Earth-based Materials Research Articles

Rejuvenating the Market for Earth-Based Building Construction Materials in a Developing Economy

Earth-based materials are useful in building and engineering construction projects globally, but they have largely remained unextracted and wasted, and their use has been limited to rural areas and avoided in modern buildings in the cities. The sustainability market in developing countries is still largely unsaturated and undertapped. This situation is blamed on lack of knowledge of the potential, benefits, and characteristics of green and sustainable building materials. This study aimed at determining the benefits of earth-based materials that could lead to the rejuvenation of the market for their adoption in building construction projects in the urban areas of a developing economy. The study adopted a structured questionnaire administered via electronic means to construction experts using the snowball sampling technique in Nigeria. With a response rate of 62.94% and a reliability index of over 0.90, the gathered data were analysed using frequency, percentage, and exploratory factor analysis (EFA). The study revealed that the main clusters of benefits of earth-based materials that can stimulate the market for these materials in urban areas are "cost and pollution-related benefits," "emissions and environmental benefits," "waste and workability benefits," "sound and fire-related benefits," and "thermal insulation and resource efficiency." The study recommended that housing investors, clients, and stakeholders should capitalise on the availability of large quantities of earthen materials to improve the quantity of housing provisions in cities and urban areas.

Potential of Earth-Based Bamboo Bio-Concrete in the Search for Circular and Net-Zero Carbon Solutions to Construction Industry

In order to limit climate change by achieving goals of cutting emissions down to net-zero by 2050, stronger efforts are needed to reduce the whole life cycle emissions of buildings. Integrating residual bio-based and earth-based solutions to concrete seems to stand out in the sector since these solutions have the potential of lowering materials embodied emissions, and enhancing building thermal performance. However, it is still unclear how environmentally beneficial bio-based and earth-based materials are and how they behave mechanically when they are both integrated into concrete. In order to know their potential applications in the sector, this study aims to evaluate and compare the mechanical performance and environmental profile of Earth-based Bamboo Bio-Concretes (EBBCs) with different earth fractions as partial replacements of the cementitious matrix, by evaluating its Greenhouse Gas (GHG) emissions. For that, it was considered the use of only bio-based aggregates (bamboo waste) instead of mineral ones at a fixed volume fraction of 45%. The methodology involved the: processing and characterization of earth and bamboo; EBBCs dosage study and mechanical testing; consideration of fixed proportions of binders of 30:30:40 (cement: metakaolin: fly ash) which were replaced gradually by earth in the volume fractions of 10%, 15%, and 20%. The Life Cycle Assessment (LCA) was used for accounting GHG emissions. LCA scope was from cradle-to-gate considering biogenic carbon methodology and avoided impacts of incinerating bamboo waste. A sensitive analysis was performed to evaluate the impact of transport distances variation of bamboo waste. Mechanical results point to an increase in EBBCs compressive strength with the increase of earth content until 15% of cementitious matrix replacement. LCA results showed negative embodied GHG emissions in all mixtures with an average of -115,7 kgCO2-eq/m3 mainly due to the high biomass content in mixtures. The increase of earth content from 0% to 20% in the mixtures reduced emissions by 59,7 kgCO2-eq/m3 since the binder’s content was reduced. With that, EBBC seems to be a promising innovative material to help achieve net-zero carbon emission targets and a circular pathway in the building and construction sectors.

Long-Term Deformations and Mechanical Properties of Fine Recycled Aggregate Earth Concrete

Earth-based materials are currently receiving high attention, as they are considered as sustainable. In addition, the reuse of waste materials and more particularly recycled aggregates can boost circular economy while reducing landfilling and mineral resource depletion. Incorporating recycled aggregates in earth concrete can be an innovative way to valorize them. However, investigations are required concerning their long-term behavior. Such an aspect is more important when fine recycled aggregates are considered. In this paper, the vulnerability to long term deformations of natural sand (NS) and recycled sand (RS) earth concrete mixtures is examined under real exposure conditions. Autogenous shrinkage, drying shrinkage, basic creep and drying creep of the different mixtures were monitored for a period of two months. Specimens were then subjected to compressive tests in order to evaluate their residual strength. Furthermore, the destructive tests were monitored in parallel with the acoustic emission (AE) technique. The results show an increase in the rate of drying creep and shrinkage for RS earth concrete mixtures. In addition, NS and RS earth concrete mixtures subjected to drying, with and without loading, reported a strength development in comparison to the reference mixtures. However, the Young’s modulus reported its lowest value for drying shrinkage of both mixtures. Regarding the AE technique, the distribution of its activity reflected the higher rate of damage of dried specimens in the pre-peak region.

