Calcium Chloride was a cement additive used to reduce the 'setting' or 'hardening' time of the cement.
Over time, Calcium Chloride has become problematic as it is water absorbent, reacts with water and produces a corrosive agent that attacks steel.
Concrete slabs often use steel reinforcement rods and as a result end up corroding or 'rusting'. Rusting reinforcement bars and the resulting damage to the surrounding concrete can seriously compromise the structural integrity of the slab.
However, concrete slabs are relatively rare in residential builds and Calcium Chloride has not been used since the late 70's in the UK. It is therefore an unusual finding on a Residential HomeBuyer Report or Building Survey.