Brick and Structural Clay

BRICK & CLAY RECORD: A Glass Act

February 1, 2007
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A recent study shows that using recycled granular glass as a grog in brick manufacturing can dramatically reduce energy consumption.

Above: Waste glass often accumulates at municipal recycling centers.


The use of recycled glass in brick manufacturing is not a new concept. In the 1970s, following the first energy crisis, the U.S. Bureau of Mines sponsored research to determine how much energy could be saved by using recycled glass as a flux in brick manufacturing.1 More recently, studies sponsored by the national government of the UK have verified that finely ground glass lowers firing temperatures.2

Indeed, glass is used today in the manufacture of brick and tile in the U.S. and the UK. However, the glass comes almost exclusively in the form of dust from the bag houses of large glass processors that generate the dust as a byproduct of processing glass for fiberglass and container manufacturers. At most, a few brick manufacturers are using 2% of glass as a raw material, which is not enough to significantly affect either recycling markets or national energy consumption.

One reason more glass is not used is that all previous studies focused on using very finely ground glass (200 mesh and finer). A number of problems arise when using very finely ground glass as a brick flux in amounts over 10%. In general:

  • High levels of fine glass reduce the workability of moist clay bodies.
  • The amount of available inexpensive fine glass is not adequate for brick manufacturing.
  • Processing glass to very fine, uniform material in the 1000-ton volumes required for manufacturing brick is expensive when compared to using mined raw materials.


Over the last two years, The Recycled Glass Feedstock Conversion Project, sponsored by the California Department of Conservation (DOC) and managed by the Center for Environmental Economic Development of Arcata, Calif., has disseminated information to both industry and artists on the use of recycled glass as a ceramic raw material. During the project, we obtained a bag of dry brick clay mix from a California manufacturer and found that the mix contained over 50% (by dry weight) grog and sand coarser than 30 mesh. The grog and sand were in the mix to accelerate drying and reduce shrinkage. Could granular glass fill this same function?

I began experimenting with 12-mesh and finer glass, which is currently being processed in California as a raw material for fiberglass manufacturing, and thus is already available in 1000-ton quantities. The glass readily blended with dry clay and formed into green brick the same as brick mixes containing grog and sand. In fact, since glass particles have zero absorption, the glass grog required a little less water for the same plasticity.

Figure 1. Fixture for testing flexure strength.

Granular Glass as Grog

We obtained permission from the DOC to do a sub-project to test the energy benefits of using glass as a grog in brick forming and firing. For the clay used in the project, we standardized on Cedar Heights Redart Fireclay. Redart has a reported absorption of 9.9% at cone 01 and 1.1% at cone 3.3 After blending 50% clay and 50% 12-mesh glass (by dry weight), 16.5% water was needed to press 1600-gram, 4 x 8-in. brick in a bottle jack press. Control brick were made using 50% Redart, 25% Christy grog 12M and 25% Christy grog 20M.

A "good" brick was defined using absorption and strength standards from ASTM C-1272-05a, "Standard Specification for Heavy Vehicular Paving Brick." The standard requires a test sample of five brick to have a minimum cold water absorption average of less than 6% and a strength (in flexure) average of 1900 lbs-force for a 4-in.-wide brick (see Figure 1).

Figure 2. The brick on the left contains 50% glass, while the brick on the right includes 50% grog. Both were fired to 1850°F.

Brick were fired in a special kiln using quartz halogen bulbs as firing elements. The quartz halogen tubes, which emit primarily in the near-infrared range, are useful because they have virtually zero thermal mass and very high black body equivalent emission temperatures. Producing immediate 100% radiation output, they reduce some of the variables that arise in energy analyses when using resistance elements during heat-up cycles.

After forming, the brick were dried for two hours on a heating pad with a surface temperature of 200°F. Then they were transferred to an oven and further dried for one hour at 350°F.

The first task was to define the maximum speed with which the brick could be fired. At ramp rates faster than 1000°F per hour, small cracks formed on the sides of both the glass/clay and grog/clay brick, so 1000°F per hour was chosen as the maximum practical ramp rate.

A series of brick were fired until the ASTM cold water absorption standard of 6% was met. The standard mix for a 4 x 8 x 1.5-in. brick included 800 grams Redart dry clay, 800 grams 12-mesh and finer glass, and 264 grams of water. The standard firing profile began with a ramp at 1000°F per hour to maximum temperature, a soak at maximum temperature, and natural cooling.

Maximum temperatures from 1700 to 1900°F were attempted. At 1868°F and higher, the glass began to bead on the outside of the brick. Additional testing showed that surface beading could be prevented with the right mix of fine and coarse glass, and by not over-firing the piece. 1850øF was the maximum temperature used to achieve good brick in a minimum length of time without glass beading on the surface.

Aside from strength and absorption standards, any product expected to be successful in the marketplace needs to be aesthetically pleasing. One of the appealing aspects of brick is the rich color of the red clay fired to maturity. One of the surprising findings of the testing was that the glass content of the clay lowered the temperature at which the rich red developed (see Figure 2).

Figure 3. Broken brick face.

The minimum firing profile required to achieve average cold water absorption of under 6% began with a ramp at 1000°F per hour to 1850°F, holding for 20 minutes at 1850°F, and natural cooling. Table 1 shows test results on a pair of brick fired at this profile. However, the brick appeared to not be fused through the middle (see Figure 3).

