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40 Cement Kiln Dust Background Cement kiln dust (CKD) is generated during the produc- tion of the cement clinker and is a dust particulate mixture of partially calcined and unreacted raw feed, clinker dust, and ash that is enriched with alkali sulfates, halides, and other volatiles (Adaska and Taubert 2008). According to EPA (2010), the definition of CKD is âa fine-grained, solid, highly alkaline material removed from the cement kiln exhaust gases by scrubbers (filtration baghouses and/or electrostatic precipitators).â The composition of CKD varies by plant and over time at a single plant. Much of the material com- prising CKD is incompletely reacted raw material, includ- ing a raw mix at various stages of burning and particles of clinker. Cement is produced using a rotary kiln to turn raw materials (limestone, clay, iron ore, silica) into a sintered product referred to as a clinker. Gypsum is added at the end of the process to manage the rate of hydration. A rotary kiln is fundamentally a long, slowly rotating cylinder tilted at a slight angle with the burner at the lower bottom end. The raw materials enter the top end of the cylinder, are heated, then exit and cool. The sintered material at the bottom end is referred to as âclinkers.â Kilns were first introduced in the 1890s and became popular in the first part of the 1900s as improvements were made to provide continuous production and a more consistent final product in larger quantities (âUnderstanding Cementâ 2010). There are three main types of kilns: â¢ Long-wet kiln â¢ Long-dry kiln â¢ Precalciner kiln. The original kiln style was the long-wet kiln, which feeds in the raw material as slurry, and the cylinder can be up to 656 ft long and 20 ft in diameter. The length is required because the material needs sufficient time to dry out the slurry water, which until recently was difficult to blend and add dry (âUnderstanding Cementâ 2010). Once in the kiln the mate- rials are calcined then sintered to form the clinker. Some of these kilns are still in use. Newer dry kiln configurations add the dry, blended raw materials after passing through a pre-heating tower using heat from recycling hot kiln gases (Figure 15). The heat exchange is accomplished by feeding the finely ground raw material, called raw meal, into the top of the pre-heater tower, then passing through a series of cyclones in the tower through which the hot gases are circulated (âUnderstanding Cementâ 2010). The high surface area and small particle size provide efficient heat transfer and about 30% to 40% of the decarbonation of the raw meal before it enters the kiln. Because the material enters preheated, the length and the diameter of the cylinder can be smaller but still produce the same quantity of clinker per hour. The precalciner kiln, the newest technology, is similar in concept to the dry kiln but with the addition of a second burner, or precalciner (Figure 16). With the additional heat, 85% to 95% of the material is decarbonated before entering the kiln (âUnderstanding Cementâ 2010). The particulates for all types of the cement kilns are cap- tured from the exhaust gases using air pollution control devices such as cyclones, baghouses, and electrostatic pre- cipitators (Adaska and Taubert 2008). The particles captured in this process are the CKD. The type of kiln that generates the dust can significantly influence the chemistry of the CKD byproduct. literature Review summary The list of the most commonly researched and used CKD byproducts include CKD, long-wet or long-dry kiln, and CKD precalciner kiln. CKD for PCC applications was most effective when there was a high concentration of calcium oxide (CaO) and a low loss on ignition. These properties were found to be a function of the type of cement kiln technology. Periodic byproduct testing was recommended to track historical changes in CKD byproduct over time, since changes in technology, burner fuel, and/or sources of raw materials can change the properties of the CKD. Post-processing of the CKD improved reactivity by the grinding of the CKD. Fresh CKD was best if kept dry prior to use in a highway application. Keeping track of the age of the CKD on the infor- mation provided to the user could help decrease byproduct variability. chapter nine manufaCtuRing anD ConstRuCtion BypRoDuCts
