Influence of carbon nanofiber clustering on the chemo-mechanical behavior of cement pastes

Abstract

The influence of carbon nanofiber (CNF) clustering on the chemo-mechanical behavior of cement pastes subjected to a decalcifying environment was studied. Portland cement pastes with and without CNFs were exposed to a concentrated solution of ammonium nitrate to accelerate decalcification. Microstructural changes and evolution of the porosity were examined as a function of exposure duration. Changes in the flexural response of the cement paste with CNFs were studied and reviewed in relation to CNF clustering and microstructural evolution. Results showed a strong coupling between decalcification, CNF clustering, microstructural evolution, and the flexural properties of the cement paste. After 7 days of decalcification by NH₄NO₃, the CNF clusters acted as weak zones that reduced the flexural strength retention of the cement paste. However, after 125 days of decalcification by NH₄NO₃, a dissolution-filling mechanism within the clusters created a better bond with the surrounding cement paste, slowing down the loss of flexural strength and providing added ductility to the cement paste.

Introduction

Carbon nanotubes/nanofibers (CNTs/CNFs) possess a number of unique properties, including exceptional mechanical properties and high aspect ratios, that make them attractive for reinforcement of cement-based materials at the nano-level [1], [2], [3]. The contribution of CNTs/CNFs to the reinforcement is strongly dependent upon their distribution and arrangement in the cement matrix – CNT/CNF clustering has proven to be a critical factor in influencing the material mechanical and electrical properties [4]. Despite much effort to homogeneously disperse CNTs/CNFs in the cement matrix [5], [6], [7], there is still evidence of the presence of localized sub-micro and micro-scale clusters and inhomogeneous distribution of CNTs/CNFs [5], [7], [8], [9], [10], [11], [12]. While the occurrence of CNT/CNF clustering may constitute a defect causing a loss of mechanical properties, it may not always be undesirable. For example, it has been shown for polymer composites to enhance certain mechanical properties [13] and to favor the formation of a percolating network for electrical conductivity [14], [15], [16]. During their service life, cement-based materials are subjected to a wide variety of weathering induced degradation. Given that perfect dispersion at the individual fiber level may not be achievable and that controlled clustering may even be desirable for certain applications, it is therefore critical to understand the effect of CNT/CNF clustering on the durability of cement-based materials exposed to aggressive environments.

Decalcification is closely associated with various types of concrete degradation [17], including during sulfate attack and leaching by exposure to neutral or acidic waters. It is a complex dissolution-diffusion process that involves the removal of calcium from the cement paste and results in both chemical and physical degradation. A loss of cohesion of the cement paste, an increase in porosity, and a decrease in the mechanical properties are all common manifestations of decalcification [17], [18]. While the addition of CNFs is anticipated to improve the cement paste durability through pore refinement [19], [20] and by increasing the cohesion of the paste, the influence of a non-uniform dispersion of the CNFs, specifically CNF clustering on the cement paste during exposure to a decalcifying environment, is unclear. Yet, CNF clustering is expected to introduce a secondary porosity into the cement paste, which could significantly impact the chemical and mechanical property evolution during decalcification. This paper focuses on CNFs in Portland cement paste and reports on the effects of CNF clustering on the chemo-mechanical behavior of cement pastes subjected to a decalcifying environment. Portland cement pastes with and without CNFs were exposed to a concentrated solution of ammonium nitrate to accelerate decalcification. Microstructural changes and evolution of the porosity were examined as a function of exposure duration. Changes in the flexural response of the paste with CNFs were studied and reviewed in relation to CNF clustering and microstructural evolution.