Study of thermal and mechanical properties of cement-based composites reinforced with vegetal sponge wastes and silica fume

Magali Fois1, ⋆, Thouraya Salem1, Rémy Manuel2, Laurent Ibos1
1 Université Paris-Est, CERTES, 61 avenue du Général de Gaulle, 94010 Créteil Cedex
2 Centre d’études et de recherche en mécanique et automatismes, Université d’Evry, 40 rue du Pelvoux, 91020 Courcouronnes,
Mots clés : Lightweight concrete, cellulosic fibers, compressive strength, flexural strength, thermal conductivity, thermal diffusivity, silica fume, cement-based composites, natural sponge.
Résumé :

As 40% of the global energy consumption and 56.7% of carbon dioxide emissions are relative to the building sector, the development of new insulating concretes based on biomaterials remains an innovative subject. [1] Biomaterials like vegetal fibers are biosourced and biodegradable, they are renewable resources which are not participating to the emission of carbon dioxide [2; 3]. A wide range of natural fibres prepared by different pulping methods were studied in various matrix systems (cement, mortars, etc. ) [2, 4]. In France, hemp concrete has been developed but the degradation of mechanical properties due to aging in humid conditions may bridle its use. [5] In specific conditions, some micro-organisms alter the hemp shiv microstructure leading to a modification of the porosity of concrete. This induces a decrease of the toughness and post-cracking strength of the cement [2]. Moreover, some constituents of natural fibers are very sensitive to the alkaline environment of cement which causes the reduction of the composite durability [6].

In this context the addition of silica fume to any vegetable fiber based concrete appears as a good alternative. Indeed silica fume reduces the concentration of hydroxyl ion in the pore solution producing a less aggressive environment for the cellulose fibres [4]. Silica fume is composed of spherical particles of silicon dioxide (SiO2) with an average diameter of 140 nm. These particles are 100 times smaller than the cement ones; they are participating to the chemical reactions of the cement producing additional Calcium Silicate Hydrate (CSH) in the voids around the hydrated cement particles. The increase of the bonds between particles and the decrease of the porosity lead to the reinforcement of the concrete [7].

This paper examines the thermal and mechanical properties of vegetal fiber cement based composites. These composites contain cement, sand, silica fume and a waste of industry composed of cellulosic fibers. This industrial waste is in fact crushed vegetal sponge. The objective was to obtain good mechanical properties combined with thermal insulation. The addition of vegetal sponge lowers the density (from 2092 to 1171  kg.m − 3), the thermal conductivity (from 1.78 to 0.47  W.m − 1.K − 1) and mechanical properties (compressive strength going from 73 MPa to 4MPa). Scanning Electron Microscopy (SEM) of the microstructure was also carried out.

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