41 Project and research data showed that using CKD in PCC applications generally reduced the compressive strength, but a combination of CKD and fly ash helped minimize the loss of strength. The best strengths were obtained when the CKD had a high CaO content and a low loss on ignition. CKD or CKDâfly ash decreased PCC workability and occa- sionally required the use of superplastizers in the PCC mix design. CKD was also used to improve soil properties by decreasing plasticity and increasing strength. Adding fly ash with the CKD resulted in further property improvement. CKD increased in the pH of water, which needs to be considered during the project selection and design phases. Reactivity of the CKD was improved with warmer and slowed by colder temperatures. The rate of improved strength owing to weather conditions and the increased strength of the soil should be considered in designing and constructing the applications. One study evaluated the properties of landfilled CKD, which were relatively consistent throughout the 12 years of the operation, although there were noticeable differences in the composition owing to hydration over time. The aged CKD reactivity was lower than fresh CKD byproducts. agency survey Results The most common use of CKD was in soil stabilization (Table 17). Eleven states indicated they have used CKD in highway applications (Table 18, Figure 17). Two of these states used a combination of cement and lime kiln dusts. No information was collected with this survey on the type of kiln used to produce the byproduct. Roofing mateRials Background From the late 1800s to the 1970s, roofing shingles were man- ufactured by saturating a thick organic mat such as cotton, asbestos, waste paper, or wood fibers with asphalt topped with protective stone coating (Figure 18; Seattle Roof Broker 2010). Although the shingles came with 15- to 20-year war- ranties, they were typically left in place from 30 to 35 years. In the 1970s, the conversion was made from organic to fiberglass Slurry Wet Kiln Burner Clinker Drying Calcining Sintering Exhaust Gases FIGURE 15 Typical long-wet kiln configurations (after âUnderstanding Cementâ 2010). Kiln Burner Clinker Preheating Towers Electrostatic Precipitators Cooling Dry Feed FIGURE 16 Precalciner kiln configuration (after âUnderstanding Cementâ 2010).
42 Byproduct Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embank. Flowable Fill HMA Pavement Surface Treatment (non-structural) PCC Soil Stability Cement Kiln Dust 0 0 0 0 0 2 0 3 7 Combination Kiln Dust 0 0 0 0 0 0 1 1 1 Embank. = embankment. TABLE 17 RESULTS FOR AGENCy SURvEy FOR CEMENT KILN DUST ByPRODUCTS USED IN HIGHWAy APPLICATIONS Number of Applications States Cement Kiln Dust Combined Dust 2 OR â 1 CO, IL, IN, IA, KY, MO, NE, NM, NY, TX IA, MA, NY TABLE 18 STATES USING CKD ByPRODUCTS IN HIGHWAy APPLICATIONS IN 2009 Miscellaneous Byproducts 2009 Kiln Dust, Cement 1 1 1 1 1 1 1 1 2 1 1 FIGURE 17 Agency survey results for cement kiln dust byproducts. backing. However, the 1974 oil embargo and the economic recession in the 1980s compelled roofing shingle manufac- turers to focus on cost savings, which led to a reduction in the fiberglass mat (expensive) and an increase in mineral filler content in the asphalt to extend the binder volume and save money. There was declining asphalt content in the newer shingle products compared with the older recycled asphalt shingles (RAS) materials. There are several types of roofing materials that are asphalt- based products available for recycling, including: â¢ Roofing manufacturing byproducts (pre-consumer); â¢ Tear-offs (post-consumer); and â¢ Built-up roofing (BUR), which is an asphalt and roofing felt product constructed in-place. No information was found on the research or use of BUR in highway applications. Regardless of the source of the shingles, RAS needs to be post-processed by shredding, sizing, and cleaning to be used in highway applications. The steps in processing RAS for use in highway applications are: â¢ Grinding â¢ Sizing â¢ Contaminate removal (tear-offs) â¢ Stockpiling. Brock (2007) described various methods of shredding RAS that have been tried over the years. Equipment needed for processing includes crushers, hammer mills, and rotary shredders, with variable success (Figure 19; Brock 2007). Brock (2007) noted that most shingles were shredded with large wood chippers with 500 hp engines that produced about FIGURE 18 Typical composition of roofing shingles (after Gevrenov 2007). Granular surface Waterproofing asphalt Waterproofing asphalt Fiberglass or organic felt Back surfacing
