In lightweight concrete, what primarily causes low slump?

In lightweight concrete, low slump is primarily caused by low aggregate density. Lightweight concrete utilizes aggregates that are often less dense than traditional aggregates, such as expanded clay, pumice, or other lightweight materials. These aggregates can lead to a mix that requires specific adjustments in water-to-cement ratios and other mix design components.

When the aggregate density is low, the overall density of the concrete mix decreases, which affects the workability of the mix. This can lead to a lower slump, as the lightweight aggregates may not retain water as effectively or create a cohesive mix, thus making the concrete stiffer. Consequently, the low slump indicates that the concrete is less fluid and more difficult to work with, a characteristic seen in mixtures using lower density aggregates.

The other choices do not fundamentally address the specific characteristics of lightweight aggregates and their impact on slump in the same way that low aggregate density does. Understanding the properties of the materials in the mix is crucial for achieving the desired workability and performance in lightweight concrete applications.