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|Federal Highway Administration > Publications > Public Roads > Vol. 63· No. 3 > New Technologies Improve Cost-Effectiveness of CMA|
New Technologies Improve Cost-Effectiveness of CMA
by W.C. Ormsby
For a long time, the Federal Highway Administration (FHWA) has been seeking an efficient, economical, and environmentally acceptable treatment of pavements to prevent or remove the accumulation of ice and snow in the snowbelt in winter. Since the 1930s, salt (sodium chloride or calcium chloride) has been the most widely used deicing/anti-icing material.
Deicing agents destroy the bond between snow/ice and the pavement. Anti-icing agents are designed to prevent the bonding of the snow or ice to the pavement.
Because of salt's corrosive effects and bad environmental impact, FHWA initiated an effort in the late 1970s to find a replacement or supplement.1 A substitute for salt was not readily identified, and as a result, FHWA sponsored a comprehensive search for alternatives by Bjorksten Research Laboratories.2 This search identified two materials — calcium magnesium acetate (CMA) and methanol — as likely candidates to replace or supplement salt in deicing operations. Methanol was dismissed as a candidate because of its flammability and propensity to corrode.
Based on the results obtained by Bjorksten Laboratories, a comprehensive research program was developed to investigate the production, properties, and performance of CMA. After an evaluation of the results, cost, and environmental factors, the conclusion was that, except for the lack of an economical production method, CMA was an excellent alternative deicer.3
The economics of using CMA instead of salt were evaluated in a congressionally mandated study conducted by the Transportation Research Board (TRB). While a clear-cut, economic evaluation was not made in this study, the investigators concluded that CMA use should be restricted to specialized applications on bridges, bridge approaches, and overpasses — at least in the near term.4 After the TRB study, FHWA teamed with several state highway agencies to sponsor two pooled-fund studies to investigate ways to produce CMA at a lower cost.
This article summarizes the results of significant studies pertaining to CMA use, the environmental aspects of using CMA, and relevant federal legislation.
Production, Properties, and Performance
Bioengineering Resources Inc. (BRI) developed a gasification/fermentation process for producing CMA from domestic wastes, such as municipal refuse and sewage sludge. Figure 1 shows the process of gasifying wastes to produce syngas — carbon monoxide, carbon dioxide, and hydrogen. The syngas mixture, in liquid medium, is processed (fermented) using a suitable proprietary bacterium. Acetic acid is produced, extracted, and reacted with dolomitic lime to produce CMA. The CMA product is then dried and pelletized. Details regarding CMA's compositional analysis and properties (ice melting, penetration, and eutectics) are given in BRI's final report, but it is sufficient to say that the properties that were measured compared favorably with commercially available CMA (Cryotech CMA™).
Ohio State University (OSU) developed a cheese-whey fermentation process to produce acetic acid. The process, shown in figure 2, involved the co-culture fermentation of waste cheese products (whey) to produce acetic acid. The acetic acid-containing broth was then treated in two ways to produce a crude and a refined product. In the first case, dolomitic lime reacted with the broth to produce CMA. In the second case, acetic acid was extracted from the broth and then reacted with dolomitic lime to produce a purer product. Both products were reduced in moisture to 5 percent. Physical properties and performance of the OSU products, described in the final report, compare favorably with Cryotech CMA.
To further characterize the CMA products, scanning electron microscope (SEM) studies were made. The SEM results and energy-dispersive X-ray fluorescence analyses (EDAX) for the OSU, BRI, and Cryotech CMA products are given in figures 3 and 4, respectively. The SEM for the Cryotech CMA shows a dense pellet about 3 mm in diameter. On the other hand, the BRI and OSU products, which were not pellets, were relatively small, irregular particles and occurred as amorphous aggregates. The EDAX spectra for the Cryotech and BRI products were very similar; the chemical analysis indicated a high degree of purity for both samples with a calcium to magnesium ratio of approximately 7 to 3. (See table 1.)
Table 1. EDAX Analysis of CMA Products
K = ration between unknown and standard x-ray intensities
Z = correction factor for atomic number
A = correction factor for absorbance
F = correction factor for fluoresence
The EDAX analyses for the OSU CMA samples showed that the products contained significant impurities. The broth sample contained significant amounts of sodium, sulfur, and chlorine, while the extracted sample contained sulfur and chlorine. Both OSU samples had higher calcium-magnesium ratios than the BRI and Cryotech products. Chemical analyses for the OSU products, derived from computerized analysis of EDAX data, are shown in table 1.
