CLAYS and ADDITIVES

Basic raw materials used for clay products and surface finishes are either top-organic soil or virgin-non organic soil.

Top or organic soils have substantial amounts of organic matters, from the decomposition of vegetation and human, animals excrete. The presence of organic matters makes a soil light in weight and dark in colour. Organic soils usually show high workability and low shrinkage characteristics. When organic soils are found below an existing layer and are old, contain gallic acid and tannin in small proportions but sufficient to act as fungicide and mild insecticide.

Virgin or non organic soils have negligible amounts of organic matters, and so reflect the basic characteristic of the predominant constituent element present, i.e., lime, silica, or alumina. However, soils do take-on the personality of the other minerals present in it. Iron oxide as ferric and ferrous is the most important colourant. Other important colourants are quartz, kaolin, mica etc. Soils show a wide range of colours from off-white to yellow, light brown and chocolate to radish tones. Non organic soils unless constituted by colloidal particles show very little plasticity. Some mineral constituents of such soils are reactive to water resulting in swelling and leaching.

Clays: Clays are fine albuminous products formed by decomposition of igneous rocks (lava activity). Clays are tenacious and plastic when wet. Clays are highly cohesive, have high capillaries and no internal friction. Clays are smooth to touch, sticky and plastic. Clays can also be classified according to their plasticity, or silt content. Hard clays or stiff clays have low sand content, and are difficult to excavate. Fine clays have medium sand content, and can be excavated with slight effort. Soft clays have coarse texture and are easy to excavate. Pure clays are mostly useless because of the high plasticity and excessive shrinkage on drying. Plastic clays are called fat clays, and less plastic clays are called lean clays. Clays are black, white, red, brown and yellow in colour. China Clay is a residual material, contaminated with silica, mica, felspar and decomposed felspar. Ball clay is a sedimentary material of fine grain size and some organic contents. It is finer than china clay. Fire clay are formed from felspar as residual and sedimentary deposit. Brick clays are high in iron content, and impurities of calcium compounds and organic matter.

Sands: Sand is made of small particles usually of sandstone / quartz. It is gritty to touch, no cohesion when dry and has no plasticity when wet. It has high internal friction and very little capillarity.

Silt: Silts are soils that are somewhere between a clay and sand. Silts are slightly gritty to touch and are darker in colour than clays.

Colloids: Colloids are gelatinous or gluey like matter found in clays consisting of ultra fine clay particles (size below .002mm) in the form of uncrystallized semi solid substances. The colloids absorb moisture and remain suspended, rather than settle down in water. Colloids have low moisture movement.

Shale: It is a compressed and laminated clay with or without organic matter. Shale is plastic when wet but disintegrates when dry.

Hard pan: A very dense accumulated mass of soil, consisting of clay, sand gravel, held together in a rock like but layered formation. Hard pan does not soften when wet.

Hoggin: A natural deposit of a mixture of small stones, grit and sand containing small amounts of clay as a binding agent.

Loam: A soft mixed deposit of sand, silt and clay. Loam is silty, sandy or clay depending on the contents.

Peat- muc: It is an accumulation of fibrous or spongy textured vegetative matter formed by the decay of plants. It is black or dark brown in colour. It is very compressible so unsuitable for heavy loads. The decomposition of organic material is more advanced in muc than in peat.

Humus: A dark brown earthy material formed by the decomposition of vegetative matter.

FILLERS for CLAY products

Fillers are generally other soils including minerals of various purity. Fillers, when added to a soil, physically and chemically change the quality of surface finish. Fillers constitute a substantial bulk compared with additives, which are used in smaller measures. However additives play a far effective role than fillers. Fillers and additives both play a role that may overlap, support or contradict each other.

ADDITIVES

A variety of substances are added to soils for one or many of the following reasons:

1. to improve the quality of basic soil material

2. to reduce or enhance the moisture content

3. to control moisture removal

4. to control plasticity

5. to achieve a desired colour / texture

6. to produce specific type of castings / mouldings

7. to improve weatherability of the final product

8. to improve upon insect vulnerability

9. to improve substrate adhesion in wet and dry states.

