Incidence of the water-soluble compounds contained into lavender and sunflower bioaggregates on the hardening process of mineral binders

Abstract

In building materials, the incorporation of plant aggregates into mineral matrix is an eco-friendly promising solution as these biobased building materials can answer to the new requirements for thermal and hydric performances. Nevertheless, plant aggregates can have deleterious effects on the hardening mechanisms of mineral binders. The aim of this study is to evaluate the incidence of the water-soluble compounds contained into bioaggregates on the hardening process of three mineral binders (CEMI 52.5 cement, metakaolin-based pozzolanic binder, limebased commercial binder). Two vegetal by-products were selected (lavender straw and sunflower depithed stalk) because of their large availability in France. Distilled lavender straw is the by-product of the steam distillation process applied to lavender plant. Sunflower bark aggregates were obtained from the fractionation of the entire stalk (i.e. bark plus pith). The chemical composition of the two bioaggregates was first studied by the ADF-NDF method of Van Soest and Wine. The cellulose, hemicellulose and lignin contents were thus determined. In addition, the water-soluble components were estimated by measuring the mass loss of the bioaggregates after 1 h in boiling water. Large differences were highlighted concerning the content in water-soluble compounds between the two plant aggregates. Model pastes were then elaborated with a solution obtained by soaking lavender or sunflower powder in demineralized water for 48 h and then filtering. The properties of the pastes were compared with those of neat pastes of the three mineral binders. The setting time (followed by isothermal calorimetry), the hardening mechanisms (followed by X-ray diffraction and thermogravimetric analysis) and the mechanical performance of the different binders were clearly and variously influenced by the lavender and the sunflower extractives at early ages. The level of the impact on the cementitious and pozzolanic binders can be correlated with the content in water-soluble compounds. Concerning the lime-based binder, it was less impacted by the water-soluble species.