Study of earth-based materials for manufacturing compaction process

The aim of this study is to develop a characterization test for the purpose of determining the consistency of the earth-based material adapted to produce compacted earth bricks (CEB). Existing tests have often been designed for plastic or fluid earth materials, which is not the case for compact earth materials of interest in the present study. This study includes the different methods of characterization for the shaping of these materials, such as: Atterberg limits, Carazas test, and dropped ball test. The earth-based mix studied has a plastic limit of 20%, a liquid limit of 42%, and a plasticity index of 22%. Therefore, the mix can be classified as a medium plasticity clay. The Carazas and dropped ball tests reveal that the manufacturing water content must be between 12% and 16% to ensure proper adaptation to shaping by compaction. However, these characterization methods are neither sufficient nor accurate enough to quantify the consistency to the manufacturing of CEB. For this purpose, to measure this consistency, a plunger penetration test with surcharge is developed. It could be shown that the plunger impact depth corresponding to the suitable consistency of CEB should be in the range of 6–10 mm. In order to confirm these results and select the suitable water content for production, the compressive strength of CEB at different water content (12%–20%) is measured; results indicate that the highest compressive strength is noted for a water content of 14%, which corresponds to a surcharged plunger impact depth of 8 mm.

An experimental approach for the investigation of the performance of non-stabilized Compressed Earth Blocks (CEBs) against water-mediated weathering

• Multidisciplinary approach to assess the performance of non-stabilized CEBs used for the restoration of heritage buildings and in contemporary construction against water-mediated weathering. • Soil and sand mineralogy affect the behavior of hardened CEBs against water-mediated weathering. • Addition of sand contributes to the optimization of CEB durability performance. • Combination of testing methodologies needed to assess CEB resistance against water attack. Compressed Earth Blocks (CEBs) have been attracting increasing research interest recently, because of their sustainable nature. While these earth-based materials have long been used in heritage and contemporary buildings worldwide, their poor performance against water-mediated weathering is still considered to be a major drawback. The aim of this study is to examine the durability performance of non-stabilized CEBs currently produced in Cyprus and to enhance their resistance against water-mediated weathering by optimizing their granular composition. To that end, four soils originating from specific areas of the island, which have been well-known for their rich tradition in earthen heritage buildings, namely adobe masonry, and a fine crushed aggregate were used to produce several CEBs. These were examined both at macroscopic and microscopic level, while spray erosion, drip, capillary absorption and wet/dry appraisal tests were carried out to assess their performance against water attack. The results show that more than one testing methodologies have to be used to fully assess the resistance of CEBs against water decay. They also suggest that the addition of sand can significantly improve the durability of non-stabilized CEBs against water-mediated weathering. This may prove to be a benefit both for restoration projects and contemporary constructions with the use of CEBs.

Indoor CO2 buffering potential of clay-based building materials

To mitigate the anthropogenic greenhouse gas emissions from the operational energy consumption in buildings, increasingly high-performance envelopes are developed. A reduction in indoor air renewal is observed that leads to a higher pollutant concentration in the built environment. Particular attention was given to CO2, an anthropogenic gas that has consequences on the Indoor Air Quality (IAQ) and the health of occupants. Developing a passive regulation system appears to be a promising solution for improving IAQ and reducing the energy consumption of ventilation systems. For that purpose, earth-based materials present interesting characteristics through the presence of clay minerals and their microstructure. In this context, this paper aims at presenting a novel experimental method to assess the CO2 retention potential of natural earth plasters. The method uses a thermogravimetric device (TGA/DSC) associated with a gas mixer and humid air generator. The mass variation resulting from the interaction of the increasing CO2 content could thereby be measured and analysed. Results show the important role of reversibility in the phenomenon: the majority of the captured CO2 being released when the CO2 concentration drops. It also highlights the role of the relative humidity on the retention capacity. As the retention of CO2 is lower when the material is wet, the water molecules may occupy part of the adsorption sites and react themselves with the CO2. This experiment provides the first values and thus evidence of the CO2 retention capacity and passive regulation potential of this material.

