Mortar
Mortar tests the selection, proportioning, mixing and use of cement, lime, cement-lime, gauged and pozzolana mortars. It covers workability, water retention, bond, curing and the difference between mortar for brickwork, blockwork and plastering.
Core rules
- Common cement mortar ratios are 1:3 for strong or damp-exposed work, 1:4 for general load-bearing brickwork, and 1:6 for internal or non-structural work; follow the specification.
- Use clean water and clean sand; sea sand needs washing and approval because salts cause efflorescence and corrosion.
- Mix dry ingredients uniformly, add water gradually and use the mortar before initial set, generally within about 30 minutes for plain cement mortar.
- Do not retemper mortar by adding water after it has begun to set.
- Lime improves workability and water retention; a plasticiser must be compatible with the mix.
Worked example
For 1 m3 of wet 1:6 cement mortar, allow dry volume = 1.00 × 1.33 = 1.33 m3. Cement volume = 1.33 ÷ (1 + 6) = 0.19 m3. At about 1,440 kg/m3, cement = 0.19 × 1,440 = 274 kg, or 274 ÷ 50 = 5.5 bags. Sand volume = 1.33 × 6 ÷ 7 = 1.14 m3. These are nominal quantities; site wastage and bulking need separate allowance.
Traps that cost marks
Adding water to stiffened mortar, using an excessively rich mix for ordinary work, or ignoring sand bulking are common. A richer mix is not automatically better: high cement content increases shrinkage and cost.
Key points
- 1:6 mortar: about 5.5 bags of cement and 1.14 m3 sand per m3 wet mortar.