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Pozzolanic reaction is a long term reaction, which involves dissolved silicic acid, water and CaO or Ca(OH) 2 or other pozzolans to form a strong cementation matrix. This process is often irreversible. Sufficient amount of free calcium ion and a high pH of 12 and above is needed to initiate and maintain the pozzolanic reaction. [2]
This reaction has been used to titrate the content of silicon in water solutions and determine their nature. In a typical preparation, monomeric orthosilicic acid was found to react completely in 75 seconds, dimeric pyrosilicic acid in 10 minutes, and higher oligomers in considerably longer time. The reaction is not observed with colloidal ...
Here, the silicic acid H 4 SiO 4, or Si(OH) 4, which is equivalent to SiO 2 · 2 H 2 O represents hydrous or amorphous silica for the sake of simplicity in aqueous chemistry. Indeed, the term silicic acid has traditionally been used as a synonym for silica, SiO 2. Strictly speaking, silica is the anhydride of orthosilicic acid, Si(OH) 4.
Hydrated silica is a form of silicon dioxide, which has a variable amount of water in the formula. When dissolved in water, it is usually known as silicic acid . It is found in nature as opal [ 2 ] (which has been mined as a gemstone for centuries), and in the cell walls of diatoms .
[5] [6] It results in the generation of silicic acid in aqueous environments. Silicic acid, Si(OH) 4, is a hydrated form of silica found only as an unstable solution in water, yet it plays a central role in the silica cycle. [1] Silicifiers are organisms that use silicic acid to precipitate biogenic silica, SiO 2.
The pozzolanic reaction can also reduce the risk of expansive alkali-silica reactions between the cement and aggregates by changing the binder pore solution. Lowering the solution alkalinity and increasing alumina concentrations strongly decreases or inhibits the dissolution of the aggregate aluminosilicates.
Water breaks the bonds between atoms in the crystal: [26] The overall reaction for dissolution of quartz is SiO 2 + 2 H 2 O → H 4 SiO 4. The dissolved quartz takes the form of silicic acid. A particularly important form of dissolution is carbonate dissolution, in which atmospheric carbon dioxide enhances solution weathering.
In addition to the anhydrous form, there are hydrates with the formula Na 2 SiO 3 ·nH 2 O (where n = 5, 6, 8, 9), which contain the discrete, approximately tetrahedral anion SiO 2 (OH) 2 2− with water of hydration. For example, the commercially available sodium silicate pentahydrate Na 2 SiO 3 ·5H 2 O is formulated as Na 2 SiO 2 (OH) 2 ·4H ...