Effects of Porosity and Hydration Heat on the Mechanical Behaviour of Foamed Concrete Incorporating Hemp Shives

The world is looking to limit the pollution emitted by the construction sector and reduce its effects by finding new solutions for construction and consumption more respectful of the environment. In this context, foam concrete, which is a cementitious matrix incorporating gas or air bubbles, represents several benefits due to its lightweight with low density (300 < ρ < 1850 kg/m3), its excellent insulation with low thermal conductivity (0.08 < λ < 1.2 Wm−1 K−1) and its high mechanical resistance in comparison to its weight. Moreover, plant materials incorporation can be an important solution in the search for renewability and carbon dioxide balance. Therefore, foam concretes incorporating hemp shives can be considered an interesting new material to investigate. In this work, we study the effect of the incorporation of hemp shives (from 0 to 15 vol%) on the resulting density, porosity, hydration heat, and mechanical behavior of the foam concrete. The control foam concrete (without hemp shives) is formulated with 70 Wight% cement, 20 Wight% Ground-granulated blast-furnace slag, and 10 Wight % metakaolin with different admixtures, and the elaboration method is the preformed method. Hemp shives introduction in the concrete decreases porosity, density, and then mechanical resistance. Pozzolanic additions increase the amount of hydration heat and the hydration speed, then accelerate the setting and ensure the stability of air bubbles in the concrete, thereby improving mechanical resistance. Meanwhile, hemp shives incorporation delays hydration without affecting the released amount of hydration heat, increasing its closed porosity. Consequently, the concrete takes longer to strengthen. Foamed concrete is usually a very porous material (with porosity larger than 70%) with more than 95% open porosity, resulting in a mechanical resistance below 3 MPa.