The main difference between concrete grade C35 and C35A lies in their composition. C35 concrete is a standard concrete mix with a minimum strength of 35 megapascals (MPa), while C35A concrete is a special mix that includes air-entraining agents to improve workability and durability. The inclusion of air-entraining agents in C35A helps to increase the concrete's resistance to freeze-thaw cycles and reduce the risk of cracking. Overall, C35A is more suitable for applications where improved durability and workability are required.