A Brief Overview of Concrete and Chemical Hardeners

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A Brief Overview of Concrete and Chemical Hardeners
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  1 A Comparison of Concrete Chemical Hardeners (Densifiers) By Roger Allbrandt, B.A. Environmental Biology    “Retroplate” (Sodium Silicate)      FGS “Permashine” (Lithium Silicate)      “Green Umbrella” (Amorphous Silica)  Background The most common building material today is concrete. It is a used in building construction, consisting of a hard, chemically inert particulate substance, known as an aggregate (usually made from different types of sand and gravel), that is bonded together by cement and water. In 1756, British engineer, John Smeaton made the first modern concrete (hydraulic cement) by adding pebbles as a coarse aggregate and mixing powered brick into the cement. In 1824, English inventor, Joseph Aspdin invented Portland Cement, which has remained the dominant cement used in concrete production. Joseph Aspdin created the first true artificial cement by  burning ground limestone and clay together. The burning process changed the chemical  properties of the materials and Joseph Aspdin created stronger cement than what using plain crushed limestone would produce. Concrete that includes imbedded metal (usually steel) is called reinforced concrete or ferroconcrete. Reinforced concrete was invented (1849) by Joseph Monier, who received a patent in 1867. Joseph Monier was a Parisian gardener who made garden pots and tubs of concrete reinforced with an iron mesh. Reinforced concrete combines the tensile or bendable strength of metal and the compressional strength of concrete to withstand heavy loads. Joseph Monier exhibited his invention at the Paris Exposition of 1867. Besides his pots and tubs, Joseph Monier  promoted reinforced concrete for use in railway ties, pipes, floors, arches, and bridges. Today we have removed the aggregate that was used 40 years ago, remove the metal that was used to reinforce concrete, we add fiber, water reducers, plasticizer, and hardeners. Today concrete is not the same as the product that was invented in 1756, or even the same  product that was being used even ten years ago. Each step in the evolution of densifiers has been in direct response to the changes in concrete, and the perceived deficiency of the older products. Older densifiers were designed to work with the concrete that was being produced at the time. As concrete has changed the products that are being used in conjunction with concrete have changed. In the beginning it was okay to just harden concrete, then customers wanted the concrete to resist oils. Today the government, employees, and customers demand safer products that perform better than the products that have been available in the past.  2 Comparison  Chemical Hardeners (Densifiers) include three basic categories of chemicals: silicates, silicinates and silica : Silicates -  penetrate and harden. They are not good sealers. Disposal of the waste material is currently an issue. 1) The oldest is Magnesium Fluorosilicates , which have been around since 1905. This type of product requires multiple applications with varying rates of dilution. 2) Sodium Silicates developed initially in Germany in the 1930’s. Application of the  product requires that it be applied at an average of 200 square feet per gallon, spread and worked until the surface tension is broken, mist with water, allowed to gel a second time and then rinsed and wet vacuumed to remove. 3) Potassium Silicates . The main difference between the sodium silicates and potassium silicates is sodium is more prevalent in the North American and potassium is predominate in Europe. 4) Lithium Silicates . Lithium silicates were developed to combat Alkali Silica Reaction (ASR). ASR is more prevalent in exterior applications where there is a constant source of water. Lithium silicates are less susceptible to solubilization than sodium or potassium. One of the by  products of this particular silicate is its ability to reduce sweating on slabs. Chemistry: Lithium vs. Sodium and Potassium (Li vs. Na and K) 1) The smaller size of the Li ion of the SiO2/Li2O molecule vs. the Na or K ion is important. 2) The location of the Li ion in the SiO2/Li2O molecule is also important. The Li ion is “ close ” , actually touchin g the SiO2, while Na and K are “ distant ” . The inter-atomic distances of  Na and K make them more available to react quickly with the available Ca or CaOH. The quicker the reaction, the less penetration is able to occur. 