This is serious and you are quite right to be worried.
It has got little to do with the environment and more to do with cost.
cement is expensive, whereas waste products are virtually free.
I am helping a Bellway buyer in his battle with the NHBC who has weak mix mortar falling out of his brickwork. He is being thwarted at every attempt by Bellway, the NHBC and Marshalls who manufactured the mortar.
It is quite complicated. He is currently awaiting an allegedly 'independent inspection' report and recommendations based on the latest series of mortar samples taken away to be analysed.
The mortar mix in question has GGBS in it, referred to as "cement substitute"
I suggest you contact (PM) the original poster
PSE and read this thread:
Soft mortar joints