Even though it has been well established that CMA is minimally corrosive toward metals and many other materials, the corrosive behavior of CMA in mortar-rebar systems (or in the more physically and chemically complex rebar-portland cement-containing system) has been the subject of some uncertainty.
According to the Salt Institute, . Milwaukee found that CMA was more corrosive than salt when used where chlorides are present, such as chloride additives in concrete..7
A special report of the Transportation Research Board concluded, "There is insufficient evidence to determine whether CMA has a passivating effect [that is, reduces the rate of corrosion] on concrete that is already contaminated with salt."8 As a matter of fact, Locke and Kennelley postulated that the addition of CMA to salt-contaminated bridge decks would tend to accelerate the rate of corrosion.9
Peart and Jacoby made electrochemical measurements (corrosion potentials and corrosion currents) on carefully prepared mortar-rebar cylinders subjected to immersion in various solutions, including CMA and sodium chloride-containing systems and sodium chloride immersion of CMA-containing systems. Corrosion potentials and corrosion currents measured after 500 days of immersion indicated little or minuscule corrosion activity in sodium chloride systems immersed in CMA. The low corrosion currents obtained for CMA-immersed sodium chloride-admixed systems corresponded to a corrosion rate of 0.1 to 0.2 milli-inches per year (mpy). This contrasts with sodium chloride-immersed systems that had corrosion rates of 2 mpy. Rebar recovered from CMA-immersed systems (and sodium chloride-immersed systems for comparison) are shown in figure 5. It is obvious the CMA has produced little or no corrosion in sodium chloride systems. On the other hand, sodium chloride immersion has produced severe corrosion. Figure 6 shows recovered rebars from CMA-admixed mortar systems and rebars from "controls," both immersed in sodium chloride solutions. It is apparent that admixed CMA has produced a beneficial effect in retarding corrosion. In other words, if sodium chloride was used to deice a bridge that had previously been deiced with CMA, harmful corrosion would be delayed or eliminated.
Another type of experiment was run by Bertocci to test the efficiency of various proprietary materials, including CMA, as corrosion inhibitors. Mortar-rebar specimens were immersed in various inhibitor-containing solutions. Sodium chloride solutions served as a basis for comparison. Various electrochemical measurements were made, including electrochemical impedance. From these experiments, conducted over a period of approximately one year, a corrosiveness parameter was derived for each of the chemicals tested. The results, shown in figure 7, clearly show that CMA is an excellent inhibitor, having a negative corrosiveness parameter. These results substantiate those obtained by Peart and Jacoby.
CMA has been touted as an environmentally friendly deicer/anti-icer. Studies have evaluated CMA's impact on the infrastructure; vegetation, fish, and aquatic life; water runoff; and soils. The impacts have been shown to be neutral or beneficial.12 Deleterious effects have not been convincingly demonstrated. Congress, as noted earlier, has implicitly recognized CMA's environmental acceptability and deicing qualities by mandating an economic study comparing it with salt, and Congress also recognized its potential positive environmental impact by legislating its subsidization.13,14
An interesting and important feature of the production processes developed by BRI and OSU is that all of the methods developed use waste materials — sewage sludge, municipal solid waste, and waste cheese whey. This not only lowers the production costs, but it also contributes to the solution of waste disposal problems. This feature is consistent with emphasis on waste management and recycling encouraged in federal legislation.15
As noted previously, federal legislation has been enacted to investigate the economics of rock salt versus CMA and to provide incentives for using CMA and other environmentally friendly deicers in highway projects.
Subsequent to the discovery of CMA in the 1970s, there was a flurry of activity, first by the government and later by the private sector, to gather information that would permit/encourage its use. Results of studies on the technical, environmental, and health and safety impacts of producing and using CMA were generally favorable. The main problem was the cost of producing CMA.
Congress' interest in CMA was first expressed in the Surface Transportation and Uniform Relocation Act of 1987.16 This legislation stated, "Congress encourages efforts to advance the research and development of alternative chemical deicers to rock salt" and urged that "once alternative deicers are commercially available, the full cost of all deicing materials, including damage to highways, vehicles, and the environment, should be considered by state and local governments in determining their snow and ice control strategies."
