Advances in the knowledge of how heating can affect aggregate stability in Mediterranean soils: a XDR and SEM-EDX approach

Abstract

Aggregate stability (AS) is a key property in the soil system and it has been previously observed that its response to the fire can be very different with patterns that can show opposite trends depending on the principal cementing agents that control the aggregation between particles. In the case of some Mediterranean soils, previous studies observed an increase of AS. However it is not clear if this increase is real or apparent since fire could destroy a part of the aggregates and, therefore, what we are measuring is the AS that remains after fire. Soils from five different areas were heated in a muffle furnace at different temperatures (300, 500 and 700°C) for 20min. The weight of aggregate size fractions of 0.25–4mm and <0.25mm before and after heating treatments was quantified. The loss of weight by the combustion, the organic matter content and AS were also measured in all samples studied. A selection of aggregates of the three studied soils (control and heated at 700°C) were also analysed by X-ray diffraction (XRD) and the surface soils were observed by scanning electron microscopy (SEM-EDX), in order to identify mineralogical and structural changes in the surface of aggregates due to heating. Results showed an initial decrease in the proportion of the macro-aggregates at 300°C with regard to control (unheated) samples for all the soils analysed (p<0.05). We presumed it is mainly due to the loss of SOM by combustion and subsequent disruption of some aggregates in lower sizes. However, this effect was not enhanced by heating the soils at 500 and 700°C compared to 300°C for any soil studied. Moreover, the heating treatments caused an increase in AS with heating temperature, despite no significant decrease for 0.25–4mm aggregate fraction and the decrease of SOM produced by the combustion. Therefore, these results are not apparent as a consequence of the selection of the most resistant aggregates because of the destruction of other soil aggregates. X-Ray diffraction patterns and SEM-EDX analyses results in selected aggregates assisted us to explain the above-mentioned results of the increase of AS with heating temperature. The relative mineral abundances of the heated aggregates at 700°C were slightly modified with regard to control. We observed some changes in the iron (hydr)oxides minerals, where the intensities had decreased and dehydroxylated to form maghemite. The presented results also show the disappearance of kaolinite for one of the soils. An increase of relative content of dolomite and calcite was also observed in the other two soils. In general, changes in compactness of soil micro-fabric units that have been produced as the consequence of the recrystallizations and micro-structural modifications observed by SEM-EDX in the surface of the soil aggregates after heating. These changes in cementing minerals may lead to a stabilization of the aggregate structure and hence an increase in the AS.