Compositions: coating or plastic – Coating or plastic compositions – Inorganic settable ingredient containing
Patent
1996-11-20
1998-06-30
Marcheschi, Michael
Compositions: coating or plastic
Coating or plastic compositions
Inorganic settable ingredient containing
106737, 106816, 106DIG1, 106819, 264DIG49, C04B 713, C04B 1404
Patent
active
057727520
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to concrete, mortar and other hardenable mixtures comprising cement and fly ash for use in construction and other applications, which hardenable mixtures demonstrate significant levels of acid and sulfate resistance while maintaining acceptable compressive strength properties.
BACKGROUND OF THE INVENTION
Acid and Sulfate-Mediated Corrosion of Concrete and Mortar
Concrete and mortar contain calcium hydroxide (Ca(OH).sub.2), which readily reacts with acids or sulfates to form ettringite. This results in reduced strength of the concrete and mortar, which manifests as surface damage, and eventually leads to complete structural deterioration. Nowhere are these problems more acute than in our cities, where buildings and roadways slowly deteriorate under the assault of acid rain and other pollutants.
Corrosion of conventional concrete due to chemical attack of both concrete and the steel reinforcement costs an enormous amount of money annually for repairs and maintenance of structures. Sulfate and acid attack are a major problem with the durability of concrete. For pH values between 3 to 6, acid attack progresses at a rate proportional to the square root of time (Neville, 1983, Properties of Concrete, 3rd. Ed, Pitman Publishing Inc.: London). Severe damage to concrete pipes in sewer systems results from the action of the bacterium Thiobacillus concreteavor, especially in warm climates. Sulfur-reducing bacteria reduce the sulfate present in natural water to produce hydrogen sulfide as a waste product. Another group of bacteria takes the reduced sulfur and oxidizes it back to sulfuric acid (Thornton, 1978, ACI J. Proceedings 75:577-584). Thus attack from sulfuric acid occurs, gradually dissolving and deteriorating concrete surfaces. This process is commonly known as "crown corrosion" in sewage collection systems.
In cement formulations, one way to minimize damage from acid or sulfate attack is to reduce the amount of C.sub.3 A (tricalcium aluminate, 3CaO.Al.sub.2 O.sub.3) present in the concrete. Such sulfate resistant cement is known as standard portland cement type V. Type V portland cement specifies a C.sub.3 A content of not more than 5%. Typically, however, the cost of standard portland cement type V is higher than standard portland cement type I.
Other strategies to increase the corrosion resistance of concrete, such as polymer concrete, are also extremely expensive. Unfortunately, the expense of making acid resistant concrete can outweigh the benefit to be gained from using such concrete.
Another possible way to increase acid resistance is to introduce fly ash into the concrete or mortar. Nasser and Lai (1990, Proceedings of the First Materials Engineering Congress, Denver, Colorado, pp. 688-97) and Irassar and Batic (1989, Cement and concrete Res. 19:194-202) reported that Class F fly ash was a good source of pozzolan, which could improve resistance of concrete to sulfate attack. The data on corrosion resistance of concrete samples monitored for more than three years indicated that concrete samples with 20% of cement replaced by fly ash protected the steel reinforcement bars from corrosion better than plain concrete (Maslehuddin et al, 1987, ACI J. Proceedings 84:42-50). The results of another study suggested that fly ash of finer particle size had greater resistance to sulfate attack (Sheu et al., 1990, Symposium Proceedings, Fly Ash and Coal Conversion By-Products: Characterization, Utilization and Disposal VI, Material Research Soc. 178:159-166).
However, the studies reported to date have not clearly revealed the degree of corrosion resistance or indicated the exact characteristics of cement or mortar containing fly ash. Partly, this was due to use of generic fly ash, which tends to be of uncertain quality from one lot to another. Without determining these characteristics, it is impossible to form any definite conclusions about the usefulness of concrete or mortar, much less risk using unpredictable materials on a construction project.
Fly Ash
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Cerkanowicz, deceased Anthony E.
Jaturapitakkul Chai
Liskowitz John W.
Wecharatana Methi
Marcheschi Michael
New Jersey Institute of Technology
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