Which materials are used as aggregates in structural lightweight concrete?

The use of shale, clay, and slate as aggregates in structural lightweight concrete is based on their inherent properties that contribute to the desired lightweight characteristics of the concrete mix. Shale and clay, when expanded through heating, create lightweight aggregate materials that help decrease the overall density of the concrete while maintaining strength and durability. Slate, being a metamorphic rock, offers similar lightweight advantages and contributes to the overall structural integrity of the concrete. These materials are specifically beneficial for applications where reducing the load on structures is critical, such as in high-rise buildings or precast concrete elements.

In contrast, other options such as wood fibers and cellulose, granular materials and quartz, or glass and metal shavings do not provide the same balance of strength and reduced weight needed for structural applications. Wood fibers and cellulose lack the necessary durability and strength when exposed to moisture. Granular materials and quartz, while useful in other concrete applications, do not have the lightweight properties of the aggregates mentioned in the correct answer. Lastly, glass and metal shavings can introduce unwanted heaviness or brittleness into the concrete, further reinforcing why shale, clay, and slate are the appropriate choice for lightweight structural concrete.