BRICK & CLAY RECORD: Using Borates in Brick and Roofing Tile

May 1, 2004
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New developments in borates are enabling brick and roofing tile manufacturers to improve the quality of their fired ware while also reducing energy consumption

Borates (B2O3) are valued constituents of the glass-making batch, offering both process and product advantages. They have long been key ingredients in fiberglass, heat-resistant borosilicate glass and vitreous enamels, and their use in ceramic glazes dates back thousands of years. In these applications, borates reduce the melting point of the glass, dissolve metal oxides and act as an important glass builder. In the finished products, they impart chemical durability and increased resistance to thermal shock. All these effects are attributed to the fluxing action of borates, which works to ensure greater vitrification.

Despite these advantages, borates have never been used in the manufacture of heavy clay brick and roofing tile-until now. New developments are making it possible to use borates in clay systems, allowing manufacturers to improve the quality of their fired ware while also reducing energy consumption.

Barriers to Brick Applications

The brick and tile industries have already mastered a high level of product quality, making further quality improvements harder and harder to find. However, adding borates to clay bodies can offer both quality improvements and reduced production costs, particularly for producers facing diminishing local clay stocks and skyrocketing fuel costs.

So why haven't borates been used before in brick and tile?

The answer has more to do with the form than the function of the borates used. Borates are naturally occurring minerals, which are refined to ensure consistent product quality and purity. Most refined borates are soluble in water and are useful for applications ranging from agricultural nutrients to insulation fiberglass, but are not productive for brick making. Tests in clay bodies with refined borates (borax and boric acid) showed a large degree of borate migration to the clay surface as the water is expelled in the drying process. The soluble borate subsequently turns to glass on firing, creating severe glazing and sticking problems on the brick surface.

While insoluble borates are available-such as zinc borates-their cost is usually prohibitive. Zinc borates are primarily formulated for high-value applications, such as flame retardants in polymers and preservatives in wood composites.

Unrefined borate minerals are another option that the brick and tile industry has investigated, due to their slow solubility and lower cost. However, full-scale trials using unrefined borates proved spectacularly ineffective-the brick containing the borates disintegrated in the kiln due to the violent release of the mineral's water of crystallization. Even grinding the mineral to a fine powder was not sufficient to prevent damage to the internal pore structure of the fired ware.

Figure 1. Firing under load curves for a clay body with and without the new borate.

A New Borate Product

A new form of borate*-a microfine sodium pentaborate in a sugar suspension-has been developed specifically to meet the heavy clay industry's economic and technical needs. The sugar prevents the borate from migrating to the surface during drying. It is so effective that it transforms sodium pentaborate's high solubility from a disadvantage to an advantage by allowing the borate to dissolve in the relatively low moisture content of the clay body. This ensures optimum distribution in the body and even vitrification. Once the clay body is dried, the sugar is burned off during firing, and the borate can begin fluxing.

Even when used at low inclusion rates, such as 1-2%, the new borate performs many different functions in brick. It reduces the eutectic at the sintering point of adjacent particles, yet it also changes the pore size distribution of the fired ware and, in some circumstances, can contribute to color development.

The most obvious effects can be seen by looking at firing under load curves for a clay with and without the borate (see Figure 1). These graphs show that the same level of vitrification is achieved with the borate present but at approximately a 50°C lower firing temperature than the control. This means that cost savings on fuel can be achieved due to the lower firing temperature. These savings might not offset the cost of the borate; however, some clays cannot normally be fired at a temperature high enough to achieve good body qualities. For these clays, a 50°C reduction in firing temperature can be a significant benefit.

*The new product was originally developed and marketed in England and is available under the brand name Evansite®.

Figure 2. Effect of the new borate and temperature on water absorption.

Improved Properties

Using the new borate in brick and tile applications can provide a number of other benefits, including reduced water absorption, enhanced freeze-thaw durability and enhanced raw material capabilities.

Reduced Water Absorption. Borates normally reduce water absorption at a given firing temperature; however, the combined effect on shrinkage and pore size distribution should also be taken into account. Figure 2 shows the water absorption of a carboniferous coal measure shale as a function of the new borate admixture and the firing temperature. The water absorption decreases with both an increase in the borate admixture and an increase in firing temperature. With the addition of 0.5% B2O3, the same water absorption was achieved at much lower firing temperatures than in the control clay body. In this case, the borate improves vitrification and replaces other, less reactive raw material fluxes.

Figure 3. Results of the freeze-thaw cycles.
Enhanced Freeze-Thaw Durability. Freeze-thaw durability is partly related to a product's pore characteristics, including pore size distribution, volume and area. A relationship also exists between freeze-thaw durability and the amount of glassy phase and mechanical strength of the fired ware. Tests have shown, however, that water absorption, which has traditionally been the key measure of durability, is not a reliable indicator. In one test, for instance, a very poor-quality Mercia mudstone (a carbonate-based Triassic system) was treated with 2% of the new borate. While the control brick started to fail after 10 cycles, the brick containing the borate went on to pass 100 cycles, making them FL (fully frost resistant) class brick (see Figure 3). A further benefit was that the firing temperature was also reduced by 50°C.

Enhanced Raw Material Capabilities. Many clay brick and roofing tile manufacturers blend imported clays with their own stocks to achieve a particular body property or color, and these clays can sometimes come from hundreds of miles away. Along with the transportation costs, producers must also finance the large stockpiles needed to ensure consistency. These imported clays can be eliminated or greatly reduced by adding the new borate. In effect, the borate acts as a powerful flux in very small quantities, compared to the weak fluxing action of some imported clays.

In some cases, a manufacturer's own clay stocks can also be improved by adding the borate. It is increasingly more difficult and costly to develop new clay extraction sites. The best option for producers is to make the most of their clays even as the quality starts to drop.

The new borate also increases the compressive strength and abrasion resistance of brick, helping manufacturers more easily meet building standards.

Figure 4. Effect of the new borate on leaching.

An Environmental Edge

Many brick manufacturers have begun investigating the use of waste products such as rock wool, fly ash and even calcined sewage in their products to save on raw material costs and capitalize on the use of recycled materials in the growing "green building" movement. However, incorporating such secondary materials into brick bodies can present problems ranging from compromising the compressive strength to actually harming the environment through the leaching of harmful elements into the soil. The new borate maintains or increases the compressive strength of the fired ware, even with these additives. Furthermore, tests have shown that borates effectively lock up leachates such as arsenic, fluorine and molybdenum, preventing them from contaminating groundwater (see Figure 4).

A Beneficial Additive

The new borate offers brick producers a wide range of benefits-from lower production costs and energy use, to higher quality brick and clay roofing tile. Measurable yields include improved durability, compressive strength, abrasion resistance, leaching resistance and reduced porosity. Additionally, manufacturers can make more efficient use of their other raw materials by adding the new borate to their batch.

For more information about the new borate, contact Michael Austerberry at U.S. Borax Inc., 26877 Tourney Rd. Valencia, CA 91355; (661) 287-5646; e-mail michael.austerberry@borax.com ; or contact Adrian Stubbs at Borax Europe Ltd., 1A Guildford Business Park, Guildford, Surrey, GU2 8XG, UK; (44) 1483-242032; e-mail adrian.stubbs@borax.com .

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