3) With silica to Li ratio of 20:1 vs. 3:1 for Na , the lithium silicate is more “  potent ,” relative to the silica content which is what reacts with the free Ca and CaOH to form C-S-H (calcium-silicate-hydrates). In addition, when LiSiO2 reacts, it does not produce free Na or  NaOH, (sodium hydroxide) which can raise the pH of the concrete surface. 4)  Na and K remain soluble in water. This solubility allows them to undergo expansion/contraction cycles with wet/dry cycles. Li becomes insoluble and remains stable throughout these environmental changes. In summary , lithi um silicate has about 1/5 less “ interfering ”  mass as a sodium silicate. This is the true beauty of the lithium and one in which size really matters. The smaller lithium ion stabilizes the silicate ions more efficiently with less mass and fewer molecules, resulting in improved performance while not contributing to higher pH levels. Naturally occurring sulfates of sodium, potassium, calcium, or magnesium are sometimes found in soil or in solution in ground water adjacent to concrete structures, or from sodium or potassium silicates added to concrete as hardeners. The sulfate ions in solution will attack the concrete. There are apparently  3 two chemical reactions involved in sulfate attack on concrete. First, the sulfate reacts with free calcium hydroxide which is liberated during the hydration of the cement to form calcium sulfate (gypsum). Next, the gypsum combines with hydrated calcium aluminate to form calcium sulfoaluminate (ettringite). Both of there reactions result in an increase in volume. The second reaction is mainly responsible for most of the disruption cause by volume increase of the concrete (ACI 201.2R): “ (b) Symptoms. Visual examination will show map and pattern cracking as well as general disintegration of concrete. ”   EM 1110-2-2002 20 June 95 (http://140.194.76.129/publications/eng-manuals/em1110-2-2002/c-3.pdf )  Leaching of sodium and potassium from concrete chemical densifiers into ground water is currently the issue of investigation by the Environmental Protection Agency (EPA).    Silicinates - excellent sealer, poor hardening characteristics. Real world typical life expectancy is 18 to 24 months, and then it should be reapplied. Disposal of the waste material is currently an issue. 1) Silicinates are applied the same way silicates, spray, scrub, mist, rinse, and vac. 2) Silicinates can offer increased abrasion resistance over silicates in the short term due to the coating effect of the silicinates. 3) Silicinates are either potassium or sodium. Silicates have been directly linked to silicosis. Silicates and silicinates have been tagged as carcinogens. Silicates and silicinates must be disposed of as hazardous material. There is significant research that documents the ill effects of sodium, potassium silicates and silicinates on reactive aggregate in concrete. Silicas  –    are the newest and most promising of the chemical hardeners: Silicas are applied simply by spraying them on the surface of the slab and allowing them to dry. The surface should be clean and void of any curing compound. Application rates are  between 400 to 600 square feet per gallon. Unlike silicates or silicinates there is no scrubbing and rewetting of the product. Unlike silicates or silicinates there is no waste material to dispose of. Silicas increase abrasion resistance over silicates or silicinates by up to twice as much  4 Silicas do not contribute to ASR Silicas do not raise the pH of the concrete since the product is neutral 6.5. Silicas have the highest increase in abrasion resistance Silicas have reduced application and labor costs Silicas have no hazardous waste to remove or dispose Silicas do not contribute to silicosis and are not carcinogenic unlike silicates which do contribute to silicosis and are carcinogenic Silicas will not contribute to sweating or efflorescence Silicas performance is not contingent on dwell time unlike silicates or silicinates In Summary,  there are features and benefits to each of these types of chemical hardeners. The upside for the silicates is that they harden better than silicinates, Silicinates seal  better than silicates. Silicates have been directly linked to silicosis. Silicates and silicinates have  been tagged as carcinogens. Silicates and silicinates must be disposed of as hazardous material. There is significant research that documents the ill effects of sodium, potassium silicates and silicinates on reactive aggregate in concrete. Currently the best technology for chemical densifiers is amorphous silica.
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