CaO in Floor Tile Glazes Affects Hardness

Chemical Education Today

Reports from Other Journals

Research Advances by Angela G. King

MgO/CaO in Floor Tile Glazes Affects Hardness Anyone building a home wants the expensive ceramic tile floors they install to last forever. Thus, consumers want to maximize abrasion resistance, hardness, and mechanical properties. Ceramic floor tiles are essentially binary products that consist of a ceramic support topped by a glass layer, typically referred to as a glaze. Many of the properties of tiles, including scratch resistance and hardness, are based on the glaze, which can be processed either as a powder or bulk. The ceramic-glass process controls the crystallization of glass, and the final tile properties are based on the formation of homogeneously dispersed fine crystals within the glass. To help achieve a favorable microstructure throughout the glaze, manufacturers often add nucleants to it.

Earlier research demonstrated that in CaO–MgO– Al2O3–SiO2 quaternary glasses, the amount of MgO is critical to the formation of ␣-cordierite crystals while CaO favors the formation of anorthite crystals. Since maximizing the amount of fine ␣-cordierite crystals with well-defined morphologies improves the mechanical properties of the resulting glazes, scientists from the University of Valencia in Spain studied the effect of varying the MgO/CaO ratio on the formation of ␣-cordierite. In addition to promoting the growth of fine ␣-cordierite crystals, they also wanted to prevent the formation of a secondary crystalline phase, ␮-cordierite.

Figure 2. FESEM micrograph of glass GC11 heated at 1160 ⬚C for 5 min. (bar = 2 ␮m). Image credits: Reprinted with permission from the J. Am. Ceram. Soc. 2004, 87, 1227–1232.

Figure 1. Comparison of X-ray diffraction patterns of glass GC11 fast fired (25 ⬚C min᎑1) at different temperatures for 5 min (• is ␣cordierite). Reprinted from Torres, Francisco Jose; Alarcon, Javier. Effect of MgO/CaO Ratio on the Microstructure of Cordierite-Based Glass-Ceramic Glazes for Floor Tiles. Ceram. Int. 2005, 31, 683– 690, with permission from Elsevier.

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Figure 3. High-magnification view of ␣-cordierite hexagonal prisms in glass GC11 heated at 1160 ⬚C for 5 min. (bar = 1 ␮m). Image credits: Reprinted with permission from the J. Am. Ceram. Soc. 2004, 87, 1227–1232.

Vol. 83 No. 10 October 2006

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Reports from Other Journals Suppression of ␮-cordierite formation facilitates densification by viscous flow and produces a higher quality ceramic tile. Led by Javier Alarcón, the research team prepared and characterized three glasses with compositions (in weight percent) 55 SiO2, 21.5 Al2O3, (16.5-x) MgO, x CaO, 3.8 TiO2, and 2.9 B2O3. B2O3 was added as a nucleant to suppress the formation of ␮-cordierite, while TiO2 was added to maximize the direct nucleation of ␣-cordierite. Glass powders with this composition were prepared using conventional techniques. Pellets of loose pressed glass powder were fast thermal treated (25 ⬚C min᎑1) with temperatures ranging between 700 and 1190 ⬚C to simulate the industrial processing of glazed ceramic tiles. The crystallization and microstructural evolution was examined using several techniques. X-ray diffraction analysis revealed the relative amounts of ␣-cordierite. Field emission scanning electron spectroscopy (FESEM) allowed scientists to observe the microstructure of thermally treated samples, while energy dispersive X-ray microanalysis allowed the scientists to evaluate the composition of ␣-cordierite crystals in the glass samples. The researchers’ success in increasing tile hardness was assessed by the Vickers microhardness test. This test uses diamond pyramid-shaped indenters and a very precise instrument to automatically apply a given load to the sample on a microscopic scale. Precision microscopes are employed to measure the resulting indentations with accuracy up to ±5 ␮m. Taken together, the results indicate that ceramic glazes contain the best-shaped crystals and the highest microhardness if they are constructed with an intermediate range MgO/CaO ratio of x = 4.6. Be sure to ask the salesperson at your local home improvement or hardware store for this ratio—and encourage them to learn the chemistry behind it!

