What rock makes a poor aggregate for Portland cement?

What rock makes a poor aggregate for Portland cement?

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

Answer: (D)

The main idea here is alkali-silica reaction. Portland cement contains alkalis that can react with certain forms of silica in aggregates when moisture is present. Chert is a silica-rich rock with silica in microcrystalline forms that are particularly reactive with those alkalis, so it tends to form an expansive gel that swells and cracks the concrete. That makes chert a poor choice for concrete aggregates. In contrast, granite and basalt are hard, stable silicate rocks that are typically durable as aggregates, and limestone is a carbonate rock that doesn’t participate in the alkali-silica reaction in this way. So the reactive silica in chert is what makes it unsuitable for Portland cement aggregates.

The main idea here is alkali-silica reaction. Portland cement contains alkalis that can react with certain forms of silica in aggregates when moisture is present. Chert is a silica-rich rock with silica in microcrystalline forms that are particularly reactive with those alkalis, so it tends to form an expansive gel that swells and cracks the concrete. That makes chert a poor choice for concrete aggregates.

In contrast, granite and basalt are hard, stable silicate rocks that are typically durable as aggregates, and limestone is a carbonate rock that doesn’t participate in the alkali-silica reaction in this way. So the reactive silica in chert is what makes it unsuitable for Portland cement aggregates.