How long should a mortar be mixed? Should all ingredients be mixed at the same time?
Most masons mix mortars using a paddle mixer. The appendix to ASTM C 270 Standard Specification for Mortar for Unit Masonry recommends that 3/4 of the required water, 1/2 the sand, and all the cementitious materials be briefly mixed together first. Then the balance of the sand and water is added. The mortar should be mixed a minimum of 3 minutes and a maximum of 5 minutes after the last water has been added to the mix. The standard states that overmixing can change the mortar's air content. The Reinforced Concrete Masonry Construction Inspectors Handbook, published by the Masonry Institute of America and the International Conference of Building Officials, says to mix mortar by first mixing 1/2 the water and 1/4 of the sand, then adding the cement, lime, color, and remaining water and sand. It recommends that all of the ingredients be mixed for not less than 3 nor more than 10 minutes.