[ggm]

my answer was an attempt to get to this point because the OP said "get some cement and sprinkle it on your garden and see what happens" and the whole point was "thats not what people are doing in the farm sector here, they're not using pre-moistened cement dust, they are using post-pour concrete dust, the chemistry is different"

altering the PH can be desirable or harmful depending. thats where the 'get a soil agronomist into the convo' comes in.

from memory, there can be a significant non-bound component of concrete, and it can persist for years. so I accept there is a continuing risk of aggregation of the dust, because it doesn't completely process in immediate time-frame of the pour (I am told that the hoover dam continued to bake off for years afterward)

[jaclaz]

Sure, I was only trying to distinguish cement dust from concrete dust, the OP mentioned concrete dust, not cement, and he mentioned it being "waste":

>Just try adding a bunch of concrete dust to your basil plant in the window sill, see what happens. Most likely this will be the source, as concrete waste is a huge issue in most population

About aggregation, there are usually big differences between the different kinds of cement, modern Portland cements tend to end hydration (and thus hardening) within months, whilst good ol' Pozzolanic went on for years.

But I wouldn't be surprised if such a massive pour as a dam would go on hardening for many years.