Last Updated 28/07/2004

Mortar

Mortar is literally the "glue" which holds masonry blocks together in a wall. A little care in the batching and mixing of mortar will help you to build strong, good-looking walls.

The three principal functions of mortar are:

• To provide an even bedding for the blocks and allow course levels by taking up small variations in unit height.
• To transmit compressive loads.
• To hold the blocks together in the wall by bonding to them, so that tensile and shear forces can be carried. (This is often referred to as a "bond strength"). This is particularly important so that units on top of a wall are not easily dislodged.

In order to provide a good bond between the units and the mortar, the following guidelines should be followed:

• An appropriate mortar mix design should be selected, see Table.
• The mortar should be batched accurately using some consistent form of volume measurement, eg. 1/2 bag of cement, 1/2 bag of lime and 18 shovels of sand for a 1:1:6 mix.
• The sand used in the mortar should be clean pit sand, masonry sand or plasterer's sand. Clayey loam or sand containing organic impurities will affect the mortar strength and should not be used.
• Mortar should be discarded and not retempered, after the initial set of the cement has taken place.
• Admixtures. Caution should be exercised when using lime-replacing additives such as plasticisers or workability agents. They should only be used if specified by the architect or engineer and then strictly in accordance with manufacturer's instructions. Detergent should never be used.

  Mortar Type	GP Portland Cement	GB Blended Cement	Building Lime	Sand	Methyl Cellulose added	where Used
  M3	1	0	1	6	Optional	General purpose application with moderate exposure including: -General purpose blockwork above the DPC. -Below the DPC in non-agressive soils. -Greater than 100m from a non surf coast. -Greater that 1km from a surf coast. -Blockwork standing in fresh water or non-saline wetting and drying. Fireplaces, BBQs and incinerators.
  	1	0	0	5	yes
  	1	0	0	5	yes
  	0	1	1	5	optimal
  	0	1	0	4	yes
  M4	1	0	0.5	4.5	optional	High durability applications with severe exposure including: -Below DPC in aggressive soils. -Severe marine environment up to 100m from a non-surf coast. -Severe marine environment up to 1km from a surf coast. -Blockwork standing in saline or contaminated water including tidal and splash zones. Blockwork within 1km of an industry in which chemical pollutants are produced.
  	1	0	0	4	yes
  	1	0	0	4	yes
  	1	0	0.25	3	optional
  	0	1	0	3	optional
  	0	1	0.25	2.25	optional

  Notes:
  

  (1) Methyl Cellulose water thickener is used to prevent the rapid drying out of the mortar thus improving its workability as well as increasing bond strength.
  It does not have the detrimental effect of the plasticisers. It is available under the trade name of DYNEX or similar.
  

  (2) While this table provides an overall general guide to where mortar is used, the Australian Standard for Masonry Structures AS3700–1998 has detailed specific requirements for where both the mortar and masonry units can be used.