Review of separation and concentration techniques for impurities in rare earth-based materials for further spectral and mass spectral analysis

The effectiveness of using materials based on rare earth elements (REE) largely depends on their impurity composition, which affects their structure and properties. Before the analytical quality control of REE-based materials and initial substances for their production, it is necessary to determine both macrocomponents and impurity elements with high sensitivity and accuracy. A complex of atomic emission and mass spectral analytical methods is often used for the determination of impurities in REE-based materials in the range from 10–5 to 5.0 wt.%. However, the analysis of such materials, even using these modern high-sensitivity methods is a difficult task due to spectral and matrix interferences. Therefore, different preliminary separation/concentration procedures are needed to determine both rare earth and other impurities. This article reviews publications is devoted to preconcentration methods for spectral and mass spectral analysis of REEbased materials and, in part, a number of other analytical techniques. It was shown that the most common approaches are liquid extraction and chromatography. Sorption, cloud-point extraction and coprecipitation are also used. There is no universal method. Each of the methods discussed in this article has its own advantages and limitations. The analytical completion of the method confirms the effectiveness of the selected separation/concentration method in each specific case.

Scaffold-free 3D printing of shells: Introduction to patching grammar

This paper describes the Patching technique for building shells without wasteful support, thanks to 3D Printing technology. The method developed in this paper extends ancient vaulting techniques and decodes and formalises them into a design Grammar. Furthermore, numerical tools such as mesh pattern topology and shape topological skeleton are used to adapt vaulting techniques to 3D Printing data. This Grammar classifies the self-standing interconnected components (Patches) and automatically decomposes the non-traditional shapes into those Patches following the Patching process. As a result, the practitioners can use the Patching technique to fabricate low-cost, efficient shells. The presented prototype demonstrates that the Patching technique can contribute to printing shells with low-strength material such as fresh clay without any scaffolding. However, more research needs to be done on the material design and rheology control, shells' cross-section optimisation, and material failure theories such as Mohr-Coulomb and their application to 3D Printing using earth-based material.

Dimensionnal stability and strength appraisal of termite hill soil stabilisation using hybrid bio-waste and cement for eco-friendly housing

Dimensional stability and compressive strength are key factors to consider when modelling earth-based materials. It defines the volumetric performance of earth-based materials upon wet and dry environment. Meanwhile, the deformation under compression loading is accessed with the compressive strength testing. This study is aimed to use locally available materials considered as waste to model sustainable construction materials through soil stabilisation technique. The utilization of biowaste in this study is aimed to reduce the amount of waste produced in the agricultural sector in addition to the promotion of this material locally in the construction field. Cement was used as stabilizer to establish the performances of the waste-based stabilizer when mixed with conventional stabilizer or partnerless. Borassus fruit ash and cement were used both in solo, and hybrid mix (5wt%, 10wt%) to stabilize termite mound soil in the mix design. The mix design was analyzed microstructurally with scanning electron microscopy (SEM)-energy dispersive spectroscopy (EDS) to understand the effect of each stabilizer on the microstructural level. Fourier transform infra-red (FTIR) was conducted to identify the functional group present in each mix design to establish the influence of both stabilisers on the bonding mechanism. The mix design was also tested for water sensitivity, linear shrinkage, and compressive strength. From the results, samples containing 10wt% hybrid borassus fruit ash/cement exhibited higher content of Silicon, Aluminum, and Iron consequently satisfactory compressive strength. For hybrid stabilisation of earth-based materials, preference is given to 10wt% stabilisation level. The results of this study are analyzed to reduce the footprint of agricultural waste and to model locally available materials into sustainable housing materials.

Potential of raffia nanofibrillated cellulose as a reinforcement in extruded earth-based materials

The production of cellulose in the nano-scale size and their application as a reinforcement in the construction materials have gained increasing attention due to their unique properties, such as high tensile strength and modulus of elasticity, and large specific surface area. This study aimed to evaluate the mechanical, physical and thermal behaviour of extruded earth-based matrix reinforced with 0.1%, 0.3%, and 0.5%wt. of nanofibrillated cellulose (NFC). The results have shown that the mechanical nanofibrillation process of fibre improves its thermal stability. Their inclusion in an earthen matrix satisfactorily increases flexural strength and specific energy, while there is no significant increase in the composite's water absorption and thermal conductivity. The inclusion of a less than 0.5%wt. of raffia NFC contributes to fill the voids and arrest the propagation of nanoscale crack and improve the packing density of the earth-based composite for the development of the eco-friendly housing solution.

