[Marcello]

"Concrete armour makes sense, actually I recall now granite being once tested for MBT armor in some countries."


Concrete isn't the best option.A controlled deformation structure using steel/rubber/aluminium layers, like the one employed in the T-55 Enigma or the T-72B is simple to make and quite effective.The BDD armor of the T-55AM should be another solution.You could use the former for the front turret armor and the latter for the glacis.
A factory churning out bulldozers should be able to churn out tanks without problems.Some sort of stabilizer should be within the capability of such facility, I guess, as bulldozers use hydraulics too.
Indicatively something roughly comparable to the T-62M1 in performances.Dead meat against an Abrams but better than no tanks.