43 50 to 75 tons of RAS per hour. To ensure that the RAS meets the Â½ inch minus particles size, the material needed to pass through the shredder a second time. Grinding could be easier in the winter when the shingles are cold and more brittle, which helps minimize agglomera- tion. The oxidation of the roofing asphalt aided in reducing the agglomeration of the shredded material (vANR 1999). A Minnesota recycler found grinding manufacturing by product was easier if the material had weathered by being stored in a stockpile for a year before grinding. Manufacturing byproducts were reportedly more difficult to process than the aged roof- ing material, which had hardened with age and was less likely to agglomerate during grinding (vANR 1999). Some shredding processes used water to cool the cutting heads and limit dust production. It can be noted that aging may help the mechanical processing but could result in a harder asphalt byproduct that might accelerate pavement cracking resulting from embrittlement. literature Review summary The list of the most commonly researched and used byprod- ucts include roofing manufacturer (pre-consumer) and tear-off shingles (post-consumer). Post-processing (grinding) of RAS is needed to size the byproduct for HMA, soil improvements, and dust control applications. Some contractors added sand during the grind- ing process to minimize agglomeration. If sand was used, it needed to be considered in the overall application design. Others reported that grinding of the RAS in colder weather was easier and minimized agglomeration of particles. Grinding processes that are used to cool the cutting heads need to evaluate the moisture contents of the stockpiles prior to use. Dust mitigation was required during RAS grinding operations. Any metals (tear-offs) were removed as the RAS was stockpiled. Recommendations were made for preparing individual stockpiles for each type of RAS byproduct. Some states required the stockpile to be tested for asbestos content (primary for tear-offs). This was not a concern for current manufacturing byproducts because asbestos is no longer used in roofing materials. When RAS was used in HMA applications, the combined RAS-aged asphalt and fresh asphalt cement PG grade occa- sionally resulted in a higher PG grade upper temperature and occasionally a warmer PG grade lower temperature. Changes in the binder properties owing to the addition of 5% RAS were similar to changes observed when using 30% to 40% RAP only (Schultz 2010). The aged RAS asphalt increased the viscosity and stiffness of the binder and the final HMA. In some cases, the moisture content of RAS required longer dwell times in HMA plants. Higher moisture content RAS HMA showed an inclination to be tender during rolling, which had to be delayed to prevent movement of the mix under the rollers. Recent research focused on the use of RAS as a means of improving the stability of poor soils or as a method of dust control. Soil stability improvements used 5% finely ground RAS to increase the California bearing ratio (CBR) values of soils with initially low values. Improvements were seen in CBR, compressive strengths, and especially tensile strengths of the modified soils. The most improvement was seen when the soil had high fines content. A combination of RAS and fly ash worked well with silty subgrade soils. RAS did not improve properties when used with base materials with initially higher CBR value (e.g., crushed limestone). A dust control study used ground tear-offs, which were spread on a gravel base and mixed with a motor grader. The result was approximately 2.5 in. of surface mix, which was somewhat friable. An emulsion fog seal was used to preserve the surface. Three states have used similar applications to reduce dust and provide improved driving conditions. Tipping fees varied widely across the country. Based on material values and operating costs in the early 2000s, the most commonly reported tipping fee was approximately $50/ton with cost grinding, sorting, testing, housing, regulator and administrative costs of about $40/ton. The cost of processing RAS was equivalent to 75% to 80% of the average tipping fees. Organic-backed manufacturer RAS and tear-offs pro- vided a cost savings of approximately 5% per ton of HMA at FIGURE 19 Typical grinding operation set up (after Brock 2007).
