Northwestern researchers invented a process to use multi-walled carbon nanotubes (MWCNTs) at very low levels in conjunction with additives and mixing methods that provide excellent MWCNT dispersion in cement matrixes and afford significant improvement in cured mechanical properties. An aqueous MWCNT (0.08 wt% cement basis) additive solution, after suitable mixing, is blended into cement under ASTM procedures, cast and cured. Scanning electron microscopy (SEM) analysis of fracture surface specimens after 18 hours reveals isolated MWCNT fibers with a high degree of dispersion in the matrix. The uniform MWCNT distribution contributes to the observed increase in cured cement mechanical properties. Over 45% increase in the 28 day flexural strength is realized at the 0.08 wt% MWCNT loading versus the unmodified control. This modest MWCNT addition inhibits cracking at the nanoscale level and provides a cement matrix essentially “crack free”. The current invention promises a cost effective technology to provide high strength cements in a direct and scalable process.
Most construction cements today are hydraulic, and generally based on Portland cement, composed primarily of limestone, certain clay minerals and gypsum. Effort to mitigate structural failures in cement is a constant endeavor that has employed a range of materials. Although microfiber reinforcement has led to significant improvement of cement mechanical properties, flaws at the nanoscale remain. Carbon nanotubes (CNTs) have been added to cementitious matrices at 0.5 to 1.0 wt% loadings to overcome such defects but suffered from poor dispersion and cost.
Northwestern researchers Surendra P. Shah, Maria S. Konsta-Gdoutos, and Zoi S. Metaxa were able to overcome the major obstacle to the manipulation and use of carbon nanotubes for reinforcement in cementitious materials which has been their poor dispersion in cement. A composite cement material was prepared from cement material and carbon nanotubes from about 0.02 wt % to about 0.10 wt % based on weight of cement material. The process for preparing such cement compositions includes sonicating a mixture of a surfactant, water, and carbon nanotubes; blending the dispersion and the cement material to form a cementitious paste. The composite cement materials are useful in a variety of cement applications where a reduction in nanoscale flaws and fractures is desired.
The nanocomposite cementitious material exhibits a reduction of autogenous shrinkage of at least 30% after 96 hours from casting as compared to the same cementitious material without carbon nanotubes. The nanocomposite cementitious material exhibits a modified nanostructure so that the average values of stiffness and hardness of C-S-H, as determined by nanoindentation tests, are higher compared to the same cementitious material without carbon nanotubes. The nanocomposite cementitious material exhibits an increase of the Young's modulus of at least 15% up to about 55% as compared to the same cementitious material without carbon nanotubes. The nanocomposite cementitious material exhibits an increase in flexural strength of at least of 8% up to about 40%. as compared to the same cementitious material without carbon nanotubes.
CNT cement composites are expected to find wide application for highway structures, bridges, pavements, runways for airports, continuous slab-type sleepers for high speed trains and in general in all applications of conventional and high strength concrete, as well as in manufactured precast elements for residential and commercial buildings.
Further details of the invention are revealed in United States Patent Application 20090229494. The technology is available for licensing from Northwestern University. Interested parties should contact Technology Transfer @ Northwestern 1800 Sherman Avenue - Suite 504 Evanston, IL 60201 Phone: 847-491-3005 Fax: 847-491-3625 E-mail: jcowan(at) northwestern.edu