As @Lehmuusa@nord.pub said. However it also has to do with temperature. I work primarily with lime mortar, so hard limit without additives is 5°C. And honestly I don’t really like to risk it when it might drop to 6° either. With cement mortar mixes you can get a little bit lower, 3 or 4°C. With cement and an anhydrous additive you can work to like -5° but you still need to wrap it with something to retain heat. Insulated tarpaulin or multiple layers of hessian with a tarpaulin over top.