Silicon and its Chemical Properties: Role in Architecture and Construction
Silicon ranks seventh as the most abundant element in the universe and second most abundant element in earth with 27.7% composition of crust by mass (Exley, 1998). Silicon is a metalloid of atomic number 14 and chemical symbol Si discovered by Swedish chemist, Jöns Berzelius in 1823. Natural silicon contains three isotopes: 92.2% of Si-28, 4.7% of Si-29 and 3.1% of Si-30. Pure silicon exists in either shiny, crystalline dark grey or amorphous powder forms. In the period table, silicon is situated under germanium in group IV. It is usually tetravalent, though sometimes exhibits bivalent properties in compounds (Exley, 1998).
Silicon exists in many dioxide forms and in natural silicates. It is present in rocks (as silicates), cement, sand, glass, silicone and ceramics (Exley, 1998). It is also a semiconductor commonly used in electronics like computers where they are formed as wafers in computer chips. Silicone is category of polymers with silicon in structure, alternating with oxygen atoms (Moretto, Schulze, & Wagner, 2005).
Chemical Properties of Silicon
The electronic configuration of silicon is given as 1s22s22px22py22pz23s23px13py1 usually represented by [Ne] 3s23px13py1. First, second and third ionization energies are 786.3 kJ/mol, 1576.5 kJ/mol and 4354.4 kJ/mol, respectively. Ionization energy refers to quantity of energy that an isolated, gaseous atom in the ground electronic state has to absorb in order to discharge an electron, resulting in a cation. Based on its electronic configuration, silicon forms tetra-positive ions of Si+4 by loss of 3s23px13py1 electrons (Exley, 1998).
Silicon bonds with hydrogen to form hydrides represented as SiáµŸ+ – HáµŸ- in organo-silicon chemistry.
It also reacts with strong alkalis to form silicate and hydrogen gas.
Si(s) + 2KOH(s) + H2O(l) ————-> K2SiO3 (s) + 2H2(g)
Si(s) + 2NaOH(s) + H2O(l) ————-> Na2SiO3 (s) + 2H2(g)
At 4500C silicon react with oxygen to form silicon-dioxide (silica) (Iler, 1979).
Si(s) + O2(g) ————-> SiO2 (l)
It reacts with halogens to form tetra-halides.
Si(s) + 2H2(g) ————-> SiH4 (s)
Silicon reacts with metals to form siliodes.
2Mg(s) + Si(s) ————-> Mg2Si(s)
Silicon reacts with hydrochloric acid on heating to form hydrogen gas.
Si(s) + 3HCl (aq) ———-> SiHCl3(s) + H2(g)
Molecular silicon has the ability to stabilize positive and negative charges and the ability to affect bond strength and lengths in molecules.
In organosilicon compounds, due to the relative inertness of the Si-C bond, the Si-X bond is usually much more readily cleaved. Organo-chlorosilanes, RnSiCl4-n, rapidly hydrolyse to RnSi(OH)4-n which are condensating, e.g.:
2Me3SiCl + H2O ———–> 2Me3SiOH + HCl
Me3SiOH + HCl ———–> Me3Si-O-SiMe3 + H2O
Silylation reaction is given by Me3Si-X + RO-H ———–> RO-SiMe3 H-X, where X is Cl, Br or I.
Stabilization of α-silyl carbanions in organic silicon compounds occurs due to (p-σ*)π overlap of p orbitals of carbon which is highly polarized bond of α metalloid species with σ* antibonding species near carbon-silicon bond. This is observed in 2,2-diphenyl-1-(trimethylsilyl)cyclopropane (Mark, Allcock, and West, 2005).
Figure 1 Stabilization of α- silylcarbanion
The β-effect of silicon is its ability to stabilize a β-positive charge. Electronegativity of silicon puts high charge density on carbon-silicon bond that facilitates conjugative stabilization of the p orbital polarized carbon-silicon bond. In anchimeric assistance in the process of solvolysis of (bromoethyl)trimethylsilane, maximum stabilization of β-positive charge occurs when there is co-planar orientation of the empty p orbital and the carbon-silicon bond (Sekiguchi, Kinjo, & Ichinohe, 2004).
Role of Silicon in Architecture and Construction
Silicon compounds play a major role in architecture and construction. Silicon is the principal constituent of natural stone, glass, concrete, sand and cement (Ca3SiO5) used in building. Sand (SiO2) is the main component in glass (Uhlmann, & Kreidl, 1991).
Asbestos used in roofing is a set of silicon compounds. It is important thermal insulation. The strength of asbestos makes it useful as addition to concrete, asphalt, vinyl materials in roof shingles, pipes, siding, wall board, floor tiles, joint compounds and adhesives. It should be used with care because of its health hazards (Brodeur, 1985; Kozumbo, Kroll, & Rubin, 1982; Selikoff, 1978; Wayne & Crump, 2003). Common minerals present in asbestos are serpentine – chrysotile ((Mg,Fe)3Si2O5(OH)4); and amphiboles – tremolite (Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2); actinolite (Ca2(Mg4.5-2.5Fe2+0.5-2.5)Si8O22(OH)2); cummingtonite ((Mg,Fe)7Si8O22(OH)2); grunerite (Fe2+,Mg)5Si8O22(OH)2); richeckite (Na2(Mg,Fe2+)3Fe3+2Si8O22(OH)2) and anthophyllite ((Mg, Fe)7Si8O22(OH)2).
Silicon organic compounds like polymers (silicones) act as bonding intermediates between glass and organic compounds, form polymers with useful properties such as impermeable to water, flexible and resistance to chemical attack. Silicones are used in waterproofing treatments, moulding compounds and mould-release agents, mechanical seals, high temperature greases and waxes. Silicone sealants are used in high performance buildings due to their good performance and long lasting capabilities. It has low temperature flexibility and high temperature stability.
Silicone enable amazing feats of architecture and the preservation of our most treasured landmarks – as well as making our homes more comfortable and energy efficient (Moretto, Schulze, & Wagner, 2005). They can solve structural glazing and weatherproofing; energy efficiency in buildings; improve in-shop productivity and reduce material waste; extend building life and reduce life cycle costs; help realize sustainable development and achieve design freedom. Silicone sealants outperform and outlast organic weatherproofing materials. They enable innovative applications that would otherwise be impossible.
For example, only structural silicone sealants have the long-term adhesion, compatibility, and strength required for structural glazing and protective glazing applications, making sheet glass skyscrapers a reality. Silicones are also inherently waterproof, and provide greater UV stability, temperature and weather resistance then organic materials. And, because they last longer, they can be replaced less often – reducing lifetime costs and contributing to sustainability.
It is concluded that silicon and its compounds have many uses in architecture and construction. More building materials with silicon as part of their structure are being developed for structural and other construction applications. Research in architecture and construction should focus on silicon and its compounds as an important frontier in developing the industry.
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