I'm pretty certain that cement mortar is alkaline so may raise the soil pH slightly. However I would imagine that the biggest problem will be that when it gets wet it is going to go lumpy. If it is only a very small amount it probably won't really matter too much but otherwise, I would suggest clearing the 10-15cms of affected soil and replacing it with top soil from another part of your garden (or buy some bags of top soil/soil conditioner from your local garden centre ) and mix it up with the existing soil.