Final Report Summary - NATURALIME (Naturally durable: Developing and testing the resilience of innovative natural admixtures for lime-based conservation mortars)
Executive summary
NatuRALiMe (acronym standing for: “Naturally durable: Developing and testing the Resilience of innovative natural Additives for Lime-based conservation Mortars”) is a two-year project (2013-2015) funded by the European Union's Seventh Framework Programme (FP7/2007-2013) and developed at the University of Oxford (UK). The overall aim of this research project was to design suitable and durable lime mixes (in particular, renders) made with lime and natural additives (i.e. derivatives of the vegetable and animal world). Interest in studying and using these additives is clearly linked to the need to use sustainable building materials and adopt processes that entail reduced pollution, CO2 emissions and energy consumption.
Three different limes have been studied: dry hydrated lime (CL90S); natural hydraulic lime (NHL3.5) and lime putty (CL90S PL).
One part of the project focused on the study of hemp-lime mixes. Hemp is one of the 10 major primary plants grown as a source of bast fibres (i.e. plant fibres collected from the stem) and shiv (i.e. inner woody part of the stem), used for various industrial purposes, such as: textiles, cordage, panelling, biofuel, paper, plastic composites and building materials. The use of hemp shiv in mortars involves important modifications to chemical-mineralogical, textural, physical-mechanical and hygrothermal properties compared to those of a typical mortar made with lime and a limestone aggregate. During these two years we have investigated all these inter-related aspects to gain an exhaustive knowledge of this peculiar biomaterial. It has been found that mixes of hemp shiv with natural hydraulic lime show the best final performance compared to those made with aerial limes in the form of dry hydrated powder or putty. Hemp-lime mixes show also good durability towards different climatic conditions, typical of Mediterranean, Semi-arid and Tropical countries. However, some preventive measures need to be undertaken to reduce the bio-receptivity of this material.
The NatuRALiMe project also focused on the study of other mixes made with lime and organic additives, in particular natural polysaccharides deriving from seaweeds. Their hydrogel properties supported the intention of using them as plasticisers and shrinkage-reducing additives in bio-based lime mortars. For this reason, special attention has been paid to study their rheological properties when mixed with lime. We have found that adding these natural polysaccharides to the lime paste increases the plasticity of the fresh mix, but in a different way depending on the mixing procedure, time and temperature.
Summary description of project context and objectives
NatuRaLiMe was conceived with the main aim of designing new sustainable and compatible bio-based mortars to be applied as rendering materials. For this purpose, two main groups of additives have been taken into consideration for this study: hemp shiv (i.e. inner woody part of the stem of the hemp plant) and three natural polysaccharides, two of them derived from seaweeds and the third one with an animal or bacterial origin.
Hemp shiv mixed with a binder (mostly lime) produces a peculiar bio-composite (also called hemp concrete or hemp lime, in which the hemp shiv acts as a lightweight aggregate) that is being increasingly used in construction, in a number of applications. Its production entails the use of a natural and renewable resource (the hemp plant), low carbon and pollutant emissions during the manufacturing process, low net waste at the construction site, increased energy efficiency in the building and improved quality of indoor air. These are all reasons why this hemp-based composite is being increasingly used as sustainable building material, as a valid and efficient alternative to traditional construction.
The characteristics listed above make hemp-lime mixes efficient sustainable building materials and at the same time they justify the use of hemp lime composites also in repair interventions, especially focused on improving the insulation and hygrothermal performance (i.e. rendering or plastering applications) of historic buildings (e.g. rammed earth, adobe and cob constructions). In particular, the use of hemp-based composites for rendering applications (i.e. covering external wall surfaces) presents several advantages, such as the decrease of the material density when hemp shivs are used instead of a stone aggregate (lightweight rendering mortar); improvement of the water retention properties of the render and consequent reduction of shrinkage; increase of the material flexibility and consequent decrease of the tensions between the render and the materials of the wall; and development of a healthier environment in the interior of the building.
