zone which is absent when repair is done with MTA as compared to calcium hydroxide:
Zones formed with calcium hydroxide (CH) pulp therapy from below CH to normal pulp are:

Zone of liquefactive necrosis
Zone of coagulative necrosis
Inflammatory response
Hard tissue barrier with vascular inclusions.

The necrosis with pure CH is due to its high PH. But dycal which is a hard setting CH formulation forms hard tissue barrier without causing necrosis.
Zones formed with MTA pulp therapy

Very narrow or even absent coagulative necrosis
Reparative dentinogenesis zone resulting in dentin bridge with fewer vascular inclusion (high quality).

MTA is a biocompatible material. The excellent bacteria-tight seal accounts for the success with MTA.
Zone of calcification
Zone of necrosis
Zone of vascular inclusion
None of the zone is absent