Which of the following rocks makes poor aggregate for use with Portland cement?

• A. Chert
• B. Limestone
• C. Sandstone
• D. Granite

Answer: (C)

When selecting aggregates to use with Portland cement, the key factor is chemical inertness. A major durability issue is the alkali-silica reaction: the alkalis in cement (sodium and potassium) can react with silica in certain aggregates to form a gel that swells with moisture and induces cracking and expansion in the concrete.

Sandstone often contains significant silica in the form of quartz grains and silica cement, and it can host silica forms that are reactive with the cement alkalis. Under moisture conditions, this reactive silica can participate in the alkali-silica reaction, leading to cracking and reduced durability, which is why sandstone is considered a poor choice for aggregate in Portland cement concrete.

Chert is also silica-rich and can be reactive, but many exam contexts highlight sandstone as a common problematic aggregate for this reason. Granite is hard and largely chemically inert, and limestone is carbonate and generally does not participate in alkali-silica reactions, making them more suitable in typical situations.