Mortar: what materials do I need?

FAQ Category: 

  • Building with bricks

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.