Engineering properties of carbon nanotube reinforced ordinary Portland cement paste

Abstract

Although ordinary Portland cement (OPC) is one of the most widely used construction materials in the world, its relatively weak tensile strength and fracture resistance limit wider structure applications. Carbon nanotubes (CNTs), having been identified as one of the strongest and stiffest materials on earth, are attractive candidates as nano-filaments in reinforcing OPC. The investigation of CNT reinforced OPC composites (CNT-OPC) is at a relatively early stage, and very limited research regarding the effectiveness of CNTs in enhancing the tensile strength or fracture toughness of OPC are available in open literature. The published results on testing of the CNT reinforcing effect often show large variations and sometimes contradict each other. Therefore, there is a significant need for further studies in this area to improve understanding of the reinforcing behaviour of CNTs in cement matrix, including dispersion of CNTs and the effect of CNT on OPC paste in terms of hydration, microstructure, tensile strength and fracture properties. This study builds on the earlier research on CNT reinforcement of the mechanical properties of OPC paste in Duan’s group in the Department of Civil Engineering at Monash University. Specifically, the objectives of this study are (1) development of high mechanical performance CNT reinforced OPC paste by considering the effect of dispersion and concentration of CNTs within OPC matrix, and (2) investigation of the reinforcing mechanisms of CNTs in cement matrix by estimating the post-peak softening behaviour of CNT-OPC paste and the interfacial bond strength between CNTs and cement matrix. In order to develop high performance CNT reinforced OPC paste, the dispersion and concentration of CNTs were studied. The combinations of chemical functionalisation (COOH functional groups on CNT surface), a polycarboxylate-based cement compatible superplasticiser (PC), and sufficient ultrasonication energy have been found essential to achieving homogeneous dispersion of CNTs in cement paste. An effective PC to CNT mass ratio of 8 is recommended to ensure effective dispersion of CNTs, at the same time maintaining satisfactory workability of CNT-OPC paste. Moreover, a CNT dosage-independent optimal ultrasonication energy for achieving mechanically superior CNT-OPC paste was found to be 50 J/ml per unit CNTs-to-suspensions weight ratio. The incorporation of CNTs (of 0.075 wt.% of cement) substantially enhanced the mechanical properties of plain cement. For example, Young’s modulus was improved by 31.5%, flexural strength by 49.9%, and fracture energy by 62.6%. Regarding the reinforcing mechanisms of CNTs within cement paste, the post-peak softening characteristics of CNT-OPC paste were investigated. It was found that the linear post-peak softening characteristics, including initial fracture energy and cohesive tensile strength of plain OPC paste (estimated using size effect tests with the assistance of cohesive crack-based finite element simulation), can be significantly improved by incorporating CNTs. Particularly, the enhancement of cohesive tensile strength may be attributed to the filling of CNTs in the nano-sized colloidal pores and bridging micro-sized capillary pores. On the other hand, the comparable brittleness numbers obtained for CNT-OPC and OPC pastes indicate the minor contribution of CNTs to the ductility of plain OPC paste. Based on cohesive tensile strength, the range of effective interfacial bond strength between CNTs and cement matrix was estimated to be from 9.5 to 24.5 MPa based on a micromechanics-based crack bridging stress-crack separation model for CNT reinforced composites developed in Duan’s group. This result suggests that the introduction of COOH functional group may enhance the interfacial bond property between CNTs and cement matrix, thereby resulting in improved mechanical performances. This project will provide key information to assist other researchers and practitioners to better understand and apply the novel CNT-cement composite with improved mechanical properties to the design of structures and codification.