One of the main reason modern concrete structures fail is because the are reinforced with steel, which expands when it corrodes causing cracks. Adding seawater

One of the main reason modern concrete structures fail is because the are reinforced with steel, which expands when it corrodes causing cracks. Adding seawater to concrete used with steel reinforcement would cause it to fail much quicker. Many other kinds of reinforcement are available, but steel rebar is by far the most commonly used as it's cheap. Roman concrete structures have little tensile strength and depends on compression to maintain their integrity, so their design is much more limited.