Compositions: coating or plastic – Coating or plastic compositions – Alkali metal silicate containing
Reexamination Certificate
2002-01-23
2004-06-15
Marcantoni, Paul (Department: 1755)
Compositions: coating or plastic
Coating or plastic compositions
Alkali metal silicate containing
C106S602000, C106S705000, C106S716000, C106S737000, C106S792000, C106S793000, C106SDIG001
Reexamination Certificate
active
06749679
ABSTRACT:
FIELD OF THE INVENTION
This invention relates in general to compositions and a method of use of such compositions to produce cement pastes, mortars and concrete, which are resistant to corrosion in an acidic environment.
BACKGROUND OF THE INVENTION
Durability is one of the most important concrete design criteria in most cases. Common durability problems include chloride ion penetration leading to corrosion of reinforcing steels, alkali-aggregate reaction, freeze-thaw attack, sulphate attack, carbonation, acid corrosion, etc.
The acid corrosion of hardened cement and concrete materials has drawn more and more attention recently due to the corrosion of concrete sewer pipes and concrete structures at municipal wastewater treatment plants, chemical plants, coke ovens and steel plants. Further the impact of animal feed and manure are of concern regarding the acid corrosion resistance of concrete. Conventional Portland cement concrete corrodes relatively quickly in an acidic environment. Some limited research results have indicated that the use of supplementary cementing materials such as silica fume, fly ash and ground blast furnace slag can improve the resistance to acid attack of concrete. pH adjustment and corrosion resistant linings are often used for concrete sewer pipes and concrete structures at municipal wastewater treatment plants at a substantial additional cost.
A recent study conducted by Shi and Stegemann entitled “Acid Corrosion Resistance of Different Cementing Materials” and published in Cement and Concrete Research, Vol. 30, No. 5, (2000) indicates that the corrosion of conventional cementing materials in acid solutions depends on the nature of the hydration products rather than the porosity of the hardened cementing materials. Up to now, the widely held belief has been that a high alkalinity of cement improves a cement's acid corrosion resistance and improves the acid neutralization capacity of the material. For example, the USEPA Toxicity Characteristic Leaching Procedure [Federal Register, 1986] examines the solubility of metals upon addition of a limited amount of acid and is usually used to evaluate the resistance of cement-solidified wastes in an acidic environment. In fact, passivation by deposition of reaction products plays an important role in corrosion resistance and prevents the matrix from further corrosion. Some cementing materials may have low acid neutralization capacity, but high acid corrosion resistance due to the passivation effect.
Acid resistant cement and concrete are known in the art. Early acid resistant cements mainly consisted of liquid sodium silicate as a binder, sodium hexafluorosilicate as a setting agent for liquid alkali silicate and ground quartz or silica flour as a filler. In the past, sodium hexafluorosilicate was a readily available by-product from production of phosphate fertilizers. Now, however, it is difficult to economically obtain this material due to changes in the production of phosphate fertilizers. Other disadvantages with presently known acid resistant cements are that they exhibit low strength if cured at temperatures over 35° C., the cement needs to be cured in a dry environment instead of moist environment, and the hardened cement does not show good resistance to water or dilute acids unless an acid treatment is carried out before being exposed to those environments.
U.S. Pat. No. 4,138,261 to Adrian et al. discloses the use of condensed aluminum phosphates as hardeners for liquid alkali silicates. U.S. Pat. No. 4,482,380 to Schlegel discloses aluminum iron phosphates as hardeners for liquid sodium or potassium silicate. The hardeners have an atomic Al/Fe ratio of 0.052 to 95 and an atomic P/(Al+Fe) ratio of 0.9 to 3, and the cement is waterproof 16 days after it is manufactured. This patent does not discuss the acid resistance of the cement. In fact, both condensed aluminum phosphates and aluminum iron phosphates are very expensive. U.S. Pat. No. 4,221,597 to Mallow discloses the use of a spray dried hydrated sodium silicate powder instead of liquid sodium silicate for the manufacture of acid resistant cement. However, it does not overcome any disadvantages as mentioned above.
U.S. Pat. No. 5,989,330 to Semler et al. discloses an acid resistant cement composition composed of a colloidal silica sol and an acid resistant particulate aggregate without any setting agent. This cement has to be pre-cured and is mainly suitable for use as a mortar in acidic autoclave environments.
U.S. Pat. No. 5,352,288 to Mallow discloses an acid resistant cement comprised of, by weight, 1 to 1.5 parts of calcium oxide material containing at least about 60% CaO, 10 to 15 parts of pozzolanic materials containing at least 30% amorphous silica and 0.025 to 0.075 parts of alkaline metal catalyst. However, after an immersion of the invented material in a 0.70 pH sulfuric acid for two weeks, a white softened skin about {fraction (1/32)}″ deep forms on the surface of the tested samples.
Alkali-activated cement and concrete using sodium silicate as an activator, and blast furnace slag, fly ash and/or waste glass as a cementing component, are well known in the art. There are many publications related to these materials used in cementitous compositions. Generally speaking, a literature review and research by Shi and Stegemann published in Cement and Concrete Research indicates that these materials provide a cement with better acid corrosion resistance than conventional cement concrete, but they still corrode in strong acidic environments.
U.S. Pat. No. 5,601,643 to Silverstrim et al. relates to a cementitious mixture comprising Class F fly ash and an alkali metal or alkaline earth metal silicate, which sets rapidly and gives high strength under elevated temperature. U.S. Pat. No. 6,296,699 to Jin relates to the production of a binder using waste glasses activated by sodium silicate with a SiO
2
:Na
2
O weight ratio between about 1.6:1 to about 2.0:1. however the weight ratio of SiO
2
:Na
2
O should be below 2, otherwise, the activated cementing material will set too fast to be useful [Jolicoeur et al., Advances in Concrete Technology, Natural Resources Canada, pp. 483-514, 1992]. Although those binders can give high strength, they are not stable in moist conditions. Also, they display serious effluence problems because of low SiO
2
:Na
2
O ratios.
The silicate anions in the liquid sodium silicate exist in different forms of polysilicate ions with the silicon atom being equal to or greater than one, depending on the SiO
2
/Na
2
O ratio, pH, concentration and temperature. The lower the SiO
2
/Na
2
O ratio, the lower the degree of polymerization of silicate ions. A detailed description in the book—
The Chemistry of Silica—Solubility, Polymerization, Colloid and Surface Properties, and Biochemistry
by R. K. Iler points out that a monomic silicate ion is highly soluble while amorphous silica consisting of highly polymerized silicate ions has very low solubility in water and strong acid solutions. The presence of sodium hexafluorosilicate essentially makes the silicate species in the solution having a low degree of polymerization form highly polymerized silicates with excellent resistance to strong acid solution.
When the SiO
2
/Na
2
O ratio in a silicate solution is lower than 2, the solution has a high pH and consists mainly of monomers and dimers. Table 1 shows the effect of SiO
2
/Na
2
O ratio on the degree of polymerization of silicate ions in a cementitious solution. It can be seen that with a SiO
2
/Na
2
O ratio of 2.2, the degree of polymerization is much higher than with a ratio of 2 or less, and that the degree of polymerization increases drastically with the ratio thereafter. Thus, according to the present invention, it is important to use a sodium silicate solution with a ratio greater than 2 in an acid resistant concrete. This enables the silicate ions in the concrete to reach a high degree of polymerization and exhibit improved resistance to acid attack.
TABLE 1
Molar Ratio and Degree of Polymerizat
Advanced Materials Technologies, LLC
Hodgson & Russ LLP
Marcantoni Paul
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