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

Chert makes a poor aggregate for use with Portland cement primarily due to its physical and chemical properties. Chert is a hard and brittle sedimentary rock that is primarily composed of microcrystalline quartz. While quartz is generally considered a good aggregate due to its strength and resistance to weathering, chert's brittle nature can lead to challenges when used in concrete applications. When chert is crushed or subjected to stress, it may break apart into angular fragments that do not bond well with the cement paste. This can result in a lower overall strength and durability of concrete.

In addition, chert may contain impurities or variations in its composition, which can negatively impact the workability and performance of cement mixtures. Its potential for high porosity can also lead to problems with moisture retention and reduce the overall quality of the concrete.

The other materials listed, such as basalt, quartz, and pumice, typically possess desirable characteristics making them suitable for use as aggregates in concrete. Basalt has a high density and strength, quartz is durable and chemically stable, and pumice provides lightweight properties which can be beneficial in specific applications. Each of these materials can effectively bond with Portland cement, enhancing the structural integrity of the resulting concrete.