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

Chert is a sedimentary rock composed mostly of microcrystalline quartz. While it has some hardness and durability, it tends to be less desirable as an aggregate in concrete applications involving Portland cement due to its high porosity and propensity for water absorption. When combined with cement, chert can lead to weakened concrete structures because the absorbed water can expand and contract with temperature changes and moisture fluctuations, potentially causing cracking and degradation over time.

In contrast, basalt is a fine-grained volcanic rock that is dense and strong, making it a suitable aggregate. Quartz, a mineral known for its hardness, also excels as an aggregate due to its resistance to weathering and its stable properties. Pumice, although lighter and more porous, can be used in specific concrete applications where lightweight properties are advantageous. Therefore, chert's characteristics make it the least suitable choice for use as an aggregate with Portland cement, highlighting its tendency to compromise the integrity of concrete mixes.