Masonry Magazine October 1997 Page. 22
Sand
Sand is an important component of mortar. Sand provides bulk to the mortar mix and acts as a filler material, reducing the amount of binder (lime or cement) needed. The type of sand used in a mortar mix can significantly affect the mortar's properties, including its workability, strength, and durability. For repointing mortar, sharp, angular sand is preferred due to the sands ability to fit together like a puzzle creating a cohesive mix. Mortars mixed with rounded sand particles roll around in the mix like small ball bearings individually "floating" as compared to "knitting together". Natural sands having rounded particle shape provide improved workability at a lower water demand and often approximate the characteristics of existing sands in historic mortars.
Gradation
The gradation of the sand (particle size distribution) plays a very important role in the durability and cohesive properties of a mortar. This means that a mortar must have a certain percentage a large to small particle sizes in order to deliver the optimum performance. (figure 2) Acceptable guidelines on particle size distribution can be found in ASTM C 144.
Void Ratios
Sand, when held in the palm of your hand, contains many small voids between the individual grains. A good performing mortar fills all these small voids with binder (lime/cement) in a balanced manner. The amount of binder necessary to fill these voids can be determined by the following simple method: Take two equal sized containers, fill one with dry sand, level the top, but do not compact. Fill the other container with clean water, again to the top, and carefully pour the water into the container of sand until the water is just visible at the top of the sand. Measure the volume of water that has been used and express this as a percent age of original water. Well graded sand generally will have a 30% void ratio by volume. Therefore if the sand has 30% voids, use 30% binder by volume. This represents the universal 1:3 binder to sand ratios often seen in mortar specifications worldwide.
Lime
Mortar formulations prior to the late-19th century used lime as the primary binding material. Lime, itself, when mixed with water (into a paste) is very plastic and creamy. It will remain workable and soft indefinitely if stored in a sealed container. Lime hardens by carbonation absorbing carbon dioxide from the air, converting itself back to limestone. Once lime and sand are combined and placed into a wall it begins the process of carbonation through frequent wetting and drying cycles. If lime mortar is left to itself without these wetting cycles after application the mortar will simply "dry out" resulting in poor adhesion and poor durability. In addition, lime mortar is slightly water soluble and thus is able to re-seal any hairline cracks that may develop during the life of the mortar. Lime mortar is soft, porous, and changes little in volume during temperature fluctuations thus making it a good choice for historic buildings.
Portland Cement
More recent mortar has used portland cement as a primary binding material. A straight portland cement and sand mortar is extremely hard, resists the movement of water, shrinks upon setting, and undergoes relatively large thermal movements. Portland cement, itself, when mixed with water (into a paste) is harsh and stiff. Portland cement is a hydraulic cement, this is, it reacts with mixing water to harden and does so within a fairly short time period. Historic lime mortars often had some hydraulic properties due to the use of hydraulic lime, pozzalane natural cement, or the effect of impurities in the lime. Thus addition of portland cement in limited quantities is often useful to approximate existing mortar properties and to eliminate total dependence upon carbonation for strength development. As more portland cement is added to the mortar formulation, the mortar becomes less workable, stiffens quicker, and develops higher strengths. White portland cement, rather than the more commonly available grey portland cement, may be required for an accurate color match.
Masonry Cement (preblended)
Masonry cement is packaged in a single bag (without sand) and is designed to produce mortars of Type N strength or higher when mixed with sand and water at the job site. Masonry cement may con-