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Recent research has shown that incorporating carbon nanotubes into cement results in composite materials with significant potential. In particular, researchers and engineers are always on the lookout for new, higher-performing concrete materials with superior durability, mechanical strength, and rheological qualities.
Taking into consideration the mechanical qualities of carbon nanotubes and the outcomes of studies on nanotube-reinforced cement materials, it is clear that such composites might be used as next-generation concrete materials.
Cement-based materials are generally considered to be relatively brittle materials with poor tensile strength and toughness, qualities which result in cracking in various structures. Reinforcement materials like cellulose, synthetic fibers, and inorganic fibers like steel have been the typical approach to manage cracking in cement forms. However, due to very robust and desirable mechanical, thermal, and electrical qualities, carbon nanotubes have captured the attention of those developing new cement materials.
The extremely small size and high aspect ratios, typically greater than 1000-to-1 and as much as 2,500,000-to-1, of carbon nanotubes, make it easy to disperse them on a finer scale than popular reinforcing fibers. When nanotubes are correctly dispersed in cement, cracks can be disrupted very rapidly during propagation. Gradually, the bridge coupling impact of carbon nanotubes ensures reduced crack dimensions and ensures load-transfer across gaps and cracks. For that reason, nanotube-reinforced composites can have greater flexural and compressive qualities, in addition to reduced strain and greater density.
One possible application of carbon nanotubes in cement is concrete rehabilitation, including exterior restoration and crack maintenance. Current materials like epoxies and grouts are somewhat effective, but epoxy repair often isn't acceptable for concrete spalling, and grouts are associated with high costs and the need for rapid heating. Considering these and other issues, nanotubes-cement composites seem to have major potential, particularly as repair mortar.
Finding an Effective Method for Dispersion
Nanotubes have a very high specific surface area and are capable of aggregating to create bundles due to their high energy; however, suitable dispersion and mixing procedures have been identified as essential factors influencing the overall performance of nanocomposites, as inadequate distribution of carbon nanotubes may bring about various defects in the resulting material, undercutting the reinforcing effect of using carbon nanotubes.
Right now, there are two techniques that are widely used for the dispersion of carbon nanotubes. Both are based on activation of the nanotube surface so that it can bond to cement.
The first dispersion approach is chemistry-based, which involves the introduction of functional groups on the exterior of carbon nanotubes via chemical reagents, ultraviolet radiation, and other methods. The second approach is physical, which utilizes mechanical operations like crushing and ultrasound to activate the exterior of the carbon nanotubes.
One recent study on the dispersal of nanotubes using ultrasonic treatment revealed how a carboxylation method could enhance the bonding action between carbon nanotubes and a cement paste, resulting in higher flexural and compressive strength qualities. When the study team used a scanning electron microscope to investigate the resulting material, they discovered the hydration of cement paste was facilitated by the inclusion of carbon nanotubes. They also observed nanotube structures and crack-bridging activities.
Importantly, the fabrication method used to create carbon nanotubes products for cement mixing, such as nanotube pellets, solutions, or powders, can impact the rheological qualities of cement-based materials. Moreover, ultrasound activates water, smashing hydrogen bonds, and boosting the pH value of water as a result. There has not been significant study of carbon nanotubes dispersal via ultrasound related to viscosity and pH value. This is a major concern as both qualities are very relevant to the effective usage of carbon nanotubes in cement materials.