Mortar admixtures typically offer properties such as waterproofing, water-repellency, efflorescence control, air entrainment using plasticizers to enhance workability, accelerating and retarding setting times etc. These mortar admixtures are used for ratios 1:3 or leaner mixes for use in masonry construction.
Admixtures for masonry mortars are different from those used in concrete mortars which are generally ineffective in case of mortars. For instance, air entrainment mortar admixture is used as a plasticizer as it lubricates the interlock between sand particles and reduces the amount of water required for mixing. It also reduces the density of mortar which in turn improves the troweling characteristics and provides freeze-thaw resistance after the mix has hardened.
The effect of admixtures in a mortar depends largely on the mortar mix, mixer, and the time in a day when it is mixed.
While accelerating mortar admixtures accelerate the setting times, the retarders are used to enhance the setting time of mortars, which is generally used for ready-mix or ready-to-use mortars. The use of retarders allows up to 36 hours for the mortar to be used after mixing or delivery.
When mortar with retarding admixture is placed between masonry units or blocks, some water and retarder chemical are sucked out by blocks; thereafter the hardening process begins. So, this retarding admixture for mortar should generally be used for masonry construction which has absorbent units.
Types and Uses of Mortar Admixtures
Types of mortar admixtures decide their respective uses, such as:
- Waterproofing admixture is used to reduce moisture movement through bedding of masonry or renders.
- Water retaining admixtures reduces the suction of water from mortar into the masonry blocks or units and enhances the workability, cohesion and reduces the bleeding.
- Air entraining mortar admixture enhances the workability.
- Accelerating mortar admixture reduces the setting times and is used where early strength gain is necessary.
- Retarding admixture increases the setting times and are generally used for ready-mixed mortars.
- Polymer dispersion admixture helps in air bonding and provides flexibility and water repellency to masonry mortars.
Benefits of Using Mortar Admixtures
- It helps to maintain water content in mortars
- Reduces efflorescence
- Improves workability/consistency
- Improves strength
- Minimizes water absorption
Proportioning of Mortar
Masonry mortars should be proportioned in accordance with ASTM C-270, Standard Specification for Mortar for Unit Masonry.
How to Use Masonry Admixtures
Masonry admixtures are used for different purposes and affect different properties in masonry construction. The use of these depends on the type of masonry construction, mix ratio, mixer and mixing times. The admixture manufactures specifications and recommendations should be followed during the use of masonry admixtures.
Standards and Tests for Masonry Admixtures
Following ASTM standards can be used for testing different properties of mortar construction using admixtures:
- ASTM C270 – Standard Specification for Mortar of Unit Masonry
- ASTM C90 – Standard Specifications for Load Bearing Concrete Masonry Units
- ASTM C1384 – Standard Specification for Admixtures for Masonry Mortars
- ASTM E514 – Standard Test Method for Water Penetration and Leakage Through Masonry
- ASTM C1072 – Standard Test Method for Measurement of Masonry Flexural Bond Strength
- ASTM C140 – Standard Test Method for Sampling and Testing Concrete Masonry Units and Related Units
- ASTM C780 – Standard Test Method for Preconstruction and Construction Evaluation of Mortars for Plain and Reinforced Unit Masonry