Regarding the choice of the correct binder, mixing hemp with lime seems to be the most convenient solution for producing carbon-negative hemp-based construction materials (both hemp and lime are efficient CO2-sequesters). However, hemp lime mixes are often produced with a formulated lime-based binder instead of lime on its own (Bevan and Woolley 2008). According to the European Standard BS 459-1 (BS-EN 459-1 2010) a formulated lime (FL) can be made with the following components in variable proportions: air-hardening lime (CL), natural hydraulic lime (NHL), cement (which can contain clinker, natural pozzolan, limestone, granulated blast furnace slag, plus minor constituents, such as calcium sulphate and silica fume, and organic or mineral additives; EN 197-1 2000). Two of the most common formulated binders available on the market for the production of hemp lime composites are Tradical® and LimeGreen®, which are both a combination of lime, cement and other additives. Although in the last decade the quantity of CO2 emitted due to cement manufacturing has decreased, as cement is increasingly manufactured from recycled materials, its production still embodies higher consumption of combustible and therefore emission of more CO2 and pollutant gases, with respect to lime. Furthermore, cement is not a compatible and suitable binder for conservation works on historic buildings, as demonstrated by the irreversible damages and acceleration of the decay processes that its application has caused to the architectural heritage. For these main reasons, lime was the only inorganic material taken into account in this project, as the most sustainable and compatible binder.
Within this context, NatuRaLiMe was especially focused on understanding:
- the microstructure and mineralogy of mixes made with lime and hemp shiv;
- the water transfer processes within the hemp-lime mix;
- the durability and bio-receptivity of hemp-lime mixes under different climatic conditions;
- the rheological properties of mixes made with lime and natural polysaccharides.
Description of the main S&T results/foregrounds
The investigations carried out in the framework of NatuRaLiMe project have demonstrated that natural hydraulic lime is the most suitable for use in combination with hemp shiv, since it slightly reduces the water absorption compared to the aerial lime and it also ensures reduced chromatic change and bio-deterioration.
Chemical, mineralogical and microstructural investigations have highlighted that the interactions between hemp, an organic component, and lime, an inorganic material, influence the normal processes of mortar hardening. We found that poor adhesion exists between the matrix and the surface of the hemp shiv (the matrix does not fill the big pores and channels constituting the hemp shiv and only a superficial deposition is observed). This is almost independent of the lime type and in part caused by the smooth external surface of the hemp shiv. Mixes showed a delayed hardening, considering that large amounts of portlandite (Ca(OH)2) and calcium silicates were found in aerial and natural hydraulic lime mixes, respectively, after three months of curing. Moreover, an unusually high amount of vaterite (polymorph of calcite, CaCO3) was also found in all mixes after three months, demonstrating a delayed transformation of this metastable polymorph into the stable one (i.e. calcite). The reason for these main findings (superficial deposition, poor adhesion, slow setting and presence of vaterite in the matrix) is inferred to be an insufficient amount of water available to the matrix of the mixes from the start. This assumption is supported by the ability of hemp shiv to absorb large amounts of water over a long time.
Due to the presence of hemp shiv, the mixes absorbed a much greater amount of water compared to a generic mortar made with lime and a stone aggregate. By studying the water absorption and desorption behaviour of a hemp shiv at micro-scale, it has been demonstrated that the shiv is not able to recover its initial volume after the swelling caused by water. Indeed, even after drying, the shiv remains in a certain way swollen and this increases its capacity for absorbing water. Although experimental conditions might play a role in the observed behaviour, this explains why the channels and pores constituting the shiv act as a nearly unlimited water reservoir.
However, the shiv did not slow down the drying rate of mortars and this is highly beneficial for a render, since it ensures the capacity of releasing water quickly when soaked, without allowing it penetrating into the wall. Furthermore, the presence of hemp shiv reduced to zero the shrinkage tensions in mortar samples, indeed no fissures were observed;
Neutron tomography imaging and 1H NMR studies were carried out to study the water transfer processes between hemp and lime in hardened hemp-lime mixes after water absorption and desorption. C NMR studies have also demonstrated that the hemp shiv suffers a partial degradation in the mix, not only due to contact with lime, but also due to the carbonation process.