Earth construction: Field variabilities and laboratory reproducibility

Building construction is a major polluting sector. As a result, there is increasing global interest in the development of sustainable building materials with low environmental impact. Earth-based materials are among the materials of interest and building with earth-based materials has thus received a particular renewal of attention. Previous research has focused on the physical characteristics and durability of these materials. The aim of this study is to assess the variability of materials made in-situ and their reproducibility in the laboratory using an automatic normal Proctor machine with different compaction energies. Both cob and light earth were investigated. Cylindrical and prismatic specimens were produced on-site and in the laboratory: cob was made of silt, silty clay, sandy silt, and flax straw; and a separate layer of light earth was made of elastic silt and reed fibres. An experimental program was designed to evaluate the properties of the materials in terms of their water content, density, porosity, compressive strength, and thermal conductivity. The results revealed that the in-situ densities could be reproduced in the laboratory with compaction energies of 0.6 MJ/m3 and 0.2 MJ/m3 for cob and light earth, respectively. These compaction energies will allow the research to produce laboratory specimens that were representative of the materials implemented on-site. Regarding the compressive strength, the values obtained in the laboratory were higher than those of the in-situ specimens. Correction factors of 0.88 and 0.67 for cob and light earth. These values should be applied to calibrate the laboratory results in relation to in-situ. Concerning the thermal conductivity, the values obtained in the laboratory were similar for cob and higher for light earth. A correction factor of 0.87 should be applied to calibrate the laboratory results to those obtained in-situ.

Barriers hindering acceptance of earth construction in the urban context of the United Kingdom

ABSTRACT Earth material is environmentally more sustainable than traditional building materials, such as bricks and blocks. The benefits of earth construction are multiple, especially with regard to the uniqueness as a way of preserving the artisanship and cultural values of architectural heritage. Unfortunately, construction professionals in the United Kingdom are unfamiliar with many aspects of earthen architecture. A number of interrelated barriers inhibit the acceptance of earth-based material and make it an unpopular choice among construction professionals in the UK. The aim of this study is to develop an understanding of such barriers by identifying and analysing them in light of perceptions held across societies, cultures, and acceptability among professionals. To get to the aim, an interpretivist philosophical stance was taken in this research and a critical analysis of up-to-date literature on various barriers was carried out. The Delphi technique and in-depth interviews were then overseen to substantiate which barriers hindering the acceptability of earth-based material in UK urban construction. A series of interconnected barriers such as, absence of supply chain networks, prejudicial conflict, reticence to changing familiar methods of construction as well as lack of knowledge, skill, and training were identified by experts as the key factors hindering the acceptability of earth-based construction material in the UK.

Thermal behavior of light earth used for building insulation: Insight on PCM introduction impact

To reduce building significant contribution to greenhouse gas emissions, architects and engineers are seeking eco-friendly construction solutions. Among investigated options, building’s thermal insulation and heat storage can be cited.In this regard, earth-based materials are attracting particular interest. These last years, there is a renewed interest in these eco-friendly building materials and techniques. This is due to many advantages that they present: excellent humidity regulation ability and high thermal inertia. Present study aims to improve light earth thermal properties. Specifically, this research work focuses on the development of an insulating and heat storing material. To achieve this, phase change materials (PCM) are incorporated in soil-natural fiber mixtures.In fact, different light earth samples are first prepared. Then, thermally characterized to highlight the impact of PCM on the light earth thermal insulating, heat storing properties and thermal response to changing boundary conditions. The incorporation of PCM showed an interesting improvement of the light earth thermal properties namely on thermal conductivity, specific heat capacity, and thermal comfort time.

