The primary use of concrete blocks is for constructing walls. These may be foundation walls for a home,sides of a barbeque, in the backyard, or complete walls for a building.
A variety of precast concrete products are used by the building trades with blocks being just one of them. Concrete blocks generally have hollow cavities on the inside to lessen weight and quantity of material used. There may be one or more such hollow cavities depending on the type of block.
The sides of the blocks may be smooth or they may have a cast design included. When in use, for building, the blocks are stacked on one another taking care to keep the rows absolutely straight and level. The joints are offset by half a block in the rows to increase strength of the wall. They are held together with concrete mortar which dries to form a hard bond.
You might think that concrete mortar is a relatively modern development but this is not the case. In ancient Roman times as early as 200 BC concrete mortar was used with shaped stones in building. In the era of 37-41 AD small precast concrete blocks were used in the Naples area of Italy for construction.
Unfortunately during the fall of the Roman empire in the 5th Century A.D.most of the concrete technology which had been developed ny the Romans became lost. Concrete technology was not seen again till the year 1824 when an English stone mason named Joseph Aspdin invented Portland cement.Thus began the modern age of concrete technology as we know it..