We have developed and applied new weathering tests specifically conceived to study the durability of hemp-lime renders. To obtain a more realistic response from the material, we have reproduced the environmental conditions of three selected climates typical of the geographic areas where hemp is mostly grown and/or applied in sustainable construction:
1) Warm temperate (named Mediterranean climate, Csa).
2) Equatorial rainforest, fully humid (named Tropical climate, Af).
3) Arid desert (named Semi-arid climate, Bwh).
For each climate, specific cycles were designed and then simulated in an environmental cabinet (one year was condensed in twelve days). The conditions in both coastal and inland areas were also simulated for each climate, taking into account the presence and absence of airborne salt (NaCl), respectively. The macroscopic characteristics of the samples (mass, volume and chromatic variations) were monitored before, during and after the weathering tests whilst chemical-mineralogical and microstructural modifications were investigated at the end of each test. Finally, microbiological tests were performed to study how the bio-receptivity of hemp-lime composites is influenced by the climatic conditions (especially RH variations) and the presence of sodium chloride (e.g. growth of different number or type of microbe species), taking into account that soluble salts may affect microorganism growth, favouring halophilic (i.e. salt tolerant) species. The influence of the type of lime (aerial dry hydrated and putty and natural hydraulic) on the durability of the hemp-lime mixes has also been studied in this work. The weathering conditions simulated in this study (Mediterranean, Tropical and Semi-arid climatic conditions) do not cause significant macroscopic changes in hemp-lime renders over the length of the experiment. Mass, volume and appearance of the material remain almost unvaried, with only slight noticeable differences after heavy rainfall (mass uptake) and in samples made with aerial lime (yellowing of the surface). Under the simulated climatic conditions the hardening process of both aerial and natural hydraulic lime mixes is improved compared to control mixes (especially under Tropical climate), because further carbonation of portlandite and, in some cases, bacterially-mediated precipitation of calcium carbonates (especially under Mediterranean climate) are promoted. The colonisation and growth of different alkaliphiles (or alkalitolerant) fungi (such as Acremonium, Penicillium and Aspergillus) and bacteria (such as Bacillus, Arthrobacter and Micrococcus) under the three climatic conditions have demonstrated that the bio-receptivity of hemp-lime cannot be disregarded when predicting its long-term durability (c.f. other mortars not used in these experiments). Although no climate-related predominance has been observed among the isolated microorganisms, the water intake (as relative humidity and rainfall) seems to be critical for the existence of a greater diversity of species in samples, as it occurs under the Tropical climate. None of the isolated microorganisms have caused either aesthetic changes or physical breakdown during or after the test and this encourages the use of hemp-lime renders under Mediterranean, Semi-arid and Tropical climates. Salt weathering due to sodium chloride does not induce breakdown in the hemp-lime renders over the timespan of the experiment under any of the climatic conditions reproduced here. The main reason is that NaCl precipitates superficially and is easily removed by leaching from the material especially when rainfall is heavy and frequent (like in Tropical areas). Under low rainfall, typical of the Semi-arid climate, more salt is absorbed but still no damage can be observed in the material.
Finally, a rheological study of the mixes made with aerial and hydraulic limes and with natural polysaccharides mainly extracted from seaweeds was carried out to study differences in the rheological behaviour of the mixes depending on the mixing and dissolving procedures, the shear rate, and the additive-to-lime proportion (% wt.). We have found an influence of temperature on the shear rate measured, meaning that the viscosity, and therefore the microstructure of the fresh mixes change depending on temperature. Moreover, a viscosity change over time has been found in the mixes made with one of the polysaccharides. This aspect needs to be controlled in order to ensure a good influence of the organic additive on the workability of the fresh mixture.
Potential impact (including the socio-economic impact and the wider societal implications of the project so far)
The topic tackled in this project has, besides scientific interest, economic and social importance because a growing consensus exists on the need for sustainable new construction and conservation and maintenance of our monumental and historic heritage. This research project strongly contributes to the reduction of energy consumption and pollution, since the investigations carried out in its framework were focused at fully understanding new bio-based building materials, made with natural and low-contaminant components. Its findings represent a step forward both in the research field and the industrial sector, with eventual benefits for the built heritage. This research will have an impact on small- and big-scale companies that produce hemp-based mortars, as well as on those developing new bio-based building materials.
