THE USE OF NANOPARTICLES TO IMPROVE THE PERFORMANCE OF CONCRETE Ismael FLORES-VIVIAN, Rani G.K. PRADOTO, Mohamadreza MOINI, Konstantin SOBOLEV University of Wisconsin, Milwaukee, USA, sobolev@uwm.edu Abstract A reduction in size of nanoparticles provides an exceptional surface area-to-volume ratio and changes in the surface energy, surface chemistry, and surface morphology of the particle, altering its basic properties and reactivity. These characteristics significantly enhance the mechanical performance of a variety of materials, including metals, polymers, ceramic, and concrete composites. Concrete can be nano-engineered by incorporating nano-sized building blocks or objects (e.g., nanoparticles and nanotubes) to control material behavior and add novel properties, or by grafting molecules onto cement particles, cement phases, aggregates, and additives (including nano-sized additives) to provide surface functionality, which can be adjusted to promote specific interfacial interactions. Nanosilica (silicon dioxide nanoparticles, nano-sio2), for example, has been shown to improve workability and strength in high-performance and self-compacting concrete. This article reviews the beneficial effects of the nanoparticles for the improvement of concrete performance. Keywords: Cement, Composites, Concrete, Nanoparticles, Nano-SiO2, Nanotechnology, Strength 1. INTRODUCTION The majority of nano-research in construction has investigated the structure of cement-based materials and fracture mechanisms in composites [1]. With new advanced equipment, it is possible to observe the structure at its atomic level and even measure the strength, hardness and other basic properties of the micro- and nano-scopic phases of materials. Considerable progress over recent years has been achieved by the employment of nano-scale characterization to understand nano-scale processes in cementitious materials The application of atomic force microscopy (AFM) for investigating the amorphous calcium silicate hydrate (C-S-H) gel discovered that at nanoscale this product has a highly ordered structure [2]. Better understanding the structure at nano-level helps to influence the important processes related to production and use of construction materials strength development, fracture, corrosion and developing new functionalities. Façade and interior applications require new finishing materials with self-cleaning properties, discoloration resistance, anti-graffiti, and high-scratch resistance. Self-cleaning tiles, window glass, and paints were developed using the TiO2 photocatalyst technology which is based on the decomposition of organic pollutants under ultraviolet (UV) light [3]. Nano-chemistry with its bottom-up possibilities offers new products that can be effectively applied in concrete technology. One example is related to the development of new superplasticizers for concrete, such as the polycarboxylic ether polymers (PCE) targeting the extended slump retention of concrete mixtures. It was proposed that, when nanoparticles are incorporated into conventional building materials, such materials can possess advanced or smart properties required for the construction of high-rise, long-span or intelligent civil and infrastructure systems [1,4]. Nano-SiO2 (Fig. 1) has proved to be a very effective additive to composites to improve strength, flexibility, and durability. The particle size and specific surface area scale related to concrete materials reflect the general trend to use finer materials (Fig. 2). For decades, major developments in concrete performance were achieved with the application of super-fine particles such as fly ash, silica fume, and now, nanosilica.
