Which aggregate is best for concrete that will be exposed to the elements?

• A. Quartzite
• B. Granite
• C. Limestone
• D. Shale

Answer: (A)

For concrete that will face the elements, you want an aggregate that resists weathering, abrasion, and chemical attack. A rock with very high hardness, low porosity, and chemical inertness fits this need because it won’t abrade easily, won’t absorb much water, and won’t react with acids or moisture that cause deterioration in outdoor conditions.

Quartzite is ideal here because it is dominated by quartz, giving exceptional hardness and chemical stability. Its low porosity means it absorbs less water, reducing freeze–thaw damage and scaling in exterior environments. Granite, while durable, contains feldspars and mica that weather more readily and can contribute to weakening minerals over time. Limestone reacts with acidic rain and CO2, leading to dissolution and surface damage, making it less suitable for outdoor exposure. Shale is often clay‑rich and fine‑grained, which can swell with moisture and weather more quickly, compromising strength and durability.

So, among these options, quartzite offers the best long‑term performance for exterior concrete.