More Information 1. Torres, Francisco Jose; Alarcon, Javier. Effect of MgO/CaO Ratio on the Microstructure of Cordierite-Based Glass-Ceramic Glazes for Floor Tiles. Ceram. Int. 2005, 31, 683–690. 2. This Journal has published resources on the chemistry of ceramic glazes. See Canty, Allan J.; Canty, Carolyn D. Copper in Apple Ash Glazes for Ceramics: An Example of Environmental Chemistry and Chemistry for Potters. J. Chem. Educ. 1981, 58, 448 and Denio, Allen A. Chemistry for Potters. J. Chem. Educ. 1980, 57, 272–275. Also Denio, Allen A. The Joy of Color in Ceramic Glazes with the Help of Redox Chemistry. J. Chem. Educ. 2001, 78, 1298–1304 was a special article for National Chemistry Week. 3. More information on hardness testing can be found at http:// www.gordonengland.co.uk/hardness/ (accessed Aug 2006).

Lemongrass Extract’s Architectural Application? Due to their biocompatibility and ease of functionalization, gold nanoparticles have applications in catalysis, sensors, and medicine. Now add glass coating to that list, thanks to the ingenious use of a natural extract. Spherical gold nanoparticles with diameters of less than 20 nm are known to absorb visible light due to surface 1420


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plasmon resonance (SPR). Open aggregated structures of gold nanoparticles and particles possessing intrinsic anisotropy are known to shift these absorbances. In some cases, the SPR can extend into the near-infrared (NIR) region of the electromagnetic spectrum. Now a team of researchers from the National Chemical Laboratory in Pune, India, has studied the control of gold nanotriangle size, which in turn controls the optical properties of the gold nanotriangles. The nanotriangles are prepared through the reaction of aqueous lemongrass extract with gold ions. The lemongrass extract acts as a reducing agent, and the scientists, led by Murali Sastry, determined that the size of the resulting nanotriangle varies with the concentration of the extract. Nanotriangles were characterized with UV– visible–NIR spectroscopy and transmission electron microscopy, atomic force microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Careful control of the amount of lemongrass extract allows the longitudinal SPR band in the NIR region measured for resulting nanotriangles to be “tuned”. Currently, many buildings in regions with plenty of solar energy utilize NIR reflective coatings. Coatings of NIRabsorbant nanomaterials offer another option for blocking heat through NIR transmission through window glass. To illustrate the effectiveness of this application, the research team measured temperature as a function of time inside a closed box fitted with a window. The box was irradiated using a 250-W tungsten filament IR lamp 20 cm away. Measurements were taken for an untreated glass window, and glass coated with spherical gold nanoparticles, gold nanotriangles (one or three layers), or gold nanotriangles after heating to 300 ⬚C for 3 h. Compared to the untreated glass, the glass with a coating of spherical nanoparticles follows a similar time versus temperature trend. This is due to the inability of the isotropic nanospheres to absorb in the NIR region. In contrast, the glass coated with gold nanotriangles showed a drastic reduction in temperature. This holds true even for the coating that had been pre-treated at 300 ⬚C, demonstrating the heat resistance of the nanotriangle coatings. Reducing the temperature inside buildings through NIR absorbing glass coatings, as modeled by this experiment, would lead to a huge savings in cooling costs in warmer climates. In addition to applications in window coatings, scientists are hopeful that the NIR absorption could be applied in cancer hyperthermia as well.

More Information 1. Shankar, S. Shiv; Rai, Akhilesh; Ahmad, Absar; Sastry; Murali. Controlling the Optical Properties of Lemongrass Extract Synthesized Gold Nanotriangles and Potential Application in Infrared-Absorbing Optical Coatings. Chem. Mat. 2005, 17, 566–572. 2. This column has previously described research involving gold nanoparticles and their applications. See J. Chem. Educ. 2005, 82, 666. 3. A teaching lab involving the preparation of stabilized gold nanoparticles has been published. See Dungey, Keenan E.; Muller, David P.; Gunter, Tammy. Preparation of Dppe-Sta-


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Total initial Hg in concrete (µg/kg)

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Figure 4. Trends in initial Hg concentration in concrete ( ) and Hg mass release rates over the first 2 days (䊐) and 28 days (䉱) of concrete curing. OPC: ordinary Portland cement; FA33: concrete for which fly ash replaced 33% of the Portland cement; FA55: concrete for which fly ash replaced 55% of the Portland cement; HgPAC: concrete with 33% fly ash plus 0.5% Hg-loaded PAC. Reprinted with permission from Environ. Sci. Technol. 2005, 39, 5689–5693. Copyright 2005 American Chemical Society.

bilized Gold Nanoparticles. J. Chem. Educ. 2005, 82, 769– 770. 4. This Journal has also published work discussing the place of NIR in the undergraduate curriculum. See Yappert, M. Cecilia. Near Infrared (NIR) Spectroscopy in the Undergraduate Chemistry Curriculum. J. Chem. Educ. 1999, 76, 315–316. 5. An article describing similar work by Murali Sastry is available online from NewScientist.com. See http://www.newscientist.com/ article.ns?id=dn3828 (accessed Aug 2006). 6. An article describing hollow cages of gold atoms is available: Bulusu, Satya; Li, Xi; Wang, Lai-Sheng; Zeng, Xiao Chen. Proc. Nat. Acad. Sci. 2006, 108, 8326–8330.

