The carbon free rock is replacing limestone in the manufacturing process, not the sand. Sand is added to cement, along with rocks and other aggregates, to form concrete.
From what I can tell, the way this might be bad is that the carbon free rock may not exist in significant quantity. If it does, it will be mined in the same way as limestone, so that's just a wash not a bad thing. If the rock they need doesn't really exist they have to buy it from someone else who makes it from readily available materials. In that case, it could be green washing, where the company can claim "our process doesn't release much carbon compared to the traditional process" but in reality the total carbon released to create the cement - from mining to processing to pouring - could be similar.