Which rock is listed as a poor aggregate for Portland cement?

Which rock is listed as a poor aggregate for Portland cement?

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

Answer: (A)

When choosing aggregates for Portland cement concrete, the key issue is alkali-silica reaction. Reactive silica in an aggregate can react with the alkalis from the cement in the presence of moisture, forming a gel that expands and causes cracking. Chert is highly silica-rich and tends to be reactive, so it is considered a poor aggregate for Portland cement. The other rocks listed—granite, limestone, and sandstone—are generally acceptable as long as they don’t contain deleterious clay or reactive silica levels, which is why they’re not flagged as poor choices.

When choosing aggregates for Portland cement concrete, the key issue is alkali-silica reaction. Reactive silica in an aggregate can react with the alkalis from the cement in the presence of moisture, forming a gel that expands and causes cracking. Chert is highly silica-rich and tends to be reactive, so it is considered a poor aggregate for Portland cement. The other rocks listed—granite, limestone, and sandstone—are generally acceptable as long as they don’t contain deleterious clay or reactive silica levels, which is why they’re not flagged as poor choices.