Role of f-electrons in determining insulator to metal phase transitions of Ca(La1−xCex)2S4 (0 ≤ x ≤ 1) solid solution: A DFT + U study

For efficient usage of the rare earth-based materials in electronic or optoelectronic applications, their 4f electrons' behaviors must be understood properly. We have presented Hubbard U corrected density functional theory (DFT+U) study of structural, magnetic, electronic, and optical properties of a rare-earth chalcogenide system, Ca(La1−xCex)2S4 (0 ≤ x ≤ 1). A unique site selection technique based on local magnetic moment arrangements was applied to build the atomic arrangements for a Ce doped Ca(La1−xCex)2S4 solid solution. The incorporation of f-electrons by Ce doping modifies the properties of the parent compound, CaLa2S4. In conjunction with the DFT + U method, we applied spin–orbit coupling to determine the magnetic ground state. The inclusion of 25% Ce transforms the non-magnetic parent compound to an antiferromagnetic (AFM) compound, and AFM magnetic ordering remains unaltered throughout the whole solid solution series. In addition, these compounds also undergo insulator to semiconducting to metallic phase transitions as Ce concentration increases. While CaLa2S4 is an insulator, Ca(La1−xCex)2S4 with x = 0.25 and 0.50 are n-type semiconductors, and on the other hand, compounds with x = 0.75 and 1.0 are found to have metallic band structures. The Ce atoms in these materials were found to be in a mixed valence state, Ce3+/4+. We explained these phase transitions from the calculated electronic structures. In addition, we have presented an explanation for the experimentally observed red-orange colors of Ca(La0.25 Ce0.75)2S4 and CaCe2S4 compounds.

Feasibility of 3DP cob walls under compression loads in low-rise construction

The rapid adoption of 3D-printing (3DP) technologies in construction, combined with an increased willingness to reduce environmental impact, has facilitated reapproaching earth materials for modern building industry. The feasibility of 3DP earth-based materials has been under investigation in recent years, with a particular focus on cob due to its favourable characteristics toward the 3DP process. Yet, there is a lack of definitive information on the construction of 3DP cob. Hence this paper investigates the structural feasibility of 3D-printed cob walls in low-rise buildings. The investigation involved experimental compression tests on 3DP cob samples to obtain key mechanical properties including the compressive strength and elastic modulus. These properties were then used as inputs for structural analyses with respect to three alternate types of 3DP cob wall patterns to evaluate their load-carrying capacity based on a limit-state design framework. Results from the analyses were implemented in modelling an idealised low-rise cob building covering a range of floor spans and wall heights. The analytical study found that 3D-printed walls have the potential to sustain gravity loads typical of residential construction. Further, since the 3DP material was shown to have similar mechanical performance to conventional (non-3DP) cob on the material scale, the 3D-printing process provides the opportunity to produce wall sections that are structurally more efficient than the solid section used in conventional cob construction. This results in lower material consumption, making 3DP cob attractive from the point of view of resource efficiency. An important outcome of the study is the demonstration of a model design technique for low-rise 3DP cob buildings that could be implemented as part of a broader optimisation procedure to satisfy structural and architectural design objectives.

Raw earth-based building materials: An investigation on mechanical properties of Floridia soil-based adobes

Raw earth, like wood and stone, is one of the oldest building materials used across the world. Nowadays, given the growing role of circular economy, researchers are ever more interested in raw earth-based building materials, because they are widely available and environmentally friendly. The use of this traditional material has positive environmental consequences, especially in traditional rural building reuse and in rural landscape preservation. In fact, raw earth is locally available and totally recyclable and, thanks to its perfect integration into the landscape, it improves site visual perception. Additives and/or chemical stabilizer agents (i.e., Portland cement) are often used in the production of raw earthbased building components in order to increase their mechanical performance and durability. This production process reduces the environmental sustainability of the base material and causes a relevant increase on the embodied energy (i.e., the total energy required for the extraction, processing, manufacturing, and delivery of building components). This research work aimed at investigating how to improve the mix-design of earth-based building materials in order to increase their mechanical properties without any addition of chemical agents. A physical stabilization was performed on an original texture soil by adding various particle sizes. Mechanical tests were carried out on five different soil mixes by changing soil composition, aggregates, and water. Specimens made with mix-design 5 offered the best results in terms of flexural and compressive strength values which were 1.65 MPa and 6.74 MPa, respectively. Mix 3 obtained the lowest linear shrinkage rate (6.04%). Since raw earth-based materials are highly sensitive to soil composition and aggregates, this study attempted to obtain a repeatable process to produce semi-industrial adobes by optimizing and controlling various natural materials (i.e., soils, aggregates, and water).

