What it is and what it is made of
Blocks are precast units for building walls. They are made of cement, water, gravel and sand — that is, traditional concrete. Their function is twofold: raising the enclosure and, when the design says so, forming part of the structural system together with the confining columns and tie beams.
Precisely because the recipe is simple, quality is decided in details that do not show: the batching, the curing and the dimensional consistency. A block that measures differently from one pallet to the next forces correction with mortar, and that is paid for in material, in mason hours and in a render that has to disguise the unevenness.
The four sizes and which wall each suits
Block #4 at 10×20×40 cm is the 10 centimetre unit: partition walls and light enclosures with no load requirement. The #5 at 12×20×40 cm is the intermediate size, common in interior walls and low perimeter walls.
The #6 at 15×20×40 cm is the most used in housing for its balance of weight, cost and wall capacity. And the #8 at 20×20×40 cm is the unit for load-bearing walls, perimeter fencing and work that needs more section. If there is a structural design, the design overrides any habit.
How many units you need
With the 20×40 cm face you get about 12.5 units per square metre before deducting openings, plus waste and joint thickness. The arithmetic seems trivial until it is applied to a whole project and the gap between drawing and reality appears.
Measure the actual wall, subtract real doors and windows and add margin. Coming up short mid-day stops the whole crew, and the freight on an emergency delivery costs more than the missing units.
How to recognise a well-made block
Complete edges that do not crumble when handled. Consistent dimensions between units from the same pallet. Even texture, with no sandy or friable patches. And a solid sound when tapped with the knuckles.
A poorly cured block is identified quickly: it scratches with a fingernail and sheds dust when lifted. Producing it at our own plant lets us control the mix and the curing, which are exactly the two variables that decide whether the unit arrives consistent.
How to lay it so the wall does not crack
Dampen the unit lightly, without saturating it. A dry block steals water from the mortar and the joint does not cure; a soaked block prevents bond. The right point is in between, and in hot weather it has to be checked unit by unit.
Use mortar with lime to gain plasticity and water retention: lime improves mortar plasticity, water retention, bond and flexibility, and helps prevent efflorescence. Keep the joint constant and confine the wall with columns and tie beams as the project specifies.
