Aggregates by relationship to concrete
Excess aggregate dust, also known as "dirty" aggregate, can cause issues in concrete at different stages. While the potential negative impact is well-recognized, it is also known that the mineralogy of the dust is critical. For example, clay coatings showed a more harmful impact on concrete performance compared with other dusts such as carbonates (limestone dust) or stone dust. Clays that weakly adhere to aggregate will be dispersed in the mixing water and therefore will be integrated into the cement paste, which could lead to the workability issue. Specific clays mixed with a particular type of air entraining admixtures (AEA) can largely neutralize the function of the AEA and make it difficult to achieve required freeze/thaw resistance. On the other hand, clays that are strongly bonded to the aggregate surface will remain mostly located at the aggregate surface after mixing process and therefore may disrupt the aggregate-paste bond (usually referred to as interfacial transition zone (ITZ)) and results in strength and durability issues. Examples of the extent of aggregate dust-related issues are dust coating observed during paving operations, residual air bubbles in the mixer after concrete mixing (indicating the potential air entrainment issue), and negative impact on mechanical properties of concrete associated with excess aggregate dust.