When the weather temperature is 10℃ ~ 18℃ and it is cloudy, sand, stone and cement are mixed and poured on the roof. How long will it solidify? Causes of "sand running" situation

10- 18℃ is not very low for concrete curing, and it is basically solidified in 24 hours, so you can go up and walk. Running sand, also known as sanding, is to wipe off a layer of powder and fine sand with a little hard hand. This situation is generally caused by improper maintenance of concrete, so water it in time to avoid sun exposure. Sometimes the surface water evaporates too quickly, and some cement can't react. Cement can't harden without water. Of course, there is another reason, that is, the vibration is too large. Nowadays, cement generally contains additives (such as mineral powder and fly ash, which are dark in color). Excessive vibration will make mineral powder or fly ash float to the surface to form laitance, which is weak and easy to sand. But these reasons will not affect the overall strength of concrete, but affect the appearance, which is not good-looking.