EP2429970A1

EP2429970A1 - Autogenous setting of non-hydraulic lime mortars by means of microbial carbonate production

Info

Publication number: EP2429970A1
Application number: EP10730732A
Authority: EP
Inventors: Willy Verstraete; Nele De Belie; Willem De Muynck
Original assignee: Universiteit Gent
Current assignee: Universiteit Gent
Priority date: 2009-05-13
Filing date: 2010-05-11
Publication date: 2012-03-21
Also published as: WO2010130712A1; GB0908268D0

Abstract

The present invention relates to the usage of micro-organisms to accelerate carbonation and to increase the compressive strength of non-hydraulic lime mortars. The micro-organisms of the present invention metabolize nutrients within the mortars in order to produce CO₂which results in accelerated CaCO₃ formation and hence increased strength of the mortar. The invention further encompasses non-hydraulic lime mortars comprising micro-organisms and relates to a process to fabricate such lime mortars.

Description

AUTOGENOUS SETTING OF NON-HYDRAULIC LIME MORTARS BY MEANS OF MICROBIAL CARBONATE PRODUCTION

Technical field of the invention The present invention relates to the usage of micro-organisms to accelerate carbonation and hardening, and to increase the performance, usability and compressive strength of non-hydraulic lime mortars. The invention further encompasses non-hydraulic lime mortars comprising micro-organisms and relates to a process to fabricate such lime mortars.

Background art

The European Mortar Industry Organization (EMO) defines mortars by considering its constituents, i.e. "a mixture of aggregates, generally with a grain size less than 4 mm, one or more binders and possibly additives and/or admixtures; mortars with inorganic binders contain, in addition, water" (EMO, 2009).

Lime (Ca(OH)₂) is probably the most versatile structural binder available and can be modified to suit a range of diverse uses and exposures. Lime has been used for the stabilization of soils, as a decorative medium for buildings, as a bonding aid of masonry units and for the improvement of wall surfaces, i.e. internal (plasters) and external (renders) coatings (Holmes and Wingate, 2006).

Based upon their setting or hardening behaviour, lime mortars can be divided into two characteristic types, i.e. hydraulic and non-hydraulic lime mortars, with 'hydraulic' referring to the ability to set under water. Non-hydraulic or air lime mortars, following an initial hardening due to drying, set entirely through carbonation. Carbonation is the process whereby slaked lime, or portlandite (Ca(OH)₂) reacts with carbon dioxide (CO₂) to form calcium carbonate, or calcite (CaCO₃). The latter is significantly stronger and less soluble than the portlandite. The process consists out of five steps: (1 ) diffusion of gaseous CO₂ through the pores of the mortar; (2) dissolution of CO₂ in the pore water; (3) dissolution of Ca(OH)₂ in the pore water; (4) chemical equilibration of the dissolved CO₂ in the pore water and (5) precipitation of CaCO₃.

Traditionally, non-hydraulic lime mortars are fabricated using lime putty, i.e. a slurry of Ca(OH)₂. Lime putty is manufactured by adding an excess of water to quicklime (CaO₂), which is the reaction product of the calcination of limestone (CaCO₃) at about 900⁰C. Nowadays, non-hydraulic lime is also available as a dry powder, i.e. hydrated lime. The latter is manufactured by slaking quicklime with a precise amount of water so that all the water either combines in the reaction or is driven off in the process. Reports on extensive damage to ancient masonry resulting from the use of cement based mortars together with the current requirements of repair mortars to be compatible with the original components of the masonry have resulted in a revival of the use of lime. Currently, lime has become one of the principal materials used in the conservation and restoration of historic buildings (Pavia et al., 2005).

Besides the compatibility aspects, the use of lime mortars offers some additional advantages over the use of cement regarding the environmental impact. Due to the lower production temperatures compared to cement, less energy is required for the fabrication of lime, resulting in 20% less carbon dioxide production. Furthermore, carbon dioxide is absorbed during the setting of lime mortars. Another advantage of lime mortars is the fact that bricks that are bonded together with lime mortar can be easily recycled, while this is not the case for cement bonded bricks (NationalGreenSpecification, 2008). Providing lime mortars with an early strength avoids failure due to environmental conditions such as frost or strong solar radiation (Pavia and Caro, 2006). Freezing of water saturated mortars induces strong fracturing and separation from the substrate as a result of the frost related expansion of water. Too rapid drying of the mortar, on the other hand, results in fracturing of the mortar through shrinkage. Setting aids such as pozzolans can be used to accelerate setting. The latter enables the mortar to acquire an early strength, resulting in a higher resistance to weathering.