44 4% RAS content (Brock 2007). Fiberglass-backed RAS pro- duced a savings of about 3% per ton of asphalt. The difference in cost savings was the result of the higher asphalt content used for the organic-(paper) backed shingles that were prevalent in the older shingle products. Recycling equipment maintenance costs could be a significant factor in the costs of operation owing to the presence of the granular component in the grind- ing mechanisms. agency survey Results for Roofing shingle Byproducts The primary use of recycled asphalt shingles was in HMA application, although several states indicated a use in fills, and one state reported experience with its use in soils (Table 19). No states were currently considering or using BUR byproducts in any highway applications. Those states using RAS in un bound applications were using tear-offs (Table 20, Figure 20). ReCyCleD founDRy sanDs Background Foundry sand is a uniformly graded, high-quality sand byprod- uct from the ferrous and non-ferrous metal casting industry (FIRST 2004). The metal casting industry uses the foundry sand in two ways: The first is as a molding material to form the external shape of the cast part; the second as a core mate- rial to fill the internal void space in products such as engine blocks. Because sand grains do not naturally adhere to each other to hold the desired mold shape, binders are added to the sand. Recycled (spent) foundry sand (RFS) can include other materials from foundry processes such as cleaning and grinding operations, slag, and dust collector equipment (i.e., baghouses) (Partridge and Alleman 1998). Most foundries have two sand systems: one for external modeling lines and one feeding the internal core lines. After the metal is poured and the cast product is cooled the green sand is shaken off of the part, recovered, and reconditioned for reuse in the molding process. Used cores are reclaimed during the cooling and shaking processes. The reclaimed core material is crushed and reintroduced into the green sand systems to replace a portion of the sand lost in the process. Broken and/or excess cores or those that do not break down when crushed are discarded. The flow chart for typical foundry processes is shown in Figure 21. Binder systems can be either clay-bonded systems (green sand) or chemically bonded systems (resin sands) (FIRST 2004). Green sands are used to produce about 90% of the casting volume in the United States and consist of 85% to 95% silica, 4% to 10% bentonite clay, 2% to 10% carbo- naceous additive (e.g., seacoal and gilsonite), and 2% to 5% water. The carbon content gives the sand a black color. Resin sands are used in core-making, where high strengths Byproduct Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embank. Flowable Fill HMA Pavement Surface Treatments (non-structural) PCC Soil Stability Roofing Shingles, Fiberglass-Backed 1 0 0 0 0 14 0 0 0 Roofing Shingles, Paper- Backed 0 0 0 0 0 13 0 0 0 Roofing Shingles, Tear- Offs 1 0 0 0 1 12 0 0 1 Roofing Shingles, Unknown Type 1 0 0 0 4 1 0 0 0 Roofing, Built-Up Roofing (BUR) 0 0 0 0 0 0 0 0 0 Embank. = embankment. TABLE 19 USE OF ROOFING SHINGLE ByPRODUCTS IN HIGHWAy APPLICATIONS Number of Applications States Fiberglass-Backed Paper-Backed Tear-Offs Unknown Type Built- Up Roofing (BUR) 2 â â ME, VA â â 1 AK, AL, DC, FL, ID, IL, KY, LA, MO, NC, NV, NY, OH, OR, WV AK, AZ, CT, DC, FL, KY, LA, MO, MS, NC, NY, OH, OK, VA AK, AZ, CT, DC, DE, ID, KY, MO, NY, OH, OK AL, MO, SC, VT, WI â TABLE 20 AGENCIES USING ROOFING SHINGLE ByPRODUCTS IN HIGHWAy APPLICATIONS
45 literature Review summary The list of the most commonly researched and used byproducts include green sands and core sands. The foundry sand byproducts are separated by their use in the casting process, which alters the physical and chemi- cal properties. These differences are a function of the type of additive used with the original foundry sand, the type of metal being cast, and the specific casting process used. Green sands are used to form the external modeling lines and are reclaimed and reused by the foundry until they fail to meet foundry sand requirements. The casting cores have been hardened by additives such as epoxies, resins, and organic binders (e.g., portland cement are needed to withstand the heat of the molten metal, and in mold making. Most of the chemical binders consist of an organic binder (e.g., oil, cereal, and wood proteins; Hughes 2002), which is activated by a catalyst, although some sys- tems use an inorganic binder such as portland cement or sodium silicate (Hughes 2002). The most common chemical binder systems are phenolic-urethanes, epoxy-resins, furfyl alcohol, and sodium silicates (FIRST 2004). The resin sands tend to be somewhat coarser in texture than the green sands. It may be important to separate the RFS byproduct streams at the foundry because of the different material characteristics of the external and core molding sands. Any metal contami- nates in the recycled sands must be removed. Large chunks of burned cores, referred to as core butts, required further crushing, separation, and screening before recycling. 2009 Roofing Shingles, Paper Backed 1 1 1 1 1 1 CT-1 DC-11 1 1 1 1 1 2009 Roofing Shingles, Fiberglass Backed 1 1 1 1 1 1 1 DC-1 1 1 1 1 1 1 1 2009 Roofing Shingles, Tear Offs 1 1 1 2 1 CT-1 DC-1 DE-1 1 1 1 1 1 2 FIGURE 20 Agency survey results for roofing shingles byproducts (numbers indicate the number of applications that use the byproduct). Miscellaneous Byproducts
