To promote shell formation in eggs ,what can be added to the diet of layers?

To promote shell formation calcium should be added more often the diet of layers.  Calcium is an important mineral for egg shell quality. It is the main component of the egg shell which consists of 95% of calcium carbonate. Deficiency of calcium in the diet of layers may lead to the reduction in eggs size and also the production, poor egg quality with high chances of breakage  and also an increased layer mortality.