Machine Learning Approaches for Prediction of the Compressive Strength of Alkali Activated Termite Mound Soil

Earth-based materials have shown promise in the development of ecofriendly and sustainable construction materials. However, their unconventional usage in the construction field makes the estimation of their properties difficult and inaccurate. Often, the determination of their properties is conducted based on a conventional materials procedure. Hence, there is inaccuracy in understanding the properties of the unconventional materials. To obtain more accurate properties, a support vector machine (SVM), artificial neural network (ANN) and linear regression (LR) were used to predict the compressive strength of the alkali-activated termite soil. In this study, factors such as activator concentration, Si/Al, initial curing temperature, water absorption, weight and curing regime were used as input parameters due to their significant effect in the compressive strength. The experimental results depict that SVM outperforms ANN and LR in terms of R2 score and root mean square error (RMSE).

Performance and Durability of Cellulose Pulp-Reinforced Extruded Earth-based Composites

This study aimed to investigate the influence of two alternative cellulosic materials: bamboo organosolv pulp and recycled waste carton pulp, on the flexural strength, fracture toughness, drying shrinkage, and durability of extruded earth-based building materials by varying the reinforcement content at 5, 7.5, and 10 wt%. The results show the flexural strength and fracture toughness achieved by the inclusion of carton pulp were between the range of 2.87–3.20 MPa and 4.22–4.52 MPa.m0.5, respectively, while the introduction of bamboo pulp gave 2.04–2.20 MPa and 3.76–4.05 MPa.m0.5, respectively. Compared with the unreinforced material, composite reinforced with 5 wt% of recycled waste carton pulp significantly enhanced flexural strength (61%), specific energy (416%), and fracture toughness (57%), while increasing the drying shrinkage (81%) and water absorption (38%) and decreasing the bulk density (15%) of earth-based matrix. Although the addition of cellulose pulp in soil matrix increased the drying shrinkage due to the high absorption capacity of pulp fibre, it decreased the wearing percentage and the shrinkage cracking of the composite which contributed to an improvement in the durability of the earth-based material. The repeated drying shrinkage test led to an improvement in the dimension stability at 87%, 83%, 85%, and 82% for 0, 5, 7.5, and 10 wt% of recycled pulp composite, respectively. Cellulose fibre reacts as crack arrester during loading and shrinkage, improves the durability, and provides a lightweight earth wall material for insulation strategies.

Combined and synergic effect of algerian natural fibres and biopolymers on the reinforcement of extruded raw earth

This paper deals with the development of sustainable building earth-based materials. More precisely, it addresses the study of the reinforcement of raw earth with natural fibres originated from Algeria (diss and date palm tree fibres) and their stabilisation with xanthan gum. The aim of this study is the design of extruded earth-based building blocks with improved mechanical properties such as compressive strength and ductility. The effects of separate and simultaneous addition of date palm trees or diss fibres (with length varying between 5 and 15 mm at dosages of 1.5 and 3% in volume) and stabiliser (xanthan gum and HMP at 1 and 2% of the dry earth mass) on the flexural and compressive strength of the stabilised and unstabilised extruded materials are compared. Results show that adding only fibers decreases the compressive strength of the earth in comparison with the unreinforced sample. It is also shown that diss fibres provides better reinforcing efficiency than date palm tree fibres and that a combined addition of xanthan gum and natural fibres create a synergic effect that greatly improved the material mechanical behaviour: higher compressive and tensile strengths and better ductile properties. These results are fully supported by microscopic observations and pull-out tests carried out on single fibres.

Impact of phase change materials on lightened earth hygroscopic, thermal and mechanical properties

Climate change is leading construction engineers and building stakeholders to improve materials thermal insulation and heat storage capacity. The development of energy efficient building materials with a low environmental impact is a real challenge. In this sense, earth-based materials are known worldwide and have been used since thousands of years. These materials present many advantages but their deployment on a large scale, while respecting the operative standards, is still limited.Considering lightened earth, present work aims to improve their thermal properties by using phase change materials (PCM). The incorporation of PCM in construction materials has aroused great interest due to their capacity to store thermal energy.In present study, mechanical, hygrometric and thermal experimental studies were carried out on soil-fiber-PCM mixtures. The PCM incorporation showed an interesting improvement of the lightened earth thermal properties and a deterioration of the hygrometric properties while keeping their mechanical properties relatively unchanged.