Specific Surface Area, m 2 /kg 1,000,000 100,000 10,000 1,000 10 0 10 Nanosilica Precipitated Silica Nano-Engineered Concrete Silica Fume Metakaolin Finely Ground Mineral Additives Fly Ash High-Strength/High-Performance Concrete Conventional Concrete Portland Cement Aggregate Fines Natural Sand 1 0.1 Coarse Aggregates Fig. 1: Spherical nano-sio2 particles of uniform distribution observed under a transmision electron microscope [5] 0.01 1 10 100 1,000 10,000 100,000 1,000,000 10,000,000 Particle Size, nm Fig. 2: Particle size and specific surface area related to concrete materials [4] 2. PROPERTIES OF CONCRETE WITH NANOPARTICLES As defined by Sanchez and Sobolev [6], concrete can be nano-engineered by the incorporation of nanosized (less than 100 nm) building blocks or objects (e.g., nanoparticles and nanotubes) to control material behavior and add novel properties, or by the grafting of molecules onto cement particles, cement phases, aggregates, and additives (including nano-sized additives) to provide surface functionality, which can be adjusted to promote specific interfacial interactions. Nano-binder with a nano-dispersed cement component used to fill the gaps between the particles of mineral additives was proposed [4]. The nano-sized cementitious component can be obtained by the colloidal milling of portland cement (the top-down approach) or by the self-assembly using mechano-chemically induced topo-chemical reactions (the bottom-up approach). The addition of small amounts of calcium silicate hydrate (C-S-H) to portland cement and tricalcium silicate (C3S) pastes was investigated by Thomas et al. [7]. It was demonstrated that nano-c-s-h particles cause a significant acceleration of the early hydration. It was proposed that C-S-H provides nucleation sites for the precipitation of hydration products out of the pore solution. The most of the experimental work on particle incorporation into cementitious systems was conducted using nano-sio2 (Fig. 1). The accelerating effect of 5% nano-sio2 on the hydration of C3S (alite, Ca3SiO5) was reported by Björnström et al. [8]. The increased rates of C3S dissolution and subsequent accelerated formation of C-S-H gel were revealed with diffuse reflectance infrared Fourier transform (DR-FTIR) spectroscopy and differential scanning calorimetry (DSC). The DSC results suggest that the addition of colloidal silica (CS) mostly affects the initial silica polymerization rates rather than the amount of ultimate product [8]. Qing et al. [9] investigated the effect of nano-sio2 (NS) addition at a dosage of 1-5% on microstructure and strength of superplasticized cement pastes with 13% of ground granulated blast furnace slag (BFS). It was demonstrated that the addition of nano-sio2, even at small dosages, reduces the amount of calcium hydroxide (CH) formed at the aggregate s interface and reduces the size of CH crystals, thereby improving the aggregate s interfacial transition zone (ITZ), Fig. 3. Li [10] performed a laboratory study of superplasticized high-strength concrete incorporating 50% of class F fly ash and 4% of nano-sio2. It was demonstrated that the pozzolanic reaction of nano-sio2 is very quick, reaching 70% of its ultimate value within three days. The addition of 4% nano-sio2 to fly ash systems accelerated the pozzolanic reaction of fly ash. It was concluded that the use of nano-sio2 increases both the early-age (with 81% higher 3-day strength) and the ultimate strength of high-volume fly ash concrete. Porro et al. [11] investigated the effect of the size of SiO2 nanoparticles used at different dosages on the performance of portland cement pastes. It was demonstrated that the compressive strength of the cement pastes increased with the reduction of the particle size (Fig. 6). This improvement was attributed to the formation of larger C-S-H silicate chains in mixtures with nano-sio2.
Fig. 3: The interaction between nano-sio2 (NS) and Ca(OH)2 at the interface between paste and aggregate with time determined by X-Ray diffraction as compared with silica fume (SF) [9] Strength increase, % Strength increase, % 140 120 100 Flexural Strength in Portland Cement Pastes with Nanosilica Aditions 100-1000 nm 20 nm 5 nm 70 60 50 Compression Strength in Portalnd Cement Pastes with Nanosilica Adition 100-1000 nm 20 nm 5 nm % increase 80 60 % increase 40 30 40 20 20 10 0 0 2 4 6 8 10 12 14 % adition 0 0 2 4 6 8 10 12 14 % adition Dosage, % Dosage, % Fig. 6: The effect of nanosilica type and dosage on the flexural (left) and compressive (right) strength (% increase) of cement pastes [11] The bottom-up sol-gel synthesis of nano-sio2 (Table 1) and the effects of this material on the performance of cement systems were reported by Flores et al. [5]. The sol-gel process involves the formation of a colloidal suspension (sol) and gelation of the sol to form a network