NatuRALiMe (acronym standing for: “Naturally durable: Developing and testing the Resilience of innovative natural Additives for Lime-based conservation Mortars”) is a two-year project (2013-2015) funded by the European Union's Seventh Framework Programme (FP7/2007-2013) and developed at the University of Oxford (UK). The overall aim of this research project was to design suitable and durable lime mixes (in particular, renders) made with lime and natural additives (i.e. derivatives of the vegetable and animal world). Interest in studying and using these additives is clearly linked to the need to use sustainable building materials and adopt processes that entail reduced pollution, CO2 emissions and energy consumption.
Three different limes have been studied: dry hydrated lime (CL90S); natural hydraulic lime (NHL3.5) and lime putty (CL90S PL).
One part of the project focused on the study of hemp-lime mixes. Hemp is one of the 10 major primary plants grown as a source of bast fibres (i.e. plant fibres collected from the stem) and shiv (i.e. inner woody part of the stem), used for various industrial purposes, such as: textiles, cordage, panelling, biofuel, paper, plastic composites and building materials. The use of hemp shiv in mortars involves important modifications to chemical-mineralogical, textural, physical-mechanical and hygrothermal properties compared to those of a typical mortar made with lime and a limestone aggregate. During these two years we have investigated all these inter-related aspects to gain an exhaustive knowledge of this peculiar biomaterial. It has been found that mixes of hemp shiv with natural hydraulic lime show the best final performance compared to those made with aerial limes in the form of dry hydrated powder or putty. Hemp-lime mixes show also good durability towards different climatic conditions, typical of Mediterranean, Semi-arid and Tropical countries. However, some preventive measures need to be undertaken to reduce the bio-receptivity of this material.
The NatuRALiMe project also focused on the study of other mixes made with lime and organic additives, in particular natural polysaccharides deriving from seaweeds. Their hydrogel properties supported the intention of using them as plasticisers and shrinkage-reducing additives in bio-based lime mortars. For this reason, special attention has been paid to study their rheological properties when mixed with lime. We have found that adding these natural polysaccharides to the lime paste increases the plasticity of the fresh mix, but in a different way depending on the mixing procedure, time and temperature.
Summary description of project context and objectives
NatuRaLiMe was conceived with the main aim of designing new sustainable and compatible bio-based mortars to be applied as rendering materials. For this purpose, two main groups of additives have been taken into consideration for this study: hemp shiv (i.e. inner woody part of the stem of the hemp plant) and three natural polysaccharides, two of them derived from seaweeds and the third one with an animal or bacterial origin.
Hemp shiv mixed with a binder (mostly lime) produces a peculiar bio-composite (also called hemp concrete or hemp lime, in which the hemp shiv acts as a lightweight aggregate) that is being increasingly used in construction, in a number of applications. Its production entails the use of a natural and renewable resource (the hemp plant), low carbon and pollutant emissions during the manufacturing process, low net waste at the construction site, increased energy efficiency in the building and improved quality of indoor air. These are all reasons why this hemp-based composite is being increasingly used as sustainable building material, as a valid and efficient alternative to traditional construction.
The characteristics listed above make hemp-lime mixes efficient sustainable building materials and at the same time they justify the use of hemp lime composites also in repair interventions, especially focused on improving the insulation and hygrothermal performance (i.e. rendering or plastering applications) of historic buildings (e.g. rammed earth, adobe and cob constructions). In particular, the use of hemp-based composites for rendering applications (i.e. covering external wall surfaces) presents several advantages, such as the decrease of the material density when hemp shivs are used instead of a stone aggregate (lightweight rendering mortar); improvement of the water retention properties of the render and consequent reduction of shrinkage; increase of the material flexibility and consequent decrease of the tensions between the render and the materials of the wall; and development of a healthier environment in the interior of the building.
