Structural Properties and Damage Detection Capability of Carbon Nanotube Modified Mortars after Freeze-Thaw

Abstract

Here we explore the structural properties and damage sensing of cementitious mortars after a freeze-thaw process (F-T) as a function of nano-modification. For this purpose, carbon nanotubes were added at 0.2–0.8 wt.% cement using two different dispersive agents. F-T resulted in reduced fracture energy in nano-modified specimens prepared using superplasticizer as a dispersant while the opposite held true for the surfactant-containing ones. All nano-modified mortars possessed significantly higher fracture energy compared to the plain specimens after F-T (up to 73% improvement). The acoustic emission activity was lower after F-T, while acoustic emission indicators revealed a more tensile mode of fracture in both plain and nano-modified mortars.

Monitoring the Hydration Process in Carbon Nanotube Reinforced Cement-Based Composites Using Nonlinear Elastic Waves

“…It should be mentioned here that during the initial stage of hydration, during the first 3 to 4 h which is considered as the period needed for the connection of small particles, it was observed an inability to detect any ultrasound signal, a fact that has been also noticed in previous investigations [2,26,[50][51][52][53]. This repeatable behavior can be attributed to the high attenuation of the fresh cement-based products.…”

Effect of Environmental Exposure on the Pore Structure and Transport Properties of Carbon Nanotube-Modified Mortars

“…CNTs were selected based on their geometrical characteristics (length > 10 μm, diameter range 20–45 nm, and specific surface area >150 m 2 g −1 [ 29 ]) and carbon purity (carbon purity >94%). Sodium-dodecyl-benzene sulfonate was used due to its high efficiency in CNTs’ separation, while poly-carboxylate-based superplasticizers (e.g., Viscocrete Ultra 300) have been previously applied in suspensions specifically targeted for nano-modified cementitious materials owing to their anionic nature and compatibility with cement [ 29 , 30 ]. An anti-foaming agent (TBP) (Tributyl-Phosphate, Sigma-Aldrich, Taufkirchen, Germany) was applied in surfactant-based nano-modified mortars to maintain the air content values constant at 5.5 ± 0.2% [ 29 ], while an additional superplasticizer (Viscocrete ultra 600, Sika Hellas SA., Krioneri, Greece) was used in all mixes to keep the workability at 16.3 ± 0.2 cm [ 29 ].…”

“…Nano-modification was introduced through the water requirement of the mixture, by preparation of aqueous suspensions of CNTs at variable concentrations. For this purpose, CNT/dispersive agent/water suspensions were sonicated according to an optimized protocol that can be found in our earlier works [ 29 , 30 ]. As already discussed, the dispersant/CNT ratio was selected based on the dispersive ability of each agent and its potential side effects on the physical properties of the prepared mortars [ 29 , 30 ].…”

“…0.5–0.6 wt.% for this type of material. Further details about the production of the CNTs suspensions, proportions of materials, and mixing procedure can be found in our previous works [ 29 , 30 ]. The fresh mortars were placed in steel molds for 24 h. After demolding, they were immersed in water baths for another 27 days.…”

“…To this end, we have recently published two studies on the durability of CNT-modified cementitious composites. One study was related to the pore structure and transport properties before environmental exposure [ 29 ] and the other related to the structural properties and damage detection capability after the environmental exposure of such materials [ 30 ]. Key parameters of interest in both studies was the effect of the amount of CNTs in combination with the type and amount of dispersant used to facilitate dispersion of the nano-filler.…”