Mortar Stiffening in a Hot Wind and the Effect on Bond
It is a dry, breezy afternoon with the temperature at 29 °C and a steady wind across an exposed site. You are laying a stretcher-bond wall of 65 mm bricks with a 10 mm joint, using a 1:1:6 cement–lime–sand mortar mixed that morning. The mortar board has been out for about 50 minutes. The bricks are moderately absorbent and were not wetted. You notice that the mortar has stiffened and you are having to 'butter' the joints heavily to get a full bed, and the last three courses show some hollow-sounding perpends when tapped. The labourer offers to add water to the board to bring it back.
1. Diagnose what is happening to the mortar and the bond, and explain why adding water to the board is not acceptable.
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Two things are happening together.
- The mortar is losing workability. On a warm, windy day with suction from absorbent bricks, the mix stiffens faster than normal. Stiff mortar does not spread into a full, even bed, so the bricklayer compensates by over-buttering - which gives an uneven bed and variable joint thickness rather than a true 10 mm joint.
- The perpends are not filling. Hollow-sounding perpends mean the mortar has not bonded to both brick faces, so the wall relies on friction rather than adhesion. That reduces flexural strength and water resistance.
Adding water to a board of mortar that has begun to set is not acceptable because the cement has already started to hydrate. Re-tempering with extra water raises the water/cement ratio, lowers strength, increases shrinkage and can leave the mortar too weak and porous. Mortar should be used within its working life and discarded once it has stiffened; it should not be brought back with water.
2. State the practical controls you would apply for the rest of the shift to protect bond and joint quality.
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Controls to apply:
- Mix smaller batches so the board is used within its working life; on a hot, windy day that means frequent small mixes rather than one large one.
- Wet the bricks if the specification permits and the brick type allows it, to reduce suction. Do not soak them; just reduce the suction so the mortar keeps its water.
- Keep the mortar board shaded and covered between uses, and never place it in direct sun or wind.
- Lay to a line and check joint thickness with a rule; reject hollow perpends and re-fill or rebuild them rather than tapping mortar into the face.
- Check the last three suspect courses: if perpends are genuinely hollow, cut out and rebuild those units rather than relying on pointing.
- Record the weather conditions and the batch times, because a hot-weather bond problem is a site condition that should be raised with the supervisor.