What is the primary characteristic of Type II (MH) concrete?

Type II (MH) concrete is specifically designed to have a moderate heat of hydration and moderate sulfate resistance. This characteristic makes it suitable for use in applications where the risk of temperature-related cracking is a concern, such as in large pours or in warm weather, while also providing durability in environments with moderate sulfate exposure.

The moderate heat of hydration is beneficial because it helps to mitigate the risk of thermal cracking that can occur when a substantial amount of concrete is placed at once, typically seen in mass concrete structures. The moderate sulfate resistance indicates that this concrete can withstand some levels of sulfate attack, making it appropriate for applications in certain soil and water conditions.

When considering the other options, high heat of hydration is not a characteristic of Type II concrete; that attribute is more closely associated with Type III concrete, which is meant for applications requiring rapid strength gain. Low sulfate resistance is contrary to the moderate sulfate resistance feature of Type II concrete. High early strength corresponds to Type III concrete as well, which emphasizes rapid curing and strength development in the initial stages. Therefore, the defining attributes of Type II concrete effectively explain why moderate heat of hydration and moderate sulfate resistance are its primary characteristics.