The Block Mould
The block mould (6” x 9” x 18”) is designed for double wall construction and is divided in half by a separator along its width. Thus, the actual block size is 3 inches (width) by 9 inches (height) by 18 inches (length). A double wall design, with 3 inches of space between the two walls, was chosen as it has many advantages in hot arid regions:
- the outer layer protects the inner layer from direct solar radiation, which first heats up the outer layer. With a reflective outer surface, this heat absorption is greatly reduced;
- only a part of the heat that passes through the outer layers reaches the inner layer by radiation or convection, and if provided with a reflective surface, it will not absorb all the heat;
- the cavity acts as an insulator
- sound transmission is reduced by the air space.
The input materials for the concrete blocks comprise of the following: an aggregate, cement and water. Readily available sand is used as the aggregate, which is compacted down to bind together with the cement. Although labour intensive and time-consuming, the batch is hand mixed on the ground using 1 bag of cement (50kg) for every 4 level wheel barrows of sand. The correct water content is important to allow for good dense compaction. Adding one wheelbarrow full of water (approximately 30L), which is hand-pumped from the borehole, provides the correct water content enabling the block to release easily from the mould. For every 50kg bag of cement, we are able to produce an average of 35 blocks.
Curing is one of the most essential parts of block making that leads to increased strength and avoids cracking where the surface dries out prematurely. With 7 days curing, in which the blocks are watered 2 times a day (in the morning and evening), the blocks will have reached approximately 75% of their final strength. In around 3 weeks, typically over 90% of the final strength is reached, though strengthening may continue for decades.