Earth as a building material is increasingly being studied for its low environmental impact and its availability. Plant aggregates and fibers have been incorporated into the earth matrix in the aim of enhancing performance for thousands of years but scientific studies began quite recently. The present paper reviews the state of the art of research on the influence of these various natural and renewable resources in unfired earth materials such as compressed earth blocks, plasters, and extruded and stabilized blocks. This review, based on 50 major studies, includes characterization of the particles and treatments, and recapitulative tables of the material compositions, and the physical, mechanical, hygrothermal and durability performances of earth-based materials. A lack of references on hygroscopic and durability properties was observed. Future research orientations are thus suggested to promote and develop this type of sustainable material, which provides a solution for saving energy and natural resources. Highlights Earth-based products with plant aggregates as a sustainable material are reviewed.  Physicochemical and mechanical properties of plant aggregates or fibers are reviewed.  Mechanical, hygrothermal and durability performances of the composites are reviewed.  Further investigations are needed to promote these materials.

Expanding the use of low-environmental impact materials in the field of building materials is a major aim in a context of sustainable development. These alternative materials should be non-polluting, eventually recycled, and locally available. Bioresources are already used in some building materials but few studies have investigated their relevance in such applications. The aim of this paper is to evaluate the suitability of three kinds of vegetal aggregates: barley straw, hemp shiv and corn cob. The availability of these bioresources, extracted from a French database, is discussed, as are their physical properties and chemical compositions. Their microstructure is described with SEM images and their particle size distributions are provided through image analysis. Sorption-desorption isotherms are measured by a Dynamic Vapour Sorption system. Bulk density, thermal conductivity and water absorption are also quantified. The results highlight a tubular structure for the three different aggregates, with low bulk density and thermal conductivity (0.044, 0.051 and 0.096 W.m-1 .K-1 respectively for straw, hemp shiv and corn cob) and high water absorption, especially for barley straw and hemp shiv (414 and 380% vs. 123% for corn cob). Their hygric regulation capacity is also sufficiently good, with a water sorption of between 20 and 26% at 95% of relative humidity. These plant aggregates could therefore be used as additions in an earth matrix, or a hydraulic, pozzolanic, air lime or gypsum binder, or just as loose-fill insulation material. However, future research should focus on their resistance to fire and bacterial growth to validate this approach.

Effect of plant aggregates on the mechanical properties of earth bricks 25 26 ABSTRACT 27A building material is mainly characterized by its mechanical performance, which 28 provides proof of its quality. However, the measurement of the compressive or 29 flexural strength of an earth-based material with plant aggregates, which is very 30 ductile, is not fully standardised. The objective of this study is to determine the 31 compressive and flexural strengths of a composite made of earth and 0%, 3% or 32 6% of barley straw, hemp shiv or corn cob. Given the manufacturing processes 33 available, cylindrical compressed specimens were studied in compression 34 whereas extruded specimens were studied in flexion. Two protocols were tested 35 for compressive strength measurements: one with direct contact between the 36 specimen and the press, and the other with reduced friction. The test with 37 reduced friction engendered a huge decrease of the stress and a slight decrease 38 of the strain. For both compressive and flexural strengths, the specimens made 39 of earth alone were the most resistant, followed by composites containing straw. 40

Physical differences between straws lead to distinct behaviour of the composites  Barley straw improves thermal behaviour but reduces mechanical strength  Lavender straw improves durability and resistance to fungal growth ABSTRACT Nowadays, for environmental reasons, the demand for bio-based building materials is increasing. The number of studies focusing on earth building bricks with addition of plant aggregates is growing. In this study, barley and lavender straws were considered as bioaggregates in an earth matrix. Their characterization revealed distinct shape, size and porosity. Composites including 3% and 6% by mass of each bioaggregates were then elaborated. Their performances were investigated and compared in terms of mechanical and thermal properties, durability of the composite materials and resistance to fungal growth. The mixture with 3% of lavender straw showed satisfactory durability results while barley straw brought better thermal properties.

Earth is a material presenting good hygric properties, which is an important point as comfort and indoor air quality have become major issues. Nowadays, the energy efficiency of a construction is also crucial, but the thermal insulation provided by an earth brick is quite low. Therefore, some plant aggregates were added to lighten the material, thus decreasing its thermal conductivity. The hygrothermal properties of seven formulations made of earth with 0, 3 or 6% by weight content of barley straw, hemp shiv or corn cob were assessed. The properties determined were thermal conductivity, specific heat capacity, water vapour permeability, and sorption-desorption isotherms measured with the saturated salt solution and DVS methods. The theoretical MBV was also calculated. The study showed a large decrease in thermal conductivity when a large volume of plant aggregates was added while plant aggregates incorporation conversely affected the thermal inertia parameters.Concerning water vapour permeability, as earth is a very permeable material, the addition of plant aggregates did not improve this property. The sorption capacity of bio-based earth materials was slightly increased in comparison with earth alone. Finally, the calculated MBV showed the excellent buffering capacity of this kind of material, with and without plant aggregates.

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Development of bio-based earth products for healthy and sustainable buildings: characterization of microbiological, mechanical and hygrothermal properties. (2015) Matériaux and Techniques, 103 (2). 206.

Laboratory test to assess sensitivity of bio-based earth materials to fungal growth. (2018) Building and Environment, 142. 11-21.