Regarding the choice of the correct binder, mixing hemp with lime seems to be the most convenient solution for producing carbon-negative hemp-based construction materials (both hemp and lime are efficient CO2-sequesters). However, hemp lime mixes are often produced with a formulated lime-based binder instead of lime on its own (Bevan and Woolley 2008). According to the European Standard BS 459-1 (BS-EN 459-1 2010) a formulated lime (FL) can be made with the following components in variable proportions: air-hardening lime (CL), natural hydraulic lime (NHL), cement (which can contain clinker, natural pozzolan, limestone, granulated blast furnace slag, plus minor constituents, such as calcium sulphate and silica fume, and organic or mineral additives; EN 197-1 2000). Two of the most common formulated binders available on the market for the production of hemp lime composites are Tradical® and LimeGreen®, which are both a combination of lime, cement and other additives. Although in the last decade the quantity of CO2 emitted due to cement manufacturing has decreased, as cement is increasingly manufactured from recycled materials, its production still embodies higher consumption of combustible and therefore emission of more CO2 and pollutant gases, with respect to lime. Furthermore, cement is not a compatible and suitable binder for conservation works on historic buildings, as demonstrated by the irreversible damages and acceleration of the decay processes that its application has caused to the architectural heritage. For these main reasons, lime was the only inorganic material taken into account in this project, as the most sustainable and compatible binder.
Within this context, NatuRaLiMe was especially focused on understanding:
- the microstructure and mineralogy of mixes made with lime and hemp shiv;
- the water transfer processes within the hemp-lime mix;
- the durability and bio-receptivity of hemp-lime mixes under different climatic conditions;
- the rheological properties of mixes made with lime and natural polysaccharides.
Description of the main S&T results/foregrounds
The investigations carried out in the framework of NatuRaLiMe project have demonstrated that natural hydraulic lime is the most suitable for use in combination with hemp shiv, since it slightly reduces the water absorption compared to the aerial lime and it also ensures reduced chromatic change and bio-deterioration.
Chemical, mineralogical and microstructural investigations have highlighted that the interactions between hemp, an organic component, and lime, an inorganic material, influence the normal processes of mortar hardening. We found that poor adhesion exists between the matrix and the surface of the hemp shiv (the matrix does not fill the big pores and channels constituting the hemp shiv and only a superficial deposition is observed). This is almost independent of the lime type and in part caused by the smooth external surface of the hemp shiv. Mixes showed a delayed hardening, considering that large amounts of portlandite (Ca(OH)2) and calcium silicates were found in aerial and natural hydraulic lime mixes, respectively, after three months of curing. Moreover, an unusually high amount of vaterite (polymorph of calcite, CaCO3) was also found in all mixes after three months, demonstrating a delayed transformation of this metastable polymorph into the stable one (i.e. calcite). The reason for these main findings (superficial deposition, poor adhesion, slow setting and presence of vaterite in the matrix) is inferred to be an insufficient amount of water available to the matrix of the mixes from the start. This assumption is supported by the ability of hemp shiv to absorb large amounts of water over a long time.
Due to the presence of hemp shiv, the mixes absorbed a much greater amount of water compared to a generic mortar made with lime and a stone aggregate. By studying the water absorption and desorption behaviour of a hemp shiv at micro-scale, it has been demonstrated that the shiv is not able to recover its initial volume after the swelling caused by water. Indeed, even after drying, the shiv remains in a certain way swollen and this increases its capacity for absorbing water. Although experimental conditions might play a role in the observed behaviour, this explains why the channels and pores constituting the shiv act as a nearly unlimited water reservoir.
However, the shiv did not slow down the drying rate of mortars and this is highly beneficial for a render, since it ensures the capacity of releasing water quickly when soaked, without allowing it penetrating into the wall. Furthermore, the presence of hemp shiv reduced to zero the shrinkage tensions in mortar samples, indeed no fissures were observed;
Neutron tomography imaging and 1H NMR studies were carried out to study the water transfer processes between hemp and lime in hardened hemp-lime mixes after water absorption and desorption. C NMR studies have also demonstrated that the hemp shiv suffers a partial degradation in the mix, not only due to contact with lime, but also due to the carbonation process.
We have developed and applied new weathering tests specifically conceived to study the durability of hemp-lime renders. To obtain a more realistic response from the material, we have reproduced the environmental conditions of three selected climates typical of the geographic areas where hemp is mostly grown and/or applied in sustainable construction:
1) Warm temperate (named Mediterranean climate, Csa).