46 and wood proteins) to form the inside of the part. This portion of the spent foundry sand required further crushing, sepa- ration, and screening before using in highway applications. Post-processing included the removal of general refuse and other contaminates, metals, and sizing. Research and pilot projects only occasionally apply RFS separating green and core sands. PCC applications showed RFS PCC mixes typically required higher portland cement contents, which was addressed during the mix design phase. Fly ash was needed to compensate for a loss of workability owing to the RFS. When RFS was used in embankments, base, or as fill the designs needed to account for a less freely draining material. Additional crushing and compaction efforts may be needed if the spent foundry sand cores are not crushed prior to use in base applications. Specific recommendations for using RFS as a base material were to place the byproduct on a prepared foundation in horizontal loose lifts not to exceed 8 in., then compact the lifts to a stable, durable condition with at least eight passes of a vibratory steel wheel roller with a minimum weight of 10 tons or the centrifugal equivalent. The compaction of the lifts needs to achieve 98% of the maximum density proper moisture content to achieve the desired in-place density. A significant amount of water was occasionally required so that compaction was achieved on the first time around, as RFS was difficult to re-wet because of the clay additive. If the sides and top of the RFS layer were exposed, rec- ommendations included covering the sides and top with natural soil with a minimum vertical cover of 3 ft, measured from the subgrade elevation and a minimum horizontal cover of 8 ft. Regional recycling facilities in one region were used to reduce the cost of post-processing byproducts. The benefits of using a recycling facility included a single disposal location for smaller foundry operations, post-processing operations for useable byproducts with consistent properties, and adequate quantities for a given application product. agency survey Results for Recycled foundry sand Table 21 shows that only six states reported experience with RFS in highway applications. Only North Carolina noted experience with sands from sand blasting operations (Table 22; Figure 22). Five states used recycled foundry sand in unbound (drainage, embankment) or semi-bound (flowable fill) applications. No distinction was made between green sand and cores in the survey questions or responses. Mold Production Core Production (in-house) Scrap Metal Storage Casting Melting Finishing Shake OutRecycledSand Recycled Scrap Metal Virgin Sands Clays Water Organic Additives (Sea coal, cellulose, starch) Externally Produced Cores Virgin Sands Graphite Wash Organic Additives (phenolics, isocyanates, petroleum distillates, amines, formaldehydes, etc.) Scrap Metal Fines, Core Butts, and Excess Sand Slag Finishing Waste FinishedCastings Foundry Byproducts FIGURE 21 Flow chart for the generation of foundry sand byproducts (after Partridge and Alleman 1998).
47 Utah was not currently using RFS in highway applications and the respondent provided information as to why, noting that the uniform size and round shape, which is a desirable property for casting metals, would make it difficult to meet the well-graded aggregate specifications and angular fines requirements for bases and HMA. Waste glass BypRoDuCts Background Waste glass from material recovery facilities is referred to by several names: glass cullet, recycled glass, soda lime glass, crushed glass, or processed glass aggregate. The term âglass culletâ is the more commonly used. This byproduct is recovered from glass containers and from breakages and inferior products made during glass manufacturing. Glass cullet from the glass manufacturing process includes such materials as broken, obso- lete, and/or off-specification glass from the manufacturing of plate, window, and analytical glassware (Wartman et al. 2004). Glass from automobiles, lead crystal, television monitors, light- ing fixtures, and electronics