(a) What are the three most common clay minerals and which one usually causes th
ID: 112270 • Letter: #
Question
(a) What are the three most common clay minerals and which one usually causes the most
problems for civil engineers? (b) What factors can cause significant changes in the thickness of
the clay double layer? (c) What kind of clay structure (dispersed or flocculated) would you
expect to see in a clay stratum deposited in a salty marine environment, and what kind of clay
structure (dispersed or flocculated) would you expect to see in a clay stratum deposited in a fresh
water lake environment? (d) Which clay would have the lowest plasticity index, a sodium
montmorillonite clay mixed with lime, which makes the Calcium cation available, or that same
sodium montmorillonite clay mixed with a fine sand, which makes no new exchangeable cation
available?
Explanation / Answer
(a) Three most common clay minerals and which one usually causes the most problem to civil engineering -
1. Montmorillonite Mineral
The bonding between the three-layer units is by van der Waals forces. This bonding is very weak and water can enter easily. Thus, this mineral can imbibe a large quantity of water causing swelling. During dry weather, there will be shrinkage.
2. Ilite Minerals
Ilite consists of the basic montmorillonite units but are bonded by secondary valence forces and potassium ions, as shown. There is about 20% replacement of aluminium with silicon in the gibbsite sheet due to isomorphous substitution. This mineral is very stable and does not swell or shrink.
3. Kaolinite: This clay mineral is the weathering product of feldspars. It has a white, powdery appearance. Kaolinite is named after a locality in China called Kaolin, which invented porcelain (known as china) using the local clay mineral. The ceramics industry uses it extensively. Because kaolinite is electrically balanced, its ability of adsorb ions is less than that of other clay minerals. Still, kaolinite was used as the main ingredient for the original formulation of the diarrhea remedy, Kaopectate.
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