PSI - Issue 28

Angeliki-Eirini Dimou et al. / Procedia Structural Integrity 28 (2020) 1694–1701 A.-E. Dimou et al. / Structural Integrity Procedia 00 (2019) 000–000

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(NHL) mortars for the first time, aiming at enhancing the properties of the traditional materials used in the rehabilitation of buildings. However, the systematic incorporation of CBNs on lime-based restoration mixtures has not been extensively investigated so far. In the above context, the aim of the present work is to study the exploitation of hydraulic binders reinforced with CBNs for the restoration of Cultural Heritage Monuments. The reinforcement at the nano-scale with CBN of appropriate type and concentration aims to transform the non-conductive binder to conductive so as to attain piezo-electric properties and thus the ability for acting as self-sensing material. In this study, two types of modified CBNs are used to reinforce an NHL paste, namely reduced graphene oxide (rGO) and the carboxylated carbon nanotubes (MWCNTsCOOH). Nomenclature CBNs carbon-based nanomaterials CNTs carbon nanotubes GNPs graphene nanoplatelets GO graphene oxide MK metakaolin MWCNTsCOOH carboxylated carbon nanotubes NHL natural hydraulic lime rGO reduced graphene oxide 2.1.1. Nanomaterials Two different types of modified carbon nanomaterials were incorporated into the matrix. Both types of nanomaterials were produced at the University of Ioannina and under the framework of the research project “Self-healing and self sensing nano-composite conservation mortars – AKEISTHAI”. The first type was carboxylated multi-walled carbon nanotubes (MWCTsCOOH), deriving from unmodified MWCNTs that were produced by NANOCYL S.A. The carboxylation of MWCNTs was achieved through dispersion in dense solution of nitric acid. The suspension was subjected to magnetic stirring (refluxed) for 24 h at 120 o C and then was left to cool. When room temperature was reached, distilled water was added. After that, the sediment was collected, centrifuged, and rinsed until the pH reached the values of 6 to 7. The carboxylated nanomaterials were collected after drying. The second type was reduced graphene oxide (rGO), obtained from the reduction of graphene oxide (GO). Aqueous GO dispersion was mixed with NaBH 4 solution and the final mixture was mixed for 2 h at 80 o C. Then, the dispersion was centrifuged in 6000 rpm for 10 min. Finally, the solid part was collected and dried at room temperature. 2.1.2. Production of binder pastes The hydraulic paste was prepared by mixing commercial Natural Hydraulic Lime (NHL 5 - St Astier Natural Hydraulic Limes) and Metakaolin (Metacal 3000 - CALTRA Nederland BV) with weight contents 80 wt% and 20 wt%, respectively. The water to binder (W/B) ratio was set at 0.55 and the nanomaterials were added at 0.15 wt% of the binder. The concentration was chosen based on the preliminary studies (Dimou et al. 2020). Before being added to the binder, the aqueous dispersions of the nanomaterials were sonicated at 65 kJ sonication energy using a probe sonicator. Three different mixtures were prepared in total: a reference paste without nanomaterials and two different nano reinforced pastes, each one with a different nanomaterial. The mix recipes are given in Table 1. After mixing, the pastes were cast moulded and stored for 24 h at room temperature, After 24 h, all specimens were de-moulded and placed in water tanks, where they were kept for 28 days. For each experimental set, cylindrical and prismatic specimens were prepared. The cylindrical specimens had a diameter of 30 mm and a height of 60 mm, while the prismatic specimens had dimensions of 80 mm in length, 20 mm 2. Materials and Methods 2.1. Materials