applications are excluded because of their composition and coatings. The Northeast Resource Recovery Association identifies suitable sources of recycled crushed glass as glass or ceramic bottles, glass jars, ceramic tableware and cookware, vases, ceramic flowerpots, plate glass, mirror glass, and residential incandescent light bulbs. Most post-consumer containers can be sorted into three categories based on color, which is achieved by different chemical compositions: â¢ Flint glass: colorless glass food, beverage, beer, liquor, and wine bottles â¢ Amber glass: brown beer and liquor bottles â¢ Green glass: green wine and beer bottles. Glass cullet can be provided by the material recovery facil- ities as unwashed, larger, broken glass particles; unwashed but crushed glass cullet; and as washed glass cullet. Wash- ing the byproduct removes most of the contaminates such as paper, plastics, and metals, which would also be considered contaminates in most highway applications. literature Review summary The byproduct categories needed for glass cullet are processed glass aggregate (any color) and powdered glass. Post-processing by washing and crushing produced accept- able physical properties and reduced material variability. For example, the specific gravity became more consistent when the glass cullet is washed, regardless of final gradation. Con- tamination by âgummyâ substances such as labels on the glass cullet wash was removed during this process. Crushing operations were needed to produce a well-graded byproduct, Byproduct Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embank. Flowable Fill HMA Pavement Surface Treatment (non- structural) PCC Soil Stability Sand Blasting Waste 0 0 0 0 0 1 0 0 0 Sand, Foundry 0 0 1 3 2 0 0 0 0 Embank. = embankment. TABLE 21 RESULTS OF AGENCy SURvEy FOR FOUNDRy SAND ByPRODUCTS USED IN HIGHWAy APPLICATIONS No. of Applications States Sand Blasting Waste Sand, Foundry 2 â WI 1 NC IA, IN, OH, PA TABLE 22 STATES USING FOUNDRy SAND ByPRODUCTS IN HIGHWAy APPLICATIONS IN 2009 2009 Foundry Sands Byproducts 1 2 1 1 1 1 FIGURE 22 Agency survey results for foundry sand byproducts (numbers indicate the number of applications that use the byproduct). Miscellaneous Byproducts
48 which could then be combined with gravel to meet specifica- tion requirements. Glass cullet greater than about 3 mm in size were visibly identifiable as crushed glass and required heavy gloves to handle safely. Glass cullet could be safely handled when it was sized to meet ASTM D448 No. 8 or finer. Handling concerns focused on the potential hazards associated with fugitive dust (eye contact and inhalation). Stockpile storage time sufficient to minimize leachable materials is important. Reclaimed glass was typically limited to containing no more than 5% of contaminates (e.g., paper, foil, metal, corks, and wood debris). Contaminates were attributed to miscella- neous waste stream differences such as glass color, chemical content of label ink, specialty glass chemistries, and waste thermometers (i.e., mercury content). Most of the design adaptations were focused on adjust- ments needed in the design of PCC mixes. It is important that mix designs consider expansive reactions that are a function of the percentage of the glass cullet. As the percentage of glass cullet increased, the water-to-cementitious material ratio was increased to maintain a consistent slump. The air content increased linearly with an increase in the percentage of glass aggregate and occasionally required adjustments to the mix design. High-range water reducers were needed to maintain adequate workability and desired slump. The amount of water reducers was similar to those necessary when using fly ash only. Expansive reactions were minimized by added fly ash or blast furnace slag to help because of the glass cullet. Work- ability was reduced somewhat, which resulted in more time and effort required to finish PCC surfaces. Segregation and bleeding were observed with glass cullet in PCC mixes. Unbound applications were less frequently used. The low CBR and limestone bearing ratio limited the use of glass cullet as a base or subbase course. Washing the glass cullet to remove contaminates improved the drainage characteristics compared with unwashed glass cullet. The cleanliness of the glass cullet is to be considered when designing embankments and fill. Glass cullet used as a drainage material works best in combination with synthetic liners, geogrids, or geotextiles when it was not placed directly on the liner material. Recommendations were made to use glass cullet drainage material when there was a minimum depth of ground water or bedrock of 4 ft, and a minimum distance of 150 ft away from any surface water body. The benefits were noted as the reduced cost of transporting