Analyzing the Role of Fe⁰ and Fe³⁺ in the Formation of Expanded Clay Aggregates

The effect of the addition of Fe⁰ and Fe³⁺ on the formation of expanded clay aggregates was studied using iron-free kaolin as an aluminosilicates source. Likewise, the incorporation of cork powder as a source of organic carbon and Na₂CO₃ as a flux in the mixtures was investigated in order to assess its effect in combination with the iron phases. An experimental protocol, statistically supported by a mixture experiments/design of experiments approach, was applied to model and optimize the bloating index, density, absorption capacity, and mechanical strength. The process of expansion and pore generation and the associated decrease in density required the addition of iron, such that the optimum mixtures of these properties presented between 25 and 40 wt.% of Fe⁰ or Fe³⁺, as well as the incorporation of 3.5-5 wt.% of organic carbon. The addition of Fe³⁺ produced a greater volumetric expansion (max. 53%) than Fe⁰ (max. 8%), suggesting that the formation of the FeO leading to this phenomenon would require reducing and oxidizing conditions in the former and the latter, respectively. The experimental and model-estimated results are in good agreement, especially in the aggregates containing Fe⁰. This reinforces the application of statistical methods for future investigations.