in a continuous liquid phase (gel). Usually, trymethylethoxysilane or tetraethoxysilane (TMOS/TEOS) is applied as a precursor for synthesizing nanosilica. The sol-gel formation process can be simplified to few stages: Hydrolysis of the precursor; Condensation and polymerization of monomers to form the particles; Growth of particles; Agglomeration of particles, followed by the formation of networks and gel structure; Drying (optional) to remove the solvents and thermal treatment (optional) to remove the surface functional groups and obtain the desired crystal structure. There are a number of parameters that affect the process, including ph, temperature, concentration of reagents, H2O/Si molar ratio (between 7 and 25), and type of catalyst. When precisely executed, this process is capable of producing perfectly spherical nanoparticles of SiO2 within the size range of 1 100 nm. The chemical reaction of nanosilica synthesis can be summarized in the Equation 1. The design and properties of nano-sio2 are summarized in Table 1 and Fig. 4. The addition of 0.25% nano-sio2 improved the compressive strength of standard mortars by 17% and 10% (vs. reference, NPC) at the age of 1 day and 28 days, respectively. The performance of mortars with commercial nanosilica, cembinder 8 (CB-8) and silica fume (SF), was over-performed by developed sol-gel nano-sio2 materials 3B3 and 3A3 synthesized at a molar ratio of TEOS/Etanol/H2O of 1/6/24 (Fig. 5). The distribution of nano-sio2 particles within the cement
paste is an important factor governing the performance; therefore, the disagglomeration of nanoparticles is essential to obtain the composite materials. The application of PCE superplasticizers, ultrasonification and high-speed mixing were proven to be an effective method to distribute nano-sio2. (1) Table 1 Design and properties of nano-sio2 [5] Specimen Type* Molar Ratio TEOS/Etanol/ H2O Reaction Time, hours Particle Size (TEM), nm Surface Area (BET), m 2 /kg 1B3 1/ 24 / 6 3 15-65 116,000 2B3 1/ 6 / 6 3 30, 60-70 145,000 3B3 1/ 6 / 24 3 15-20 133,000 4B3 1/ 24 / 24 3 5 163,000 1A3 1/ 24 / 6 3 5 510,100 2A3 1/ 6 / 6 3 <10 263,500 3A3 1/ 6 / 24 3 <10, 17 337,100 4A3 1/ 24 / 24 3 5 382,200 * BET- Brunauer Emmett Teller (BET) theory; TEM- Transmission Electron Microscope * Sample coding: ABC First number denotes molar ratio combination as per as experimental matrix Last number corresponds to the reaction time in hours Letter denotes reaction media: A- for acid and B- for base a) b) Fig. 4: The morphology of nano-sio2 (by transmision electron microscope) synthesized using sol-gel method at a molar ratio of TEOS/Etanol/H2O of 1/6/24 with ph=2 (a) and ph=9 (b) [5] Fig. 5: Compressive strength of investigated mortars at different ages [5] Collepardi et al. studied the performance of low-heat self-compacting concrete (SCC) produced with 1-2 % of nano-sio2 [12]. It was reported that the addition of nanosilica makes the concrete mixture more cohesive and reduces bleeding and segregation. The best performance was demonstrated by concrete with ground fly ash, 2 % nanosilica, and 1.5 % of superplasticizer. This concrete had the highest compressive strength of 55
MPa at the age of 28 days and the desired behavior in fresh state: low bleeding, cohesiveness, higher slump flow, and very little slump loss. 3. DEVELOPING NEW FUNCTIONALITIES The photocatalytic particles were applied in white cement-based concrete introducing self-cleaning and airpurification features [3]. Architectural concrete needs to maintain the aesthetic characteristics such as color over the entire service life, even in highly polluted urban environments; however, different pollutants adhere to the concrete surface and alter the appearance. The application of photocatalytic materials is a solution for destroying the organic pollutants and removing inorganic matter from the surface of architectural concrete. One of the most popular photocatalytic materials used in cement is the nano-sized anatase polymorph of TiO2. Under solar radiation, nitrogen oxide (NO) in the air is oxidized and converted to nitrate. The cement matrix entraps the NO2 and the nitrate salts are formed in alkaline environment. The complete process can be described by the reaction Equation 2 and 3 [3]. Accelerated 8-hour irradiation tests (with the wavelength more than 290 nm stimulating intensive solar light) of samples with 5% of nano-tio2 treated with phenanthroquinone solution in methanol (at a density of 0.1 mg/cm 2, resulting in homogeneous yellow color) almost completely restored the original reflectance spectra of the samples and turned their color back to white. Such treatment corresponds with approximately one month of sunlight exposure. It was reported that the photocatalytic nano-tio2 embedded into cement matrix is effective for NOx abatement (Fig. 6). NO + OH - NO2 + H + (2) NO2 + OH - (NO3)ads + H + (3) Fig. 6: The photocatalytic effect of TiO2 - cement composite: left, in the dark and right, under UV light [3] 4. FUTURE DEVELOPMENTS Vast progress in concrete science is expected in coming years by the adaptation of new knowledge generated from the rapidly growing field of nanotechnology. Development of the following concrete-related nanomaterials is on the way or can be anticipated [4]: Catalysts for the low-temperature synthesis of clinker and accelerated hydration of conventional cements; Grinding aids for superfine grinding and mechano-chemical activation of cements; Binders with enhanced/nanoengineered internal bond between the hydration products; Binders modified by nano-sized polymer particles and their emulsions, or polymeric nano-films; Bio-materials (including those imitating the structure and behavior of mollusk shells); Next-generation superplasticizers for total workability control and supreme water reduction; Binders with controlled internal moisture supply to avoid/reduce micro-cracking; Cement-based materials with engineered nano- and micro- structures exhibiting supreme durability; Eco-binders modified by nanoparticles and produced with substantially reduced volumes of portland cement component (down to 10-15%); Self-healing materials and repair technologies using nano-tubes and chemical admixtures; Materials with self-cleaning/air-purifying features based on photocatalyst technology;
Materials with controlled electrical conductivity, deformative properties, non-shrinking and low thermal expansion; Smart materials, such as temperature-, moisture-, stress-sensing or responding materials. 5. CONCLUSIONS Portland cement, one of the largest commodities consumed by humankind, has significant, but not completely explored potential. Better understanding and precise engineering of an extremely complex structure of cement-based materials at the nanolevel will result in a new generation of concrete that is stronger and more durable, with desired stress-strain behavior and possibly possessing the range of newly introduced smart properties, such as electrical conductivity, temperature-, moisture-, and stress-sensing abilities. Nanoparticles, such as silicon dioxide, were found to be a very effective additive to polymers and concrete, a development recently realized in high-performance and self-compacting concrete with improved workability and strength. Nano-binders and nano-engineered cement with nano-sized cementitious component or other nano-sized particles are the next ground-breaking development. LITERATURE [1] Bjornstrom, J., Martinelli, A., Matic, A., Borjesson, L., Panas, I. (2004) Accelerating effects of colloidal nano-silica for beneficial calcium-silicate-hydrate formation in cement. Chem Phys Lett; 392(1-3):242-248. [2] Cassar, L., Pepe, C., Tognon, G., Guerrini, G.L., Amadelli R.(2003) Proc. 11 th International Congress on the Chemistry of Cement (ICCC), Durban, South Africa, p-2012. [3] Collepardi, M., Ogoumah-Olagot, J.J., Skarp, U., Troli, R., (2002) Influence of Amorphous Colloidal Silica on the Properties of Self-Compacting Concretes, Proceedings of the International Conference, Dundee, UK, 473-483. [4] Flores I., Sobolev K., Torres L.M., Valdez P.L., E. Zarazua, and Cuellar E.L., (2010) Performance of Cement Systems with Nano- SiO2 Particles Produced Using Sol-gel Method. TRB International Conference on Nanotechnology in Cement and Concrete, Irvine, USA. [5] Li, G., (2004) Properties of high-volume fly ash concrete incorporating nano-sio2, Cement and Concrete Research, 34, 1043-1049. [6] Plassard, C, Lesniewska, E, Pochard, I, Nonat, A. (2004) Investigation of the surface structure and elastic properties of calcium silicate hydrates at the nanoscale. Ultramicroscopy;100(3-4):331-338. [7] Porro, A., Dolado, J.S., Campillo, I., Erkizia, E., de Miguel, Y., Sáez de Ibarra, Y., Ayuela, A. (2005) Effects of nanosilica additions on cement pastes; Applications of nanotechnology in concrete design; Thomas Telford; London. [8] Qing Y, Zenan Z, Deyu K, Rongshen C. (2007) Influence of nano-sio2 addition on properties of hardened cement paste as compared with silica fume. Construct Build Mater; 21(3):539-545. [9] Sanchez, F.; Sobolev, K. (2010) Nanotechnology in concrete - A review. Construction and Building Materials, 24(11), 2060-2071. [10] Sobolev, K., Ferrada-Gutiérrez, M. (2005) How Nanotechnology Can Change the Concrete World: Part 2. American Ceramic Society Bulletin, 11, 16-19. [11] Thomas, J.J., Jennings, H.M., Chen J.J. (2009) Influence of Nucleation Seeding on the Hydration Mechanisms of Tricalcium Silicate and Cement J. Phys. Chem. C, 113, 4327 4334. [12] Trtik, P., Bartos, P.J.M., (2001) Nanotechnology and concrete: what can we utilise from the upcoming technologies? Proceeding of 2 nd Annamaria Workshop, 109-120.