(149f) On Ultra High Performance Nanostructured Concrete by Polymer Impregnation

There are 200 million automobiles and other vehicles on the road.  Federal highway infrastructure needs to be rebuilt.  The interstate highways were built in the 1950s under President Dwight E. Eisenhower.  President Barack Obama has called for extensive infrastructure building in the transportantion sector under his stimulus and jobs plan.  Concrete with compressive strength greater than 207 MPa (30,000 psi) needs to be developed. Nanostructured concrete may be a way to attain this goal.  President John F. Kennedy presented his vision to send a man to the moon by the ned of the decade and bring him back safely during his presidency between 1961-1963.  Lunar concrete is another goal. 

Concrete is the most used material by mankind next to water.  About 7.5 km³ of concrete is prepared every year.  This is roughly 1 m³ for every man, woman and child on the face of this planet, earth.  It is a 35 billion dollar industry employing about 2 million workers in United States of America.  Over 55,00 miles of highways are paved with reinforced concrete and pre-stressed concrete. Concrete is used in road and bridge construction, for support of elevated road beds and highways, pilings, pillars, etc. They are also used in skyscrappers, hgh rises, etc. 

The next generation concrete is expected to: (i) offer a service life of over 100 years; (ii) possess compressive strength upto 207 kPa; (iii) require less construction time; (iv) by watertight; (v) crack-free; (vi) versatile; (vii) durable; (viii) corrosion resistant; (ix) abrasive; (x) resilience to impact; (xi) engineering properties comparable to stainless steel, cast iron and ceramics. UHPC, ultra-high-performance-cement can be used to make thinner sections, larger spans and taller high rises.  Formulation to make cement composites include portland cement, silica fume, quartz fluor, fine silica sand, high range waste reducer and steel or organic fibers. 

The methods of preparation of concrete has changed since the days of the Roman empire when opus caementicium or cement was made from quicklime, pumice and pozzolan.  J. Aspdin in 1824 invented portland cement.  Raw mix is prepared this time and age followed by the clinker product and cement.  Raw materials in the form of slurry are made from CaO, calcium oxide,  SiO_2, silicon oxide, Al₂O₃, alumina, Fe₂O₃, ferric oxide and MgO, magnesium oxide.  Quarried from local rock, they are ground together in a raw mill. Milled down to less than 100 µm the raw mixture is heated in a cement kiln with temperatures of 1400 - 1450 ⁰C. A complex set of chemical reactions leads to a sintered product.  The enthalpy of formation of clinker from CaCO₃ and clay minerals is 1700 kJ.Kg^-1.  Emergence of nanotechnology (Sharma, 2010) has enhanced the expectations for: (i) stronger concrete; (ii) development of super plasticizer; (iii) reduction of clinkering temperature; (iv) cold sintering of clinker materials in mechano-chemical reactions; (v) nanocatalysts for quicker hydration reactions; (vi) grinding aids for superfine clinkers; (vii) binders reinforced with nanotubes and nanorods; (viii) biomimetic materials; (ix) cement based composites; (x) step change improvement in ductility and toughness; (xi) reduced micro-cracking;  (xii) self-healing materials using fullerenes.  The use of an effective super plasticizer can effect dispersion stability in nanosilica products and thereby improve the strength of Portland cement mortars at all ages of hardening.  Longevity of concrete can be improved by introducing reinforcing carbon fibers.  These fibers can come from bituminuous coals (Renganathan and Zondlo, 1988).  Improved flexural, deflection and ductile properties of conrete can be effected by proper introduction of carbon fibers. Voids in concrete may be filled usinng nanofibers of carbon. Void fillage can result in reduction of crack formation as energy gets deflected and dissipated during propagation.

