Mortar Mixing and Preparation
This domain covers mortar materials, batching by volume, mixing, consistency, pot life, retempering, and protection of fresh mortar. The exam tests whether you can follow a specified mix, produce a workable mortar, and recognise when mortar must be discarded rather than revived with water.
Core rules
- Use the specified cement-to-sand ratio and measure materials with a gauge box, not by eye.
- Use clean sand, clean water, and the approved cement type; do not add admixtures without approval.
- Mix dry ingredients first, then add water gradually to a workable consistency.
- Use mechanical mixing where required and avoid over-mixing.
- Use mortar within its pot life; do not retemper mortar that has begun to set.
- Protect mortar from sun and wind, and keep tools and boards clean.
Worked example
A specification requires a 1:4 cement-to-sand mortar by volume. A 0.03 cubic metre gauge box is used. If the batch needs 0.12 cubic metre of sand, divide by four: cement volume is 0.03 cubic metre, about one 40 kg bag. Measure one box of cement and four boxes of sand, mix dry, then add water gradually. The reasoning is that volume batching only works if every part is measured with the same box and the ratio is applied consistently.
Traps
- Adding water to stiff mortar after it has started to set.
- Guessing the sand quantity instead of using a gauge box.
- Using dirty sand or unapproved admixtures.
- Leaving mortar on the board until it dries out.
Key points
- Batch by volume with a gauge box.
- Follow the specified cement-to-sand ratio.
- Add water gradually to a workable mix.
- Discard mortar past its pot life.
- Protect fresh mortar from sun and wind.