Who invented mortar? What were the initial ingredients of this product?
The Romans used lime mortar. They heated powdered limestone (calcium carbonate), which decomposes it to calcium oxide and carbon dioxide.
Mixing calcium oxide with water hydrates it to form calcium hydroxide. A paste of calcium hydroxide in water is lime mortar, which you can use to cement bricks together. Carbon dioxide in the air reacts slowly with the mortar to form calcium carbonate again. It takes years for lime mortar to set properly.
Good mortar can be stronger than the bricks it holds together. Apparently the reason mortar isn’t used instead of bricks in those situations is, among other things, that it’s more expensive. So in these cases bricks are in a sense “filler”.
A similar situation pertains to fortifications down through the ages, when somebody wants to economize on material. Rubble has little or no structural integrity. On the other hand, building a huge castle entirely out of stone (and mortar) is expensive. To make it cheaper, walls could be filled with rubble, loose stones, etc. The only problem with that is once the structural outer walls are destroyed, the rubble just falls out, possibly taking the rest of the intact structural wall with it.
The Romans didn’t always use good mortar. When they did, the results could last thousands of years. There’s mortar in Rome that looks like it might have been set down a few decades ago.