Fly ash (one type of CCB) is frequently used and often required, by state departments of transportation and/or design engineers, in concrete structures such as:
The round shape of fly ash particles offers several advantages:
Since fly ash reacts more slowly than cement, the amount of heat generated is reduced as the cement sets and hardens. The fly ash particles react over a long period of time, which allows the heat to dissipate.
Probably the most significant advantage of using fly ash in concrete is improvement in durability. Since fly ash is so fine, it produces a very dense concrete which will not allow water to penetrate.
Furthermore, fly ash consumes calcium hydroxide in the cement/water paste to form additional cementitious material which also reduces permeability. When water penetrates micro-cracks during winter, it expands when frozen and makes the cracks larger, which can eventually lead to premature failure. This is particularly important to structures such as roads and bridges. Using fly ash can significantly increase the useful life of these structures.
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Additionally, concrete is often used for home projects such as setting fence posts or building sidewalks. Since these projects are frequently small, the user typically purchases pre-packaged concrete from a local building supply store. The pre-packaged concrete contains the proper proportions of sand, cement and aggregate and all that is necessary is to add water. For many years, pre-packaged concrete used only locally available aggregate, sand and cement. A recent innovation by Charah, Inc. and Quickcrete uses bottom ash as the aggregate in pre-packaged concrete. The product uses totally sealed plastic bags with handles instead of traditional paper bags, which are easier to lift and carry while being less prone to breaking and leaking. This product is now available in limited markets.
Concrete blocks are made using cement and aggregate (sand and crushed rock). Since blocks are so commonly used in construction, manufacturing facilities are spread throughout the world to help minimize transportation costs.
The aggregate used is whatever is available locally. Bottom ash is frequently used if it is locally available and of the right quality. In order to be used, bottom ash must be:
Roofing Granules - The roofs of most homes in the U.S. are constructed of wood and covered with shingles in order to protect the underlying wood roof from weather. The most common roofing shingles used are composed of a fiberglass matt covered on both sides with asphalt and the top surface is usually coated with granules to protect the shingles from sunlight and provide weather resistance. Boiler slag is often used as roofing granules because it is uniformly colored, dark and quite durable. Before it can be used, the boiler slag must be crushed and sized to meet manufacturing specifications. It must also meet stringent chemical and color specifications.
As the shingles age and the asphalt degrades, roofing granules are dislodged and washed away during heavy rains. If you look closely at the ground where the rain gutters of your house empty onto the ground, or in the gutters themselves, you will probably see an accumulation of granular material, which is quite likely to be ground boiler slag, particularly if your roof is black.
Wallboard - The synthetic gypsum formed in the scrubber is similar to natural gypsum that is mined. Both types of gypsum can be used in the same products, such as plaster and wallboard, which are commonly used for walls and ceilings in home and commercial construction. Gypsum is produced in the scrubber as fine-sized particles that are collected, classified and washed to remove impurities, and de-watered to the consistency of damp sand. The de-watered gypsum is then transported to the wallboard plant where it is calcined (heated) to remove residual carbonates, mixed with various additives and pressed into sheets of wallboard that will eventually be used in building construction.
See what a wallboard plant looks like - photos of Lafarge Gypsum Wallboard Plants in Silver Grove, KY and Palatka, FL - (courtesy of the Baltimore Contractors, Inc. web site) .
Agricultural - If the scrubber solids are not of sufficient grade to produce wallboard, there are other uses. One interesting area is as soil amendment/conditioner, particularly if the soil is acidic or deficient in calcium. The calcium from the scrubber solids can help neutralize the soil pH and provide supplemental calcium which is an essential nutrient for some plants.
Some companies and/or utilities process fly ash to remove unburned carbon so the ash can be used in concrete. In several of the processing technologies, the unburned carbon is concentrated into a product containing as much as 40% to 60% carbon. While this is lower than the amount of carbon contained in coal, the concentrated unburned carbon can contain approximately half the heating value of the parent coal. As such, it can be used as supplemental fuel that is recycled back to the utility boiler.
Across the US, several utilities are doing this. Rather than disposing of the concentrated, unburned carbon, they blend it with feed coal at a ratio of 5-10% unburned carbon and 90% to 95% coal and recover the heating value in a second pass through the boiler. This results not only in a fuel savings by burning less coal, but it also eliminates the need to dispose of the concentrated carbon.
The unburned carbon in fly ash is very different from the carbon found in parent coal. An elaborate pore structure is developed as volatile matter escapes during combustion. Once the volatile matter escapes, most coal particles combust in the boiler. However, some exit the boiler before they have a chance to completely combust. This is the case when low NOx burners are used, since these burners use a lower-flame temperature and more air than conventional burners. The highly-developed pore structure of unburned carbon aids in combustion when these particles are fed back into the boiler as supplemental fuel.
Find out more about Coal Combustion Products
CCP Factsheets from ACAA - Topics include: Safety Issues; Non-hazardous Status; Environmentally Beneficial; Economic Concerns; Increase Utilization; Recycled Products