Cow dung is the most popular filler for clay type of surface finishes, in India and other developing countries. A typical dry season fresh cow dung consists of, 33% solids and 67% of water+gases etc., by weight.

The solids in a cow dung are as follows:

             Soluble organic          7.5 parts

             Insoluble organic       76.0 parts

             Soluble inorganic       4.5 parts

             Insoluble inorganic 12.0 parts

             Total                              100.0 parts by weight

A matured or rotted dung is a better filler then a fresh one. Rotting and consequent decomposition leaves an odourless mass that does not leach out with the addition of water. Rotting also generates fungicidal and insecticidal agents like gallic acid and tannin. Best way of maturing a dung is to mix it thoroughly with 1/3 of all the soil to be used and then allow the slurry to remain in a dark, warm, impermeable pit for at least 72 hours. The clay to cow dung proportion vary according to the type of use such as: 

Quality of dung                                    dry of summer or wet of monsoon

Type of soil                                            organic or mineral

Type of plaster                                      plain decorative, mural

Substrates to be plastered                  smooth or rough

Dung to clay ratios of 1:4 to 1:8 are common for plaster work, but 1:1 ratio is often used for flooring and art work. Cow dung provides homogeneity, improves workability, retards shrinkage on drying. Clay+cow dung surfaces are fairly impermeable to water.

Other dungs: Horse, donkey and other domestic and wild animals’ excretions are largely formed of fibres, and for that reason are eminently suitable as fillers for excessively plastic clays. But such dungs or excretions do not rot or decompose as readily as a cow dung.

Other additives for dung+clay mix: Chopped grass, hay, husk etc. reduce plasticity while providing fibrous reinforcement. Hay and grass are vulnerable to white ants, but husk of rice due to presence of toxic oils is almost immune from it. Long fibres of jute, coir, human and animal hair and short fibres of lint, viscose, glass wool and asbestos are selectively used for providing reinforcement against cracking.

Sand stone dust, shell and lime kanker (pebble or gravel like hard nodules) provide ‘body’, improve workability and to an extent reduce shrinkage. Calcined, hydraulic and non hydraulic limes and calcined gypsum (plaster of Paris) are used for better initial setting and over all strength. Whiting and china clay are mainly used to impart lighter colour tones. China-clay, because of its hydrophilic nature helps the mixing of water and `false'-initial setting of the mass. 

Fly ash, a fine residue of from pulverized burnt coal, collected from chimney stacks and boilers, contain 55 % SiO₂, 30 % Al₂O₃, 5% CaO and 7 % Fe₂O_3. These crude forms of tri calcium silicate and tri calcium aluminate in the presence of water form a gelatinous compound, binding the clay particles. Fly ash is an excellent additive for mineral types of soils. Mineral coal ash, if fine and free from unburnt coal and sulphur can be used as filler provided black colour is not objectionable.

Pozolana is not cementious by itself, but mixed with lime and water it sets. It is an active siliceous material that reacts with hydrated lime to form a gel, which on drying becomes insoluble and stable. Slag is a siliceous waste taken off from the molten ores of metals. If slag is quenched immediately on its removal from a furnace, crystallization of silica into glassy structure is stopped. Slag also needs hydrated lime to form a gelatinous set able compound with water. Slags contain sulphur and can be used to neutralize alkaline soils.

Surkhi is a manufactured siliceous compound to which addition of lime is not required. All these materials, pozolana, slag, surkhi are used with organic plastic clays to achieve initial setting and with mineral soils for greater homogeneity.

Portland cement 5 % to 18% on dry clay weight basis is used for quick setting, better wear-tear properties and overall mass strength. Sandy or mineral soils require lesser amount of cement then organic or silt soils.

Additives like protein glues, vegetable gums and chemical binders are used to improve the plasticity and homogeneity. Such additives are of little use with plastic clays but are most suitable for sandy soils. Most of these are generally water thinnable and soften up every time they come into contact with slightest amount of humidity. But some chemical binders, though are water thinnable, harden on drying into a water insoluble matter. Typical agglutinates are guar gum, arabic gum, casein, soluble starches, cooked starches, molasses, sodium alginate, acrylate and other polymeric resins, amino resins etc. For optimum result the quantity required of agglutinate is small, but their high costs prohibit the use.

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