glass to a landfill or distant disposal site, reduced use of landfill air space, reduced amount of virgin aggregate consumed, and improved environmental awareness and attitudes. The costs listed as associated with glass cullet use were the costs of curb- side collection, crushing glass, and mixing with aggregate. agency survey Results for Waste glass Table 23 shows that only six states indicated they used waste glass in more than one highway application. Fifteen states used this byproduct in a single application (Table 24). Figure 23 shows the geographical distribution of the states that indicated experience with this byproduct. The only western states with experience were Alaska, Hawaii, and Idaho. sulfuR anD sulfate Waste BypRoDuCts sulfur Byproducts Background A major byproduct from the oil and gas industries is brim- stone, which is essentially elemental sulfur (Shell 2010aâe). Sulfur, in the form of sulfuric acid, is also a byproduct of ferrous and nonferrous metal smelting. The use of sulfur as a binder to produce a construction material has been explored for more than a century (McBee et al. 1985). These early efforts used the sulfur as the binder in mortars and con- Byproduct Number of States Using Byproduct in a Given Highway Application Asphalt Cements or Emulsions Crack Sealants Drainage Materials Embank. Flowable Fill HMA Pavement Surface Treatment (non- structural) PCC Soil Stability Any Type 2 1 4 9 2 8 2 3 1 Embank. = embankment. TABLE 23 RESULTS FOR AGENCy SURvEy FOR GLASS PROCESSING ByPRODUCTS USED IN HIGHWAy APPLICATIONS No. Applications States 9 ID 3 PA 2 MA, MN, NY, VT 1 AK, CT, FL, HI, IA, ME, NC, NH, NJ, SC, VA, WI TABLE 24 STATES USING GLASS ByPRODUCTS IN HIGHWAy APPLICATIONS IN 2009
49 cretes to produce acid-resistance mixes with good strength. Research in the mid-1930s discovered that thermal proper- ties of the sulfur mixes could be improved by adding an ole- fin polysulfide, marketed under the name of Thiokol. In the 1940s, sample preparation and specifications for sulfur poly- mer concrete were standardized by the American Society for Testing and Materials as ASTM C1312 and C1159. Sulfur was first used in asphalt cements in the early 19th century as a product that was minimally sensitive to tempera- ture changes and weathered well. The original use fell out of favor with the marketing of air-blown asphalts. The sub- stitution of sulfur for a portion of the asphalt cement was investigated in the late 1930s, but additional development of sulfur-extended asphalts did not arrive until the mid-1970s when the oil embargo increased the cost of crude oil and lim- ited the availability of asphalt cement. Highway applications for sulfur include sulfur-extended asphalt and sulfur concrete. Sulfur, a naturally occurring com- ponent in asphalt, can be substituted for the more expensive portland or asphalt cement. Sulfur was most commonly com- bined with polymers and aggregates to produce sulfur polymer concrete starting in the early 1990s. The main uses were as a rapid repair mix and to encapsulate hazardous materials (Mattus and Mattus 1994). Literature Review Benefits to using sulfur in concrete were (Micropowder 2010): â¢ Sulfur polymer concrete (SPC) â Gained strength rapidly (about 80% within a few hours of placement) â Resistant to acids such as sulfuric, hydrochloric, and nitric acid â Durable in corrosive environments â High density â Resisted cracking â Resisted plastic deformation. Benefits to using sulfur-extended asphalt in HMA mixes were (Mattus and Mattus 1994; Shell 2010a): â¢ Increased stiffness without becoming brittle at cold temperatures â¢ Allowed the use of softer, lower viscosity asphalt cements to be used in cold climates while minimizing rutting prob- lems during hot summer seasons â¢ Better performance than conventional HMA in extremely hot or cold climates â¢ Improved the overall structural capacity of the pavement system â¢ Could be reheated since the hardening process is thermo- setting â¢ Potential for reducing pavement thickness and therefore cost â¢ Performance appeared to be comparable to conventional HMA. Disadvantages to using sulfur byproducts included: â¢ Required modifications to field mixer to provide heated material on-site (SPC) â¢ Worker safety concerns because of formation of hydro- gen sulfide or sulfur dioxide gas if mixing temperature is too high â¢ Sulfur mix becomes difficult to work with at temperatures greater than 320Â°F owing to increased viscosity â¢ Although not flammable on its own, sulfur still meets the criteria of U.S.DOT of a hazardous material. Agency Survey Sulfur was not included in the agency survey as the resurgence of the use of sulfur in highway construction applications had not been observed before 2009. sulfate Waste Byproducts Background Sulfate rich byproducts, fluorogypsum and phosphogypsum, are the result of the production of hydrofluoric and phosphoric acid. The fluorogypsum byproduct (RMRC 2008; TFHRC 2009) is the result of combining fluorspar and sulfuric acid and is discharged in slurry that solidifies over time in the holding ponds, and then must be crushed and separated if the byproduct is to be used. The resulting byproduct is sulfate-rich with a 2009 Waste Glass 1 2 1 2 1 1 1 9 1 3 1 1 1 MA-2 CT-1 NJ-1 NH-1 VT-2 FIGURE 23 Agency survey results for glass byproducts (numbers indicate the number of applications that use the byproduct). Miscellaneous Byproducts
