Design and multi-physical properties of a new insulating concrete using sunflower stem aggregates and eco-friendly binders

In the interests of sustainable development, building materials using lignocellulosic aggregates like hemp concretes are more and more used in green construction. This type of concrete made of plant aggregates has particularly attractive thermal and hygroscopic properties. The hemp produced by agricultural holdings represents 10,000 ha in France whereas sunflower production is about 700,000 ha. The purpose of the article is to bring scientific elements which demonstrate that aggregates from the sunflower stem can be valorised in the same way as hemp hurd for the manufacturing of building materials. This study could encourage the development of new outlets for agricultural crops. Moreover, the sunflower aggregates could be available at low costs and promote a local economy as compared to hemp hurd. The experimental program includes the physical and thermal characterisation of the raw materials and the design of concrete where aggregates, obtained from an industrial grinding process, are associated with a binder. The characterisation of the aggregates mainly consists of the granulometric study by image analysis, the water absorption and the determination of thermal properties using especially the hot wire method. Two binders are used to prepare cylindrical specimens of sunflower concrete. Two kinds of mixture are tested in compression after 60 days, in thermal conductivity and capillary water absorption. The first one, called wall mixture, has a binder on aggregates mass ratio of 2 ( B / A = 2) and the second mixture, called roof mixture, has a B / A mass ratio of 1. Hot wire test results demonstrate for the wall mixtures an average dry thermal conductivity of 0.096 ± 0.003 W m −1 K −1 for the two binders. Compression results show an average compressive strength value at 60 days of 0.50 ± 0.1 MPa for the wall mixtures and 0.10 ± 0.01 MPa for the roof mixtures after an average 17 % strain. The results obviously show that the sunflower concrete reaches thermal and mechanical properties very close from those of hemp concrete. The capillary absorption test confirms the high absorbent potential of the sunflower aggregates but also the role of the binder towards the water uptake behaviour of lignocellulosic concretes.