Assessment of distilled lavender stalks as bioaggregate for building materials: Hygrothermal properties, mechanical performance and chemical interactions with mineral pozzolanic binder

Abstract

The building construction sector has a major impact on sustainable development, mainly in terms of raw material use, greenhouse gas emissions and waste production. Studies on alternative materials are nowadays clearly a priority to improve energy consumption and optimize waste management. Bio-based construction materials could take up this environmental challenge. Bio-based aggregates are renewable, they are mainly produced locally and they constitute an important way to store carbon dioxide. Most of the time, these bio-based aggregates are sub-products from local industries located near the factories producing building materials. Their use thus contributes largely to a decrease in the waste produced.Moreover, many studies have underlined the very interesting hygrothermal properties of bio-based materials, although some interactions of the bioaggregates with their surrounding environment have to be considered. Various authors have shown that cement (e.g. Portland cement), used as a mineral binder sometimes presents compatibility problems with bio-based aggregates, which can impact the setting and hardening processes of the binder.Among the several bioaggregates that can be considered, the waste derived from the production of essential oils is becoming an important issue. Essential oils are increasingly used in industrial countries today, mainly in medicinal or beauty products for their aromatic, medicinal and biological properties. These oils are primarily obtained by distillation of aromatic and medicinal plants. The effectiveness of these industrial and traditional transformations is very low, about 1% by mass, and this leads to large amounts of solid and liquid waste. The processing of these waste materials is inexistent in some countries like France, so waste constitutes a real obstacle to the development of this industrial sector.The objectives of this work are to determine the physical characteristics of a specific aromatic and medicinal plant residue (lavender straw) and the performances of a composite including this straw in the form of bioaggregates within a pozzolanic matrix previously designed by combining metakaolin with slaked lime. The characterization of lavender aggregates was conducted through Scanning Electron Microscope observations and their bulk density, water absorption capacity, bulk thermal conductivity and particle size distribution were assessed by an image analysis method. The thermal conductivity, water vapour permeability, moisture buffer value and mechanical properties of the composite were assessed.Finally, a study was conducted in order to understand the impact of lavender aggregates on the hydration mechanisms of the pozzolanic binder. Model pastes were elaborated with a solution obtained by soaking lavender particles in demineralized water for 72h and then filtering. The properties of the pastes were compared with those of neat pozzolanic paste.The setting time, hardening mechanisms (followed by X-ray diffraction and thermogravimetric analysis) and mechanical performance of the pozzolanic matrix were clearly influenced by the lavender extractives.