50 primarily well-graded sand silt particle size. Phosphogypsum (RMRC 2008) is a solid byproduct from phosphoric acid production and is a byproduct from a wet process that uses hydrochloric acid to treat phosphate rock. The process is outlined in Figure 24. Literature Review Summary Sulfate and sulfur types of byproducts included fluorogypsum, phosphogypsum, and sulfur. Only a limited amount of informa- tion was found for these byproducts, no specific test methods were found in this information. Louisiana is the only state that has evaluated blended calcium sulfate, the fluorogypsum byproduct in cementitious blends, as a base material. These byproducts are to be bound to minimize undesirable leach- ates. No additional information was available with regard to materials handling, quality control, design changes, or construction guidelines. Agency Survey Results for Sulfate Byproducts No states indicated they were currently using sulfate byproducts in highway application in this survey. Waste papeR mill sluDge Background Waste paper mill sludge is the byproduct of the paper production process. The major byproducts from the pulp and paper waste stream are as follows (Bird and Talberth 2008): â¢ Waste water treatment plant (WWTP) residuals â¢ Boiler and furnace ash â¢ Causticizing residuals. The primary residuals, approximately 40% of the WWTP, including de-inking residuals (paper recycling operations), consist mostly of processed wood fiber and inorganic or mineral materials (e.g., kaolin clay, CaCO3, and TiO2). Secondary residual (activated waste sludge) is mostly bac- terial biomass (nonpathogenic) and makes up about 1% of the WWTP. Dewatering the WWTP residual produces a byproduct with between 30% and 40% solids, and once dewatered the ma terial is not considered hazardous as defined by RCRA. A few facilities can dry the WWTP to produce a byproduct with 70% to 95% solids. Chlorinated organic compounds tend to concentrate in the solids, which can be an environmental concern. FIGURE 24 Schematic of phosphate process (after Deshpande 2003). Phosphate Ore Phosphatic Clay Slimes Beneficiation Sand Tailings Wet Process (Reaction with H2SO4) Elemental Phosphorus Thermal Process (Electric Arc Furnace) Phosphate Rock FertilizersPhosphoric Acid ScaleFerro-Phosphorus R226 U238 Slag Phosphogypsum R226 U238 R226 U238 R226 U238 R226 U238 R226 U238 R226 U238 Pb210 Po210
51 Boiler and furnace ash (energy recovery) is produced from wood, coal, or a combination of wood, coal, and other solid fuels (most common) used in the pulp and paper processes. Causticizing residues have three components: lime mud, green liquor dregs, and slaker grit. Lime mud (calcium carbonate and water) is burned in a lime kiln to regenerate the byproduct to lime (CaO). This byproduct may also contain unreacted calcium hydroxide and unslaked calcium oxide, magnesium, and sodium oxides. The lime mud is approximately 70% to 80% solids. Green liquor dregs are composed of nonreactive and insol- uble materials remaining after inorganic process chemicals (smelt) from the recovery furnace are mixed with water. The dregs are removed by gravity clarification, resulting in a byproduct with 45% to 55% solids. The major components are carbonaceous material along with calcium, sodium, mag- nesium, and sulfur. Slaker grits are produced by mixing lime (burned or unburned) with the green liquor dregs, and contain between 70% and 80% solids. The solid portion is about 50% fibers and up to 50% minerals with a pH of about 12, which is neu- tralized before disposal. The solids can also contain titanium oxide and calcium sulfate. literature Review summary About 50% of these byproducts are used in land applica- tion, for energy production (incineration), or landfilled. Currently there has been little research for use in high- way applications and only one agency indicted using this byproduct. Potential use of these byproducts will likely focus on soil modification (lime mud), cement or concrete additives, or as an aggregate replacement (bottom ash). agency survey Results for pulp and paper Byproducts Only Kentucky indicated they had used paper pulp or lime mud in HMA applications.