2) Equatorial rainforest, fully humid (named Tropical climate, Af).
3) Arid desert (named Semi-arid climate, Bwh).
For each climate, specific cycles were designed and then simulated in an environmental cabinet (one year was condensed in twelve days). The conditions in both coastal and inland areas were also simulated for each climate, taking into account the presence and absence of airborne salt (NaCl), respectively. The macroscopic characteristics of the samples (mass, volume and chromatic variations) were monitored before, during and after the weathering tests whilst chemical-mineralogical and microstructural modifications were investigated at the end of each test. Finally, microbiological tests were performed to study how the bio-receptivity of hemp-lime composites is influenced by the climatic conditions (especially RH variations) and the presence of sodium chloride (e.g. growth of different number or type of microbe species), taking into account that soluble salts may affect microorganism growth, favouring halophilic (i.e. salt tolerant) species. The influence of the type of lime (aerial dry hydrated and putty and natural hydraulic) on the durability of the hemp-lime mixes has also been studied in this work. The weathering conditions simulated in this study (Mediterranean, Tropical and Semi-arid climatic conditions) do not cause significant macroscopic changes in hemp-lime renders over the length of the experiment. Mass, volume and appearance of the material remain almost unvaried, with only slight noticeable differences after heavy rainfall (mass uptake) and in samples made with aerial lime (yellowing of the surface). Under the simulated climatic conditions the hardening process of both aerial and natural hydraulic lime mixes is improved compared to control mixes (especially under Tropical climate), because further carbonation of portlandite and, in some cases, bacterially-mediated precipitation of calcium carbonates (especially under Mediterranean climate) are promoted. The colonisation and growth of different alkaliphiles (or alkalitolerant) fungi (such as Acremonium, Penicillium and Aspergillus) and bacteria (such as Bacillus, Arthrobacter and Micrococcus) under the three climatic conditions have demonstrated that the bio-receptivity of hemp-lime cannot be disregarded when predicting its long-term durability (c.f. other mortars not used in these experiments). Although no climate-related predominance has been observed among the isolated microorganisms, the water intake (as relative humidity and rainfall) seems to be critical for the existence of a greater diversity of species in samples, as it occurs under the Tropical climate. None of the isolated microorganisms have caused either aesthetic changes or physical breakdown during or after the test and this encourages the use of hemp-lime renders under Mediterranean, Semi-arid and Tropical climates. Salt weathering due to sodium chloride does not induce breakdown in the hemp-lime renders over the timespan of the experiment under any of the climatic conditions reproduced here. The main reason is that NaCl precipitates superficially and is easily removed by leaching from the material especially when rainfall is heavy and frequent (like in Tropical areas). Under low rainfall, typical of the Semi-arid climate, more salt is absorbed but still no damage can be observed in the material.
Finally, a rheological study of the mixes made with aerial and hydraulic limes and with natural polysaccharides mainly extracted from seaweeds was carried out to study differences in the rheological behaviour of the mixes depending on the mixing and dissolving procedures, the shear rate, and the additive-to-lime proportion (% wt.). We have found an influence of temperature on the shear rate measured, meaning that the viscosity, and therefore the microstructure of the fresh mixes change depending on temperature. Moreover, a viscosity change over time has been found in the mixes made with one of the polysaccharides. This aspect needs to be controlled in order to ensure a good influence of the organic additive on the workability of the fresh mixture.
Potential impact (including the socio-economic impact and the wider societal implications of the project so far)
The topic tackled in this project has, besides scientific interest, economic and social importance because a growing consensus exists on the need for sustainable new construction and conservation and maintenance of our monumental and historic heritage. This research project strongly contributes to the reduction of energy consumption and pollution, since the investigations carried out in its framework were focused at fully understanding new bio-based building materials, made with natural and low-contaminant components. Its findings represent a step forward both in the research field and the industrial sector, with eventual benefits for the built heritage. This research will have an impact on small- and big-scale companies that produce hemp-based mortars, as well as on those developing new bio-based building materials.