Table 1. Test results for a pair of brick made of 800 grams Redart dry clay, 800 grams 12-mesh and finer glass, and 264 grams of water.

The Thinner the Better?

The fact that the brick had passed strength standards by a wide margin but were not completely fused raised the idea that thinner brick might be just as strong if the heat penetrated more fully. Since the purpose of this project was to determine potential energy savings in making brick that meet the ASTM paving standard, we decided to see how thin the brick could be made.

Table 2. Test results for five brick made of 700 grams Redart Fireclay, 700 grams 12-mesh glass and 231 grams water.

The breaking strength, determined in flexure, should be proportional to the square of the thickness, so a brick as light as (1900/2535)^.5 * 1600 grams = 1385 grams may be possible. A set of five brick were made using 700 grams Redart Fireclay, 700 grams 12-mesh glass and 231 grams water. The test results are shown in Table 2.

Figure 4. Brick absorption using ceramic grog.

We had made glass/clay brick that averaged 1310 grams dry weight and met ASTM specs for absorption and strength. The next step was to produce conventional brick using the same methods and compare the energy used in production. As a control, brick were made from 800 grams Redart, 400 grams Christy grog 12M, 400 grams Christy grog 20M and 264 grams water. A series of brick were fired until 6% absorption was achieved (see Figure 4).

Table 3. Test results for conventional brick made from 800 grams Redart, 400 grams Christy grog 12M, 400 grams Christy grog 20M and 264 grams water.

The minimum firing temperature meeting absorption standards using Christy grog was 2100°F. Strength tests were then conducted for the brick fired at maximum temperatures of 2100°F and 2125°F (see Table 3). The 2125°F brick met both standards, so they were used as the reference for energy used to make a brick.

Figure 5. Energy used to make pairs of glass/clay and glass/grog brick.

The test kiln was set to record cumulative energy consumed. The energy used to make pairs of glass/clay and glass/grog brick is shown in Figure 5. The two types of brick absorbed almost exactly the same amount of energy until the glass/clay brick reached its fusing range.

Table 4. Energy savings resulting from the glass/clay brick composition.

As the glass softened and the brick mass became more conductive, though, the glass/clay brick actually used slightly more energy than the grog/clay brick. However, the grog/clay brick had to be fired to a higher temperature and for a longer time to reach full density. The glass/clay brick composition resulted in an energy savings of 38.9% when compared to the grog/clay brick (see Table 4).

Glass cullet being used as backfill in a trench.

Economics and Supply

Recycled glass markets vary greatly across the country. California, with bottle deposit laws, minimum content requirements for containers and fiberglass, and a variety of incentive programs, undoubtedly enjoys the most robust recycled glass market in the country.

Competition for clean, processed 12-mesh glass might make it uneconomical for brick making. However, the installation of automated optical sorting equipment at glass processing plants in California over the last 10 years has resulted in the generation of thousands of tons per year of "negative sort" rejection glass. This negative sort glass currently has zero value. It contains about 2% organic contamination by loss on ignition tests, as well as some inorganic ceramic contaminants. Neither of these contaminants would matter to brick manufacturers. With a small amount of reprocessing for size reduction, this negative sort glass would be an economical feedstock for brick manufacturers.

Throughout much of the rest of the U.S., recycled glass has little, if any, net positive value. For example, according to Justin Stockdale, recycling manager for the city and county of Santa Fe, N.M., most of the recycled container glass collected in New Mexico is being processed as construction fill in Albuquerque and Santa Fe. With a population of just under 2 million, glass consumption in New Mexico is about 80,000 tons per year. However, recyclers are not aggressively pursuing collection, especially in rural areas, due to the absence of end users. New markets are needed in these areas.

Equipment of all sizes is available to process containers into grades that work as a grog. For example, according to Cynthia Andela of Andela Systems in Richfield Springs, N.Y., the company's 10-ton-per-hour glass processors have a total amortized cost of under $10 per ton. Substantial post-industrial sources also exist for recycled glass, including window and door manufacturers and tempering plants. However, the largest potential sources are municipal recycling collections.

Leadership in Energy and Environmental Design (LEED) is the predominant green building certification system. The first regional brick manufacturers to commit themselves to taking advantage of the energy- and materials-saving potential of recycled glass will also be in an excellent position for green marketing to architects.

Best Practices

When firing brick containing substantial amounts of soda lime glass, it's important to keep several factors in mind. Glass is more sensitive to both time and temperature than conventional ceramic raw materials. Fortunately, improved control systems and the growth of roller hearth furnaces make the processes in this study possible. Roller hearth furnaces also provide faster firing, lower emissions, higher productivity and less greenhouse gases.

In addition, enough fine glass should be included in the mix to prevent surface beading. Having the proper amount of fine glass in the mix creates a uniform glassy matrix throughout the brick that resists surface beading. It is also important not to over-fire. Higher-temperature clays will enable higher firing of glass-containing bricks, but only within limits. Above 1850°F, soda lime glass becomes quite soft. Finally, be bold. Substituting for 50% of the brick body works better than substituting for 10%.

For additional information regarding the use of recycled glass in brick manufacturing, contact the author at kirbgood@earthlink.net or visit recycledglassceramics.googlepages.com/home. For more information on the Recycled Glass Feedstock Conversion Project, visit www.ceedweb.org/glass.

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