Recycling or downcycling? The difference between raw and stabilised earth explained

Abstract

The development of innovative earthen materials necessitates a thorough evaluation of their recyclability. Although stabilisation is often regarded as a barrier to the recyclability of earthen materials, this study investigates the feasibility of recycling compressed earth blocks (CEBs) stabilised with 5% CEM III A or NHL 3.5. To evaluate this property, the study involved repeated cycles of material characterization, compaction using the modified Proctor method, compressive strength testing, water durability tests and subsequent crushing. The raw material remained unchanged through the recycling process, retaining its clay activity, manufacturing properties, and compressive strength. In contrast, the stabilised material showed a significant decline in clay activity, rendering it negligible. After three recycling cycles, the Methylene Blue Value (MBV) of the cement- and lime-stabilised materials decreased from 0.7 to 0.1, while the materials do not show any plastic behaviour whatever their water content. Furthermore, the use of these stabilisers resulted in a decrease in the optimal dry density by 80 kg/m³ for CEM III A and 50 kg/m³ for NHL 3.5. Despite this reduction, there was no corresponding decrease in compressive strength following the first recycling cycle. This indicates the binder's effectiveness may remain similar because of the diminished clay activity. These results also indicate that stabilised materials must be re-stabilised during recycling to prevent a loss of compressive strength compared to the raw material. Considering these experimental results, recycling stabilised earthen materials should rather be defined as a downcycling process or a recycling process that needs additional stabiliser while raw earth displays an infinite recyclability.