The truth about mortar is scary. Most contractors mixing mortar have no idea what they are actually mixing. This is a result of trying to make the process easier and faster. However, it made the tradesmen less knowledgeable. One hundred years ago, it was required knowledge for masons and their mud-tenders to know the exact proportions of Portland cement and lime to mix for the needs of the mortar being produced. At that point in time, there was no such thing as pre-blended mortar of any type. All mortars were field mixed and most were dominantly lime mortar.
In the last one hundred years, the most commonly used mortar shifted to Portland based mortars. The two most commonly used are type ‘N’ and type ‘S’ mortar, as these mortars became more popular due to their durability. Many companies began to produce them in pre-blended bags. Have a look at Masonry Brick Contractors for more info on this.
This change in the market was intended to make production faster and more consistent, and it did. There were other results from their production as well. Since there was no longer any need to field mix mortars, and it was easier to use the pre-blended bags, knowledge in the field was slowly lost. Contractors began teaching and passing down the knowledge of how to use pre-blended mortar mixes instead of how to blend them.
Eventually, the understanding of what actually created these blends was lost within the industry. This became a major problem that most contractors today are still not aware of. Many of them have been in the industry for many years, and believe they know it well. This is unfortunately not accurate, because the information they learned was incomplete and in some cases completely wrong.
Mortar repairs have been performed as long as there have been mortar bonded structures. This was easy when mortars were composed of only lime. It was still fairly easy until pre-blended mortars were produced because the contractors possessed the knowledge needed to blend mortars. Now however, repairing mortars is not as easy as most people think.
Most contractors believe that they can perform these repairs using portions out of these pre-blended bags of mortar mix. Many of them think that they can mix small amounts of these bags to match the aging of mortar (match the grey tone). Both practices are wrong. These pre-blended bags contain proportions of Portland and lime that require mixing of the full package to maintain this ratio. Therefore, mixing small amounts from these bags is not acceptable and will not maintain the same ratio of Portland to lime that the bags will accomplish when mixed properly.
This ratio within these bags is required to meet the ASTM standards that regulate this industry. Each bag is labeled with instructions to mix the full bag with up to 3 cubic feet of sand. This works well for producing large amounts of mortar, but is not designed for smaller projects. These bags only come in one size (70 pounds or 1 cubic foot). In addition, these bags of mortar mix are only available in grey and white. This is because they were only intended for new construction. Repair mortars require finer tuning and are generally produced in much smaller volumes. At the time these mortars were produced, the knowledge required in the field to produce tuck point mortars was common. However, in today’s market that knowledge is not as common and it has become necessary to develop new products that are made for repair.
I don’t know that the market will ever truly regain the knowledge that has been lost, but through products like Tuck-Point Mortars the quality of these repairs can be maintained. Some companies produce a variety of products specifically for tuck pointing and repair. They produce different tones of mortar between white and grey, and these mortars come in a one gallon bag. This is a fairly small volume of mortar mix and is perfect for repair projects. In addition, there is no need to re-blend them to achieve different tones of grey. You simply get the tone you need, and add pigments as needed to match the original mortar.