Clay Products

Good brick earth is mainly clay with the following constituents.

Silica gives the skeleton, alumina the plasticity, and lime and iron oxide fuse the particles into a strong mass during burning.

The following tests are done at the site to check the quality of bricks.

1. Colour: should be uniform, bright and cherry red (or as required).
2. Shape and size: bricks should be rectangular with sharp edges and straight faces. Standard size 230 × 110 × 55 mm (Nepal, IS), checked by measuring 20 bricks laid in a row.
3. Soundness test: two bricks are struck together; a clear metallic ringing sound shows good quality, a dull sound shows a poor brick.
4. Hardness test: a scratch with a fingernail should leave no impression on a good brick.
5. Structure: a broken brick should show a uniform, compact, fine-grained structure without voids, lumps or cracks.
6. Water absorption test: a dry brick is weighed, soaked in water for 24 hours and weighed again. Absorption should not exceed 20% of its dry weight for first class bricks (15% for facing bricks).
7. Efflorescence test: a brick is kept in a dish of water for 24 hours and dried; white patches should be not more than slight or moderate.
8. Drop test: a brick dropped from about 1 m on a hard surface should not break.
9. Compressive strength: done in the laboratory, minimum 3.5 N/mm² for common bricks as per NBC.

A good brick has the following characteristics.

1. Shape and size: well burnt, regular rectangular shape, with sharp edges and plane faces. Standard size 230 × 110 × 55 mm.
2. Colour: uniform, bright cherry red.
3. Structure: homogeneous, compact, with fine grains, free from cracks, voids, lumps of lime and stones.
4. Sound: gives a clear metallic ringing sound when two bricks are struck.
5. Hardness: a fingernail makes no mark.
6. Strength: compressive strength not less than 3.5 N/mm² (higher for load-bearing and machine-made bricks).
7. Water absorption: not more than 20% of dry weight after 24 hours immersion.
8. Efflorescence: free or only slight.
9. Durability: should withstand weathering, fire and frost.
10. Weight and thermal behaviour: light in weight, a poor conductor of heat and sound.
11. Mortar bond: the surface should have a rough texture for proper bond with mortar.

Burnt clay bricks are manufactured in four steps.

 Clay preparation -> Moulding -> Drying -> Burning
 (dig, weather,      (hand or    (in air    (clamp or
  temper)             machine)    sheds)     kiln)

1. Preparation of clay: the top soil is removed and clay is dug out, cleaned of stones and roots, weathered for some weeks and then tempered with water to a uniform plastic mass.
2. Moulding: the clay is pressed in wooden or steel moulds by hand (ground or table moulding) or by machine (stiff-plastic or wire-cut).
3. Drying: green bricks are stacked in the open, under shade, for 1 to 3 weeks until the moisture is below about 2%. It avoids cracking in the kiln.
4. Burning: done in clamps (temporary) or kilns such as Bull's trench kiln, Hoffman or zigzag kiln at 900-1100 °C. Dehydration, oxidation and vitrification take place and the bricks harden.

Finally, bricks are cooled slowly, sorted and stacked.

Glazing is the process of applying a thin glassy (vitreous) coating on the surface of ceramic or clay products and firing them so that the coating melts and fuses.

Objectives (functions)

• Makes the surface impervious to water and gases.
• Gives a smooth, glossy and attractive finish in various colours.
• Makes the surface easy to clean and hygienic.
• Increases strength and resistance to abrasion and chemicals.

Used on tiles, sanitary ware, pipes and pottery.

Harmful ingredients in brick earth reduce the quality of the finished brick.

Burnt clay bricks are widely used in Nepal because:

1. Raw material (clay) is easily available all over the Terai and valleys.
2. Low cost and simple, labour-intensive manufacture.
3. Good strength and durability for walls and foundations.
4. Small size and light weight, so they are easy to handle and lay.
5. Good thermal and sound insulation and fire resistance.
6. Easy to bond with mortar and to plaster, and they allow varied designs.
7. Do not need skilled labour and can be reused or broken for aggregate.
8. A well established brick industry provides ready supply.

Bricks are classified in several ways.

1. By manufacture

• Unburnt (sun-dried) and burnt bricks.

2. By quality (burning)

• First class: well burnt, uniform red colour, regular shape, sharp edges, ringing sound, absorbs not more than 20% water, strength not less than 10-14 N/mm² as per IS (3.5 N/mm² minimum in Nepal). Used for superior work: load-bearing walls, brick flooring, face work and exposed structures.
• Second class: slightly irregular shape and rough surface, less uniform colour, water absorption up to 22%, strength 7 N/mm². Used for walls to be plastered, for general and interior work and non-load-bearing partitions.
• Third class: under-burnt, soft, absorbs up to 25% water; used for temporary work and in dry places.
• Fourth class (jhama, over-burnt): used as aggregate in lime concrete, road metal and foundations.