Alternative approaches for the accelerated hardening of lime mortars aimed at the accelerated carbonation of the lime binder. The carbonation reaction depends mainly on the humidity and CO₂ concentration of the surrounding atmosphere. The chemical and physical characteristics of the mortars, on the other hand, also have an important effect on the carbonation process. Because carbonation proceeds from the outside inwards, the thickness and permeability of the mortar will significantly affect the rate of carbonation. The permeability of the mortar is affected by several parameters, which can be controlled to enhance carbonation. The mix proportions, more specifically, the water to binder ratio, are well known to influence the final porosity and permeability. In addition to the mix proportions, the choice of the constituents of the mortar also has an important influence on the rate of carbonation. The addition of aggregates is known to impart a certain degree of porosity to the mortar, and hence, to assist carbonation (Elert et al., 2002). Furthermore, it should be clear that the diffusion of CO₂ through the mortar is controlled by its pore structure. Others have tried to accelerate carbonation by the introduction of CO₂ or CO₂ generating compounds in the mortar mixture. Direct introduction of carbon dioxide to lime grouts by means of CO₂ saturated water, however, proved to be unsuccessful (Baglioni et al., 1997). Ferroni and Baglioni (EP 0411583) proposed the setting of lime mortars by means of additives such as urethanes or carbamates that slowly produce CO₂ under alkaline conditions, i.e. autogenous setting. However, ethyl carbamate is known to be carcinogenic which represents a serious health issue. More recently, Medici and Rinaldi (2002) proposed the addition of additives such as polyaminophenolic compounds tailored to boost the absorption of CO₂ to accelerate the carbonation of hydrated lime. Fisher (1999) mentioned the ability of a mixture of additives to improve the carbonation of lime mortars in the short and long term. The mixture (known as Solubel, Hasit Trockenmortel GmbH, Germany) comprises borax, dextrine, alumina acetate, natron, natural resin, potash, proteins, talc and sugar.

Table 1 gives an overview of different additives used in the past for modifying the workability and setting speed of lime mortars (Holmes and Wingate, 2006).

Table 1 Overview of different additives used in the past for modification of the properties of lime mortars (after Holmes and Wingate, 2006).

Additive types Examples

Accelerators Alum Borax Bone ash Calcium chloride

Gypsum Pozzolans

Air entrainers Ash residue Beer Broken brick Chalk and and tile limestone particles

Charcoal

Bonding agents Acrylic Molasses Polyvinyl Styrene emulsions acetate butadiene rubber

Sugar White of eggs

Hardeners Alum Bone ash Crushed shells Dextrin

Forge ashes or Granite dust or Marble dust Molasses furnace cinders chippings

Oils Pozzolans Pulverized Rye flour soapstone (talc)

Styrene Sugar Gaggery) butadiene rubber

Hence, it is clear from the above that several strategies to accelerate carbonation, which results in an earlier strength of non-hydraulic lime mortars have been used. However, there is clearly still a need for alternative, well- or better-performing and/or user-friendly strategies resulting in an earlier and high compressive strength of these lime mortars.

Brief description of Figures

-Figure 1 Grading curves of the hydrated lime binder (A) and different types of aggregates

(B) -Figure 2 Indication of the locations on the prism (4 cm height) where the measurements of the wave velocities of ultrasonic pulses (sensors: 2 cm diameter) have been performed. The upper surface of the scheme corresponds with the troweled surface. -Figure 3 Effect of the duration and type of curing regime on the compressive strength of lime mortars with limestone sand 0/2. -Figure 4 Influence of the concentration and type of ureolytic microorganisms on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars with limestone sand 0/2 and a W:B ratio of 1.5. Bacillus sphaericus (BS) and Sporosarcina pasteuήi (SP) are a source of urease and were used in this experiment to stimulate the autogenous setting of lime mortars by means of the microbial hydrolysis of urea. In order to investigate the influence of ex situ formed carbonate ions on the autogenous setting, microorganisms have been added to the mixtures in their culture medium (Med.) or after centrifugation (C). Autoclaved centrifuged BS (BS A.C.) were included as microbial control series. In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors.

