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Chemical Hardeners

A History of Chemical Hardeners

Chemical Hardeners include three basic categories of chemicals: silicates, silicinates and si lica. Silicates - penetrate and harden. They are not a 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 German 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, allow to gel a second time and then rinse and wet vac 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 soluablization than sodium or potassium. One of the by products of this particular silicate is its ability to reduce sweating on slabs.

 

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 touching 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, lithium 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 in sodium or potassium silicates added to concrete as hardeners]. The sulfate ions in solution will attack the concrete. There are apparently 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 (www.usace.army.mil/inet/usace-docs/eng-manuals/em1110-2-2002/c-3.pdf)

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.


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.

The hazards of Silicates and Silicinates have come to light in recent years and have spurred development of safer alternatives. Some of the hazards are listed below.

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: The newest and most promising 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
  • 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 reduce labor
  • Silicas 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

 

Conclusion

  • Currently the best technology for chemical densifiers is amorphous silica.