending tests on notched specimens, were substantially increased by adding a very low amount of CNT, 0.025% to 0.08% by mass of cement. The use of CNT at this very low percentage makes the cost of the material very attractive. This small quantity also enables the control of matrix cracks at the nanoscale level, as shown by scanning electron microscopy results. It was found that in addition to reinforcement benefits, CNTs can also improve the transport properties of cementitious materials by increasing the early age strain capacity of the cementitious matrix [25] and the incorporation of CNTs has led to a substantial reduction of the autogenous shrinkage [24]. F UNCTIONALIZATION amples having up to 10 wt% of as such MWCNTs (Tab. 1) were prepared by Tulliani et al [26] (Fig. 5). In order to minimize the size of the aggregated MWCNTs, they were first dispersed in water by means of an ultrasonic probe for four hours, prior to cement and sand additions. Moreover, a superlasticizer (Mapei, Dynamon SP1) and a viscosity modifying agent (VMA, Mapei, Viscofluid SCC/10) have been added to the mixture during the stirring stage, to help increasing cohesion and homogeneity of concrete mixture and also to avoid segregation and bleeding phenomena. Both were added in amounts recommended for self-compacting concrete (SCC) preparation, for carbon nanotubes content up to 1.75 wt% with respect to cement. While, for higher CNTs amount, the superplasticizer content was increased to favor CNTs dispersion (Tab. 2). The mortars showed only a slight increase of tensile strength, determined from “brazilian” tests, for a 2 wt% addition of pristine carbon nanotubes (Fig. 6) with respect to reference samples [26]. These results seem to indicate that the nanotubes are well dispersed within the cement matrix, but are weakly or not linked to it, as illustrated by Fig. 4, where