Mortar: what materials do I need?
The cost and quality of masonry work is significantly affected by the mortar used. Mortars may account for as little as 7% of the volume of the walls, but the role it plays and the influence it has on performance are far greater than the proportion indicates. Mortar provides a bed for laying; bond units together to give compressive and flexural strength and seals joints against rain penetration.
Four types of building mortar are detailed in SABS 0164.
COMMON CEMENT |
Sand |
COMMON CEMENT |
Lime: sand |
COMMON CEMENT |
Sand plus mortar plasticizer |
MASONRY CEMENT |
Sand (common = Portland) |
Approximate proportions for mortar:
MORTAR CLASS |
COMMON CEMENT lt |
LIME lt |
SAND MEASURED LOOSE & DAMP litres mix |
MASONRY CEMENT kg |
SAND litres |
II |
50 |
0-40 |
200 |
50 |
170 |
III |
50 |
0-80 |
300 |
50 |
200 |
- Class II: Normal load bearing applications, as well as parapets, balustrades, retaining structures, free-standing and garden walls and other walls potentially exposed to severe damp conditions.
- Class III: Lightly stressed (e.g. single storey) bearing walls where exposure to dampness is not severe but check NBR and NHBRC.
Cementitious Materials
CEM 1 32.5 (ordinary Portland cement) and CEM 11/A (S,V or W) 32.5 (Portland cement 15) may be used in mortar.
It is not advisable to use CEM 111/A 32.5 (PBFC), unless the mortar sands are good quality. Mortar with common cement lacks plasticity, may bleed, and will be harsh to work with. This deficiency may be overcome by using masonry cement. The use of lime in the mortar mix is beneficial but is difficult to obtain. Masonry cements are readily available.
When sealed in airtight drums, cement remains the same in strength for up to 3 years. When packed in sacks, even under good conditions, deterioration in the strength of the cement will occur, with a prolonged shelf life, for example: 20% loss after 3 months, a 30% loss after 6 months and a 40% loss after 1 year. The arrangements for storing or stocking cement should be such that batches are used in the same order in which they were received. First in - first out.
Sand
Sand for mortar should comply with SABS 1090 and be well graded from 5mm downwards. Sand should be evenly graded and should not contain an excess of dust or other fine material. The use of fine sands, that are more or less uniform in particle size, may contribute to workability, but frequently leads to excessive shrinkage and cracking of the joints. Sands containing high percentage of clay, tend to give a conveniently plastic mix, but also leads to undue shrinkage.
Lime
Lime used in mortar is hydrated lime (commercial bedding lime) and not quicklime or agricultural lime. Lime give the best results when used with coarse sands. Lime with clayey sands can make the mortar over-cohesive and difficult to use. Lime should not be used with masonry cement.
The use of limes added to cement mortars is recommended as the improved workability and water retentively will lead to superior brick to mortar adhesion, with improved resistance of the brickwork to rain penetration.