Let's look at the characteristics of reinforced concrete as a first step to understanding how and why the cancer happens.
|concrete cancer eating away at a balcony causing a structural issue|
|typical slab pour with re-mesh|
|diagram showing correct placement of steel reinforcing for concrete|
If the steel is less than 40mm (to outside) from the surface then spalling occurs.
Spalling is when the steel inside the concrete rusts and expands causing the concrete to flake or chip away, causing more moisture to penetrate and more cancer to form.
As a building product the manufacture of cement is possibly the most impacting on Nature but if done properly it can outlast many other building materials and therefore be better for the environment in that instead of re-building every 30 to 50 years, the concrete structure will last hundreds or even thousands of years.
Having fixed many remedial concrete cancer issues in a variety of buildings I can say that the 40mm rule is not being adhered to one hundred percent, making concrete and the cement used to make it the worst material for building from an environmental standpoint, ( And what other standpoint is there?)