Raw materials and building materials
A raw material is something taken from nature before we change it. Common ones are rock (limestone, granite), sand, gravel, clay, wood and water. We turn them into building materials: cement, bricks, concrete, mortar, steel, glass and tiles.
Every material has properties, which tell us where to use it:
- Strength: how much load it takes before it breaks. Concrete is strong when squeezed; steel is strong when pulled.
- Density: mass in one cubic metre (kg/m³). Steel is about 7850 kg/m³, concrete about 2400 kg/m³.
- Durability: how long it lasts in rain, heat and salt.
- Water absorption: how much water it soaks up.
- Thermal property: how well it passes heat. Brick and aerated blocks keep rooms cooler.
Aggregates are the grains in concrete. Coarse aggregate is gravel (bigger than 4.75 mm). Fine aggregate is sand (smaller than 4.75 mm). A binder (cement, lime) is the glue that holds the grains together.
Sampling and testing raw materials
A quarry or truck may hold hundreds of tonnes. We cannot test it all, so we take a sample: a small part that stands for the whole. A good sample is taken from many places (top, middle, bottom) and mixed. If you take it from one corner only, it can fool you.
Tests that are often done (specific analyses):
- Sieve analysis: how much stays on each sieve, to find the grain sizes.
- Moisture content: wet mass minus dry mass, divided by dry mass. Wet sand carries extra water that changes the mix.
- Silt and clay content: dirty sand weakens concrete.
- Strength and hardness of rock; density and water absorption of bricks and stones.
- Chemical analysis: for example how much calcium carbonate a limestone holds before it goes to a cement plant.
Crushing, grinding and separating by size
Crushing breaks big rock into smaller pieces using a jaw crusher or cone crusher (squeezing and hitting). Grinding makes powder in a ball mill, where steel balls tumble and smash the grains. Smaller pieces have more surface, so they mix and react faster. Cement is ground very fine for this reason.
Separating granular materials means sorting grains by size. A sieve is a mesh with holes of a fixed size: smaller grains pass, bigger ones are retained. A stack of sieves gives coarse, medium and fine grades. Other ways: washing (removes clay and silt), air separation (blows light dust away) and magnetic separation (pulls out iron).
Percentage retained = (mass on the sieve ÷ total mass) × 100.
Storing, dosing and mixing
Homogenising means making the material the same all through, so every batch behaves alike. Heaps are built in thin layers and taken out from the side to the bottom. Transport uses conveyor belts, trucks and pipes; drops from a height make coarse and fine grains separate, so we keep them short. Storage: cement stays dry in bags or silos (damp makes lumps); aggregates sit on a clean, hard floor in separate bays.
Dosing is measuring each ingredient. A mix like 1 : 2 : 4 means 1 part cement, 2 parts sand, 4 parts gravel by volume. Water-cement ratio (w/c) = water mass ÷ cement mass. Less water gives stronger concrete but is harder to place; more water is easier to pour but weaker and cracks more. Most ordinary concrete uses w/c between 0.4 and 0.6. After mixing, concrete is placed, packed and cured (kept wet) for about 7 to 28 days.
Processing semi-finished products
A semi-finished product is a material that is not yet the final part: a clay slab before it is fired, a steel bar before it is bent, a board before it is cut. Processing operations include cutting, bending, shaping, pressing, drying and firing.
- Bricks: clay is ground, mixed with water, moulded, dried and fired in a kiln (about 900 to 1000 °C).
- Concrete blocks and pipes: mix is poured into moulds or pressed by a machine and cured.
- Steel bars: cut and bent into stirrups and cages with a bar cutter and a bending machine.
- Stone and tiles: cut with saws, polished by grinders.
How to carry out an operation well: read the drawing, check the material, set up the machine, make a trial piece, measure, then work in batches and inspect each batch.
Where materials are used: buildings and public works
A building is made for people to live or work inside. Public works serve everyone: roads, railways, bridges, tunnels, dams, water pipes. Both must be safe, strong, long-lasting and affordable.
- Structural elements carry the load: foundation, columns, beams, slabs (concrete, steel).
- Closing elements close the space: walls, doors, windows, roof (brick, block, glass, timber).
- Finishing elements make it neat and protected: plaster, paint, tiles, flooring.
Execution phases of a road or railway: survey and marking, clearing and levelling, earthwork (cut and fill), compacting the base, laying layers (gravel, crushed stone, then asphalt or rails on sleepers and ballast), drainage, and finishing. Bridges and tunnels follow: foundation, supports, deck or lining, then testing.
Waste, saving material and safety on site
Saving material: measure exactly, order the right quantity, store things dry and covered, reuse offcuts and use leftover concrete for small jobs. Broken bricks and concrete can be crushed and used again as filling. Sort waste into reusable, recyclable and hazardous (paint tins, oils) and send each to the right place. Never burn or dump waste in drains.
Safety rules:
- Wear a helmet, safety shoes, gloves and a dust mask (cement dust harms lungs; wet cement burns skin).
- Use a safety belt on heights, and keep scaffolds firm and railed.
- Keep walkways clear; switch off machines before cleaning them.
- Lift with a straight back; never carry too much alone.
- Keep fire extinguishers and a first-aid box near, and know the exits.
Key formulas and definitions
- Water-cement ratio (w/c) = mass of water ÷ mass of cement
- Mix 1 : s : g (by volume): cement = total ÷ (1 + s + g)
- Percentage retained = (mass on sieve ÷ total mass) × 100
- Moisture content = (wet mass − dry mass) ÷ dry mass × 100
- Compressive strength = load ÷ area (N/mm² = MPa)
- Density = mass ÷ volume
Worked examples
1. A mix is 1 : 2 : 4 by volume. You need 0.7 m³ of dry mix. How much cement, sand and gravel?
Parts = 1 + 2 + 4 = 7. One part = 0.7 ÷ 7 = 0.1 m³. Cement 0.1 m³, sand 0.2 m³, gravel 0.4 m³.
2. You use 40 kg of cement with w/c = 0.45. How much water is needed?
Water = 0.45 × 40 = 18 kg = 18 litres (1 kg of water is 1 litre).
3. A 200 g sample of gravel is sieved. 120 g stays on the 4.75 mm sieve. What percentage is retained?
120 ÷ 200 × 100 = 60 %. The rest (40 %) passed through.
4. A concrete cube of side 150 mm breaks at a load of 540 kN. Find its compressive strength.
Area = 150 × 150 = 22 500 mm². Load = 540 kN = 540 000 N. Strength = 540 000 ÷ 22 500 = 24 MPa.
5. 1060 g of wet sand dries to 1000 g. What is its moisture content?
Water = 1060 − 1000 = 60 g. Moisture = 60 ÷ 1000 × 100 = 6 %. This water must be taken off from the water added to the mix.
6. A site ordered 120 bags of cement and used 108. Find the wastage percent and say one way to reduce it.
Wasted = 12 bags; 12 ÷ 120 × 100 = 10 %. Keep bags dry and raised off the floor, and order by exact measured need.
Common mistakes
- Adding extra water to make the mix easy to pour. It makes the concrete weaker and crack more.
- Testing one handful from the top of the heap and calling it a sample. Take it from many places and mix.
- Forgetting that wet sand already holds water. Count it when you decide how much water to add.
- Skipping curing. Concrete needs days of moisture to gain strength, not just a day to dry.