39 different nanostructuring methods that may be used in preparation of nanostructured concrete are reviewed. These methods are as follows: (1) sputtering of molecular ions;(2) gas evaporation; (3)  process to make ultrafine magnetic magnetic powder; (4) triangulation and formation of nanoprisms by light irradiation ; (5) nanorod production using condensed phase synthesis method;  subtractive methods such as; (6) lithography; (7) etching; (8)  galvanic fabrication; (9) lift-off process for IC circuit fabrication; (10) nanotips and nanorods formation by CMOS process; (11) patterning Iridium Oxide nanostructures; (12) dip pen lithography; (13) SAM, self assembled monolayers; (14) hot embossing; (15) nanoimprint lithography; (16) electron beam lithography; (17) dry etching; (18) reactive ion etching;  (19) quantum dots and thin films generation by; (20)  sol gel; (21)  solid state precipitation; (22)  molecular beam epitaxy; (23)  chemical vapor deposition; (24)  CVD; (25)  lithography; (26)  nucleation and growth; (27)  thin film formation  from surface instabilities; (28) thin film formation by spin coating;(29) cryogenic milling for preparation of 100-300 nm sized  titanium; (30) atomic lithography methods to generated structures less than 50 nm; (31) electrode position method to prepare nanocomposite; (32) plasma compaction methods; (33) direct write lithography; (34) nanofluids by dispersion. Thermodynamic miscibility of nanocomposites can be calculated from the free energy of mixing. The four thermodynamically stable forms of Carbon are diamond, graphite, C₆₀, Buckminster Fullerene and Carbon Nanotube. 5 different methods of preparation of CNTs, carbon nanotubes are discussed.

     Polymer impregnated concrete specimens can improve fuel efficiency of automobiles, i.e., mpg.  20 different block copolymer precursor morphologies can lead to 20 different nanostructures post structuring. The structures are a spectrum ranging from layers to orderly spherical dispersions. Streaks may also be found. It is not clear why certain morpholgies lead to better performance.  Thermodynamic considerations of miscibility and phase separation are also pursued. Minimum allowable size estimates based on Gibbs free energy and surface tension need to evaluated for agglomerative stability.

A concrete specimen can be designed as follows: (i) PAn, polyacrylonitrile fibers are formed by electrospinning. Nanofiber formation can be better controlled. Prototypical concrete comprises of 15% cement, 18% water, 8% air, 28% fine aggregate and 31% coarse aggregate.  PAN fibers and nylon fibers can be impregnated into the cement composite.  Silicate platelets may be used with the intergallery distance in the nanodimensions ( < 100 nm).  The bond between fibers and the cementicious portion may be strengthened by pozzalanic reations. Slakes lime, Ca(OH)_2, calcium oxide may be reacted with amorphous siliceous materials to form calcium silicate hydrate, CSH. The general formula for calcium silicate hydrate (CaH₂SiO₄.2H₂O). Consequence of reaction products may be swelling. Interfacial adhesion between the fiber and the cementicious portion is an important consideration.  Interface changes with time as teh cement matures or undergoes time dependent volume changes.  Optimal volume fraction of nanofibers for better performance of nanocrete can be arrived at from mathematical modelling.  Current practioners use 0.5% PP, polypropylene fibers. 

Cost reductions in nanostructuring are proposed. The cost has come down from $200,000 per pound in the Arizona process to $200 per pound in the second generation combsution synthesis process.  HeIM, helium ion microscopy or gamma ray characterization are proposed to study the nanostructured speciment. The Raleigh resolution criterion is 200 nm. So optical devices are not sufficient. 

The fundamental structure of cement is not clear.  A new molecular model for cement hydrate was unveiled at MIT, (Pellenq. et. al., 2009).  Previous models based on natural mineral analogs indicated that cement nanoparticles consist of long SiO₂ chains interspersed with neat layers of CaO and H₂O.  The new model based on neutron scattering data is a hybrid of crystalline and amorphous CaO-SiO₂-H₂O.  Hybrid model contains shorter chains of SiO₂ with flaws that undercut the CaO layers leading to voids that are filled by water molecules. Minor disorder provides allowance for cement to stretch or compress rather than snapping. With molecular models similar to this cement chemical structure can be manipulated in order to obtain the desired strength of the concrete, durabilioty and environmental qualities such as low CO₂ emissions.