Dirty Sand and Re-tempered Mortar on a Blockwork Job
On a 200 mm hollow block wall job in Kurunegala the batching is one bag of cement to five barrows of river sand. The afternoon is 34 degrees with a dry wind. The mason mixes a large batch in the mixer and the helpers carry it to the board in wheelbarrows. By the time a batch has stood on the board for two hours it has stiffened; the helper adds water and mixes it back to a workable paste so it is not wasted. A handful of the sand squeezed in the palm leaves a muddy stain. Shaken in a bottle of water, the sand settles with a silt layer about 8 percent of the sample volume. Cement bags are stacked on bare ground beside the mixer, two of them open.
1. Is the sand acceptable for this mortar, and what would you actually do about it on site today?
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Verdict: not acceptable as delivered
- The muddy stain and the settled silt layer of about 8 percent show clay and silt fines well above the roughly 5 percent normally accepted for masonry sand.
- Why it matters: fines coat the sand grains so the cement paste cannot grip them, the mix demands more water for the same workability, and the result is weaker mortar, poor bond to the blocks and more shrinkage cracking.
What not to do
- Do not just add extra cement to compensate. You cannot buy back dirty sand with cement, and a richer mix makes shrinkage worse.
What to do today
- Repeat the bottle settling test properly on a sample and keep the result.
- Tell the supervisor and reject or return the load, or ask for washed sand.
- If this is the only sand available, have it washed and re-tested before it goes into mortar for load-bearing walls.
- Record the rejection and the reason; do not let the helpers keep batching while the question is open.
2. The helper adds water to a two-hour-old batch to make it workable again. Is that acceptable? Explain what happens to the mortar and how the batching should change.
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No.
- By two hours the cement has already begun to hydrate and set. Adding water makes the batch look workable again, but the gel bonds that had formed are broken and cannot re-form; strength and bond are reduced and the joints can end up weak and dusty.
- This is different from adding a little water in the first few minutes to correct workability, which is normal practice.
Correct practice on this site
- Mix smaller batches that can be used within about one to one and a half hours in this heat, sooner in a hot dry wind.
- Keep the mortar board shaded and damp, and cover the batch between lifts.
- Discard mortar that has stiffened; never add cement to a batch that has started to set.
- Record the change of practice and make sure the whole gang knows the rule, since the helper was trying to save material, not damage the job.
3. The cement is stacked on bare ground with two bags open. What is the risk, and what storage and use rules would you set?
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Risk
- Cement on the ground pulls moisture from the soil and from the air; open bags do so much faster. It sets in lumps, loses strength and gives variable mortar from bag to bag.
Rules
- Stack on timber pallets or planks about 100 to 150 mm clear of the ground, in a dry, covered place.
- Keep the stack covered with a tarpaulin but not sealed tight against the ground; keep it away from the mixer, water drums and the wheelbarrow washing area.
- Stack to a sensible height so bags do not burst, and keep the stack off the walls.
- First in, first out; use the open bags first and close them after each use.
- Reject hard lumps. Do not crush them and use them as if nothing happened.
- Check the delivery date and the bag condition on arrival and record any damaged bags.