Heavy Metal Concrete? Fly ash is the inorganic, incombustible matter present in coal that is fused during combustion in power plants into a glassy, amorphous structure. Fly ash, the resulting small spherical particles, is lifted up by flue gases and collected from the exhaust by electrostatic precipitators or filter bags. It consists mostly of silicon dioxide (SiO2), aluminum oxide (Al2O3), and iron oxide (Fe2O3) and reacts with calcium hydroxide to form cementitious compounds. Coal fly ash is used in the U.S. in both concrete and grout. It enhances desirable properties of both freshly poured and hardened concrete when used to replace some cement and also reduces some carbon dioxide emissions associated with the production of cement. Since fly ash is a low-cost by-product of generating electricity, it reduces the cost of concrete. However, fly ash contains some heavy metals, including 0.1–0.2 mg of mercury per kg fly ash, mainly as HgCl2 and HgO. This concentration will increase in response to the EPA Clean Air Mercury Rule. To lower mercury emissions, powdered activated carbon (PAC) will be injected into flue gas streams at some power plants. Small amounts of PAC conwww.JCE.DivCHED.org

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Figure 5. Air sampling apparatus. Reprinted with permission from Environ. Sci. Technol. 2005, 39, 5689–5693. Copyright 2005 American Chemical Society.

taining mercury (HgPAC) will be added to particulates captured from the flue stream and re-used as fly ash. To understand the effect of HgPAC on the use of fly ash in concrete, a research team based at The Ohio State University studied the release of mercury into the air from concrete prepared with fly ash and with HgPAC. The project was sponsored by the Electric Power Research Institute, the research arm of the electricity generation industry. Led by Linda Weavers and Harold Walker, the team measured mercury release into air during early curing, standard maturation, and extended curing periods by a purge-and-trap approach. The researchers analyzed the headspace above concrete samples containing fly ash that replaced 0–55% of the cement, and a sample containing 33% fly ash replacement plus 0.5% HgPAC. The air sampling apparatus they developed included iodated carbon (IC) traps to collect Hg species released into the headspace from the curing concrete samples. The mercury content varied for different concrete samples, and was estimated from Hg contents of ingredients (sand, limestone aggregate, MicroAir 100m, high-purity water, and possibly fly ash and/or HgPAC). The mercury content of the ingredients was measured through dual amalgamation preconcentration and cold vapor atomic fluorescence spectrometry (CVAFS), with Hg-loaded PAC, fly ash, sand, and coarse aggregate being the main mercury sources. The scientists’ work showed that concretes that contained fly ash or fly ash–HgPAC had average mass release rates (ng/day/kg concrete) of Hg greater than the ordinary Portland cement concrete (OPC) control. These same samples displayed lower release rates over the first two days of curing than over the 28-day curing period, possibly due to significant amounts of water present in concrete immediately after pouring. During the 28-day curing period, a very low percentage of the mercury in any sample of concrete was released. This percentage dropped further during days 28–56.


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Reports from Other Journals While the percentage of Hg in concrete that is released may be low, an estimated 65–110 kg year᎑1 worldwide of mercury may be released from freshly poured and curing concrete! The impact that this amount of mercury has on the environment—and in particular the accumulation in concentrated urban development areas—merits further study.

More Information 1. Golightly, Danold W.; Sun, Ping; Cheng, Chin-Min; Taerakul, Panuwat; Walker, Harold W.; Weavers, Linda K.; Golden, Dean M. Gaseous Mercury from Curing Concretes That Contain Fly Ash: Laboratory Measurements. Environ. Sci. Technol. 2005, 39, 5689–5693.

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2. Definition and background information on fly ash from Wikipedia. See http://en.wikipedia.org/wiki/Fly_ash (accessed Aug 2006). 3. Weavers’ research is described at http://www.ceegs.ohio-state.edu/ ~lweavers/ (accessed Aug 2006). 4. Background information of the chemistry of cement has been published in this Journal. See MacLaren, Douglas C.; White, Mary Anne. Cement: Its Chemistry and Properties. J. Chem. Educ. 2003, 80, 623–635.

Angela G. King is Senior Lecturer in Chemistry at Wake Forest University, P.O. Box 7486, Winston-Salem, NC 27109; [email protected].


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CaO in Floor Tile Glazes Affects Hardness

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