Nanocomposites and nanomaterials

The effects of MWCNT-SiO2 core-shell structures on the mechanical and microstructural properties of cement pastes exposed to elevated temperature
Sikora P.1,2, Abd Elrahman M.2,3, Chung S.-Y.2, Cendrowski K.4, Mijowska E.4, Stephan D.2
1 Faculty of Civil Engineering and Architecture, West Pomeranian University of Technology, Szczecin. Al. Piastow 50, 70-311 Szczecin, Poland
E-mail: pawel.sikora@zut.edu.pl
2 Building Materials and Construction Chemistry, Technische Universität Berlin, Gustav-Meyer-Allee 25, 13355 Berlin, Germany
3 Structural Engineering Department, Mansoura University, Elgomhouria St., 35516 Mansoura City, Egypt    
4 Nanomaterials Physicochemistry Department, Faculty of Chemical Technology and Engineering, West Pomeranian University of Technology, Szczecin. Al. Piastow 45, 70-311 Szczecin, Poland

This study is aimed to investigate the effect of silica-coated carbon nanotubes admixture on the behaviour of cement pastes exposed to high temperature (300, 450 and 600 °C). Pristine multi-walled carbon nanotubes (MWCNTs) were coated with solid silica shell to form a core-shell nanostructure (MWCNT-SiO2). To obtain MWCNT-SiO2, low-cost green synthesis method was proposed in order to decrease the core-shell nanostructure synthesis costs. For comparison pristine MWCNTs were used.
The cement pastes were incorporated with nanomaterials so that the weight of pristine MWCNTs content was equal in both types of pastes to 0.125, 0.25 and 0.5 wt.-% of cement. Hydration kinetics and compressive strength of cement pastes prior and after heating were evaluated. To characterize the microstructural changes in the specimens after heating X-ray computed tomography (CT) method was incorporated. 
The results demonstrated that incorporation of optimum amount (0.125 wt.-%) of MWCNT-SiO2 is much more beneficial than pristine MWCNTs for improving the mechanical and microstructural properties of cement pastes both in unheated and heated state. Functionalization of MWCNTs with silica shell improved the binding ability between cement matrix and nanotubes which was reflected in compressive strength retention of specimens as well as decreased micro-cracking. Thin silica shell is capable to protect the carbon nanotubes from calcination and enlarge the temperature range with positive influence of the carbon nanotubes on the residual compressive strength of cement pastes. Samples containing MWCNT-SiO2 exhibited compressive strength retention up to 600 °C, while pristine MWCNTs shows retention only up to 450 °C. However, exceeding optimum amount of nanotubes leads to creation nanotubes agglomerates. Therefore carbon nanotubes agglomerates decreases the thermal resistance of cement pastes resulting in lower strength values as well as increased micro-cracking of specimens.
Acknowledgement: This research was supported by the National Science Centre within the project No. 2016/21/N/ST8/00095 (PRELUDIUM 11).