Mortars - General Guidance

Airtec, Ultralite, Insulite and Dense Concrete block surfaces offer an excellent substrate for accepting mortars and no pre-treatment is required other than ensuring that block surfaces are clean and free of dust and debris.

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Generally, in order to avoid unsightly cracking, the weakest mortar mixture appropriate to the structural requirements should be selected as per BS EN 1996-1-1.
For most applications above dpc, we recommend that a Class iii (M4) designation mortar is used.

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There is no need to wet the blocks before applying mortar. The consistency of the mortar mix should be adjusted to suit the suction of the block. For example, a slightly thinner mortar mix would suit Airtec blocks whose suction is higher than that of an Ultralite, Insulite or Dense Concrete block.

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• Cold or wet weather can adversely affect the results by hindering the adhesion and setting of the mortar. Building in such conditions should be avoided.

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• Block surfaces should be clean and free of dust, residues and moss-like substances.

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• For lightweight and dense concrete blocks, the suction of blocks will increase as they dry out. Do not wet the blocks but rather adjust the mortar consistency to suit the suction (i.e. the drier the block, the mortar should be made slightly wetter).

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• In the case of Airtec aerated blocks, when the blocks are dry the surfaces accepting the mortar can be lightly brushed with water immediately prior to application of the mortar.

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• Always use a full bed of mortar and fully fill the perp ends (vertical joints).
  

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• Protect newly laid blockwork from frost and inclement weather using suitable weatherproof sheeting and stabilise the wall wherever strong winds pose a risk.

• As a rule of thumb, weaker mixes should be used with Airtec aerated blocks to avoid cracking.

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• Stronger mixes which are unable to accommodate differential movements may produce some cracking - weaker mixes are able to accommodate the small movements producing only hair-line cracks in joints, which are less noticeable

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• Grade iii mortar which will assist in accommodating small movements, is suitable for most locations and exposed conditions and may be considered the most generally suitable for concrete blockwork.

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• It should be noted that improvements in adhesion and resistance to rain penetration are indicated for lime-based mortar.

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• Improvement in resistance to frost attack is indicated for mortars containing a plasticiser.

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• Insulating mortars, such as Limelite and Pearlite, may be used to improve the thermal performance of the blockwork.

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• For thin-joint masonry (Airtec aerated blocks only) water is added to the bagged powder and simply mixed. Always follow the instructions supplied with the mortar.

Airtec Aerated block walls can be constructed using either conventional 10mm mortar joints with mixes as shown in the table above or using 2mm Thin Joint mortar.

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Thin Joint mortar is widely available in the form of 25kg bags of dry, pre-mixed powder. Mixing is simply done by adding water to the powder according to the instructions on the bag and if made correctly will ensure that the correct mortar strength is achieved for use with aerated blocks.

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Click here to download our guide to Thin Joint Construction.