In 1989, Congress ordered a study comparing the relative costs of rock salt and CMA. The Transportation Research Board, which conducted the study on behalf of FHWA, concluded that because of the high cost of CMA, its use should probably be limited to specialized applications.4
The Intermodal Surface Transportation Efficiency Act of 1991 (ISTEA) specified CMA's eligibility for subsidization in federal projects. The federal subsidy is 80 percent of the cost of CMA for authorized applications, which include highways, bridges and approaches, and the mitigation of damage to wildlife, habitat, and ecosystems caused by a transportation project funded by ISTEA.13
Continued interest in CMA and other environmentally friendly deicers/anti-icers was expressed by Congress in the latest highway act, the Transportation Equity Act for the 21st Century (TEA-21). TEA-21 reaffirms the eligibility of CMA and other environmentally acceptable, minimally corrosive anti-icing and deicing compositions for subsidization.14
The material cost of CMA has prevented its widespread use. Selling CMA on the basis of difficult-to-quantify environmental, corrosion, and health and safety benefits has not been effective in increasing use. Traditional conservatism and skepticism in using a new product have added to the problem. Nonetheless, a certain attractiveness has prevailed for CMA since its discovery, and it has found a small niche market.
Projected costs for producing CMA were made by BRI and OSU. These costs, found in table 2, are significantly lower than most other costs estimated from various experimental production methods. The costs are roughly 20 percent of the price of commercially available CMA.
Viable methods for producing low-cost CMA using cheese whey and other solid wastes have been developed by BRI and OSU. OSU's methods involved co-culturing of cheese whey to produce acetic acid, followed by reaction with dolomitic lime to produce CMA. An acetic acid extraction step may be used before reaction with lime; this produces a purer CMA. BRI's method involved gasification of wastes to produce syngas, fermentation of the syngas in a liquid medium to produce acetic acid, and treatment with dolomitic lime to produce CMA. Projected cost of CMA products were less than $200 to $328 per short ton. CMA produced by these methods had acceptable deicing properties, comparing favorably with commercially produced CMA.
Based on their purity, the CMA products produced by OSU and BRI should have negligible adverse safety and health impacts. Undesirable environmental impacts should also be negligible. In fact, using waste materials in the processes, in addition to lowing the cost of CMA, has a positive overall environmental impact.
The economics of implementing large-scale CMA production, as outlined in the BRI and OSU final reports, are quite favorable. Also, the cost of CMA in specialized applications, in accordance with TEA-21, could be lowered by 80 percent. In these situations, CMA is totally cost-competitive with conventional deicers.
Electrochemical corrosion experiments on mortar-rebar systems demonstrated conclusively that salt-containing mortar-rebar systems, when immersed in commercial CMA solutions, have acceptable corrosion potentials and very low corrosion currents. Further, examination of the interiors of the systems discovered that CMA has a passivating or inhibiting effect on salt-containing systems.
The excellent corrosion-inhibiting properties of commercial CMA products have been clearly demonstrated using electrochemical impedance spectroscopy. This demonstration included not only comparison with conventional deicing materials, but also an evaluation of the inhibition power of CMA compared to a spectrum of commercial inhibitors, all based on chloride-containing formulations.
Continued congressional support for environmentally friendly, low-corrosion deicers/anti-icers, such as CMA, is provided by TEA-21.
Implementation of the new technologies described in this paper should promote the acceptance and increased use of CMA as an environmentally friendly, technically viable, and economically competitive deicer/anti-icer.
3. B.H. Chollar, Douglas L. Smith, and J.A. Zenewitz. . The Involvement of the Federal Highway Administration With Calcium Magnesium Acetate,. Calcium Magnesium Acetate, An Emerging Bulk Chemical for Environmental Applications, D.L. Wise, Y.A. Levendis, and M. Metghalchi (eds.), pp. 1-20, Elvsevier, N.Y., 1991.
5. R. Basu, et al. Calcium Magnesium Acetate at Lower Production Cost: Production of CMA Deicer From Biomass, Publication No. FHWA-RD-98-055, Federal Highway Administration, Washington, D.C., January 1999.
6. Shang-Tian Yang, et al. Calcium Magnesium Acetate at Lower Production Cost: Production of CMA Deicer From Cheese Whey, Publication No. FHWA-RD-98-174, Federal Highway Administration, Washington, D.C., April 1999.
W.C. Ormsby is a researcher in geotechnology and chemistry. He is employed by Salut Inc. and works onsite at FHWA's Turner-Fairbank Highway Research Center. He retired from FHWA in 1992 after 40 years of government service.
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