3. By use

• Common bricks, facing bricks, engineering bricks, fire bricks, perforated and hollow bricks.

4. By strength (IS 1077)

• Classes 35, 30, 25 ... 5, according to compressive strength in N/mm².

Efflorescence is the whitish crystalline deposit of soluble salts (sulphates of sodium, calcium, magnesium) that appears on the surface of bricks and brick walls when water evaporates. It spoils the appearance and can cause spalling.

Good brick earth should have the following characteristics.

1. Plasticity: when mixed with water it should be workable and hold its shape on moulding. Alumina provides this.
2. Proper composition: silica 50-60%, alumina 20-30%, lime 5-10%, iron oxide 5-6% and magnesia below 1%.
3. Free from harmful substances: no lumps of lime, pebbles, iron pyrites, gravel, organic matter or excess alkalies.
4. Low shrinkage: should not warp, crack or shrink excessively on drying and burning.
5. Uniform fineness: fine-grained, with a balanced sand to clay ratio.
6. Binding power: strength after drying so that the green brick can be handled.
7. Good fusibility: it should fuse slightly (but not melt) at burning temperature to give a hard and strong brick.

The clay is prepared in these operations.

1. Unsoiling: the top 200 mm of soil containing impurities and organic matter is removed.
2. Digging: clay is dug up and heaped in lumps of about 300 to 400 mm.
3. Cleaning: stones, roots, pebbles and lime lumps are removed by hand.
4. Weathering: the clay is exposed to rain, sun and frost for a few weeks (up to a season); this makes it more uniform and breaks lumps.
5. Blending: different earths are mixed and sand or ash is added if needed to improve composition.
6. Tempering: water is added and the clay is kneaded by feet, spades or pug mill until a uniform plastic mass is obtained, ready for moulding.

Drying removes most of the moisture from moulded (green) bricks before burning. If wet bricks are burnt directly, the water turns to steam, causing cracks, warping and breakage, so proper drying is important for a good and economical product.

Causes

Efflorescence is caused by soluble salts (sulphates and carbonates of sodium, potassium, calcium and magnesium) present in brick earth, in mortar water or in ground water. When moisture rises or enters the brick and evaporates, the salts remain as white crystals on the surface. Under-burnt bricks, poor quality water and salty soil make it worse.

Laboratory test

1. Place the brick vertically in a shallow dish with 25 mm depth of distilled water, in a room.
2. Let all the water be absorbed and evaporate. Then add the same quantity of water again and let it evaporate.
3. Examine the brick for white patches.

Rating: nil (no deposit), slight (up to 10% of area covered), moderate (10-50%), heavy (more than 50%, without powdering) and serious (heavy, with powdering and flaking). Good bricks should show nil to slight efflorescence.

The brick is placed on end in a shallow dish containing about 25 mm of distilled water, in a well ventilated room. After all water is absorbed and evaporated, the same quantity of water is added again and allowed to evaporate. The brick is then checked for white salt deposits.

Rating: nil (no deposit), slight (up to 10% of surface covered), moderate (10-50%), heavy (more than 50% with no powdering) and serious (powdering and flaking). A good brick should show not more than slight efflorescence.

In Nepal, as per NBC 109 and the Nepal Standard NS 1, the minimum compressive strength of common burnt clay bricks is about 3.5 N/mm². Local hand-moulded bricks are mostly 3.5 to 5 N/mm², while good machine-made (and fly-ash, Hoffman kiln) bricks reach about 7 to 10 N/mm² or more. Values vary with the kiln and manufacturer.

Qualities of good floor tiles

1. Uniform shape and size, with true plane surface and sharp edges, and no warping or cracks.
2. Hard, dense and strong, with a smooth, uniform surface and a good colour and finish.
3. High abrasion and wear resistance for foot traffic.
4. Low water absorption (not more than about 3 to 10% depending on type).
5. Good resistance to stains, acids, chemicals and frost.
6. Adequate flexural/breaking strength, and a ringing sound when tapped.
7. Slip resistance on the surface and easy cleaning.
8. Durability and fire resistance, with a low cost.

Properties of over-burnt brick

• Dark brown or blackish colour, with a vitrified (glassy) surface.
• Irregular, distorted, shrunk shape and brittle nature because of fusion.
• Very hard, high crushing strength but low impact resistance.
• Very low water absorption.
• Poor bond with mortar because of the smooth surface.
• Used as aggregate for lime concrete, in foundations and road metal (jhama brick).