-Figure 5 Influence of the use of sand as microbial carrier on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars produced with a W:B ratio of 1.5. Bacillus sphaericus (BS) and Sporosarcina pasteurii (SP) are a source of urease and were used in this experiment to stimulate the autogenous setting of lime mortars by means of the microbial hydrolysis of urea. The sand was immersed for 24 hours in a fully grown culture of microorganisms to allow the formation of a biofilm on the aggregates. -Figure 6 Influence of the concentration and type of alkaliphilic microorganisms on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars with limestone sand 0/2 and a W:B ratio of 1.5. Bacillus alcalophilus (BA) and Alkalibacillus haloalkaliphilus (AH) were used in this experiment to stimulate the autogenous setting of lime mortars by means of the oxidation of acetate. In order to investigate the influence of ex situ formed carbonate ions on the autogenous setting, microorganisms have been added to the mixtures in their culture medium (Med.) or after centrifugation (C). Autoclaved centrifuged BA (BA A.C.) were included as microbial control series (^* not determined) In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors. -Figure 7 Influence of the use of sand as microbial carrier on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars with limestone sand 0/2 and a W:B ratio of 1.5. Bacillus alcalophilus (BA) and Alkalibacillus haloalkaliphilus (AH) were used in this experiment to stimulate the autogenous setting of lime mortars by means of the oxidation of acetate. The sand was immersed for 24 hours in a fully grown culture of microorganisms to allow the formation of a biofilm on the aggregates. In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors.

-Figure 8 Influence of the type of organic acid on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars with limestone sand 0/2 and a W:B ratio of 1.5. Bacillus alcalophilus (BA) was used in this experiment to stimulate the autogenous setting of lime mortars by means of the oxidation of organic acids. In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors. -Figure 9 Influence of glucose and yeast cells on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars with limestone sand 0/2 and a W:B ratio of 1.5. Saccharomyces cerevisae (SC) was used in this experiment to stimulate the autogenous setting of lime mortars by means of the oxidation of glucose. In order to investigate the influence of ex situ formed carbonate ions on the autogenous setting, microorganisms have been added to the mixtures in their culture medium (SC Med.) or in water (SC H₂O). Autoclaved centrifuged SC (SC A.C.) were included as microbial control series. (^* not determined). In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors. -Figure 10 Influence of the use of sand as microbial carrier on the 28 day compressive strength (CS), flow and carbonation depth (CD) of lime mortars with limestone sand 0/2 and a W:B ratio of 1.5. Saccharomyces cerevisae (SC) was used in this experiment to stimulate the autogenous setting of lime mortars by means of oxidation of glucose. The sand was immersed for 24 hours in an active culture of yeasts to allow the formation of a biofilm on the aggregates. The latter was expected to enhance the survival of the microorganisms in the mortar. In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors. -Figure 11 Overview of the results from the ultrasonic measurements performed on prisms containing 8.3% urea and/or Bacillus sphaericus in culture medium (A and B), Bacillus alcalophilus and/or 25% acetate (C and D) and Saccharomyces cerevisae and/or glucose (E and F). The graphs on the left (A, C and E) present the wave velocities measured at the middle of mortar prisms of different ages. Notice the increase of the wave velocities with time, indicative of the hardening of the mortar. The graphs on the right (B, D and F) present the ratios of the wave velocities measured at the end (V_a) and middle (Vb) of the mortar prisms. Note that values of V_a/V_b around one are indicative of a homogenous hardening of the mortar across the complete length of the prism. Bars with an asterix are significantly different from the reference bars at a given time, i.e. 14, 21 or 28 days: ^**p < 0.05 and^* p<0.01 as determined from independent groups T-tests. In order to allow an easy comparison of the mortar characteristics between mixtures with varying composition, it was decided to use identical ranges on the respective axes. The error bars in the graphs indicate standard errors.

Description of the invention

The present invention relates to the usage of micro-organisms to accelerate carbonation and hardening, and to increase the performance, usability and compressive strength of non-hydraulic lime mortars. It is thus an object of the present invention to provide the use of micro-organisms for accelerating carbonation resulting in an increase in the compressive strength of non-hydraulic lime mortars. The term 'non-hydraulic lime mortars' refers to air-setting mortars, i.e. mortars which rely on the reaction of the binder with CO₂ (from the air) for setting/hardening of the mortar. Examples of such binders are (but not limited to) metal hydroxides (Ca(OH)₂, Mg(OH)₂, Ba(OH)₂...) and metal oxides (CaO, MgO,...). In particular, the present invention relates to mortars which contain at least 20%, 30%, 40%, 50%, 60%, 70%, 80% or 90%, or 100% of such a binder. More specifically, the invention relates to mortars which contain at least 20% 30%, 40%, 50%, 60%, 70%, 80%, or 90%, or 100% of Ca(OH)₂ (lime) . Definitions for the terms 'binder' and 'setting' are given on page 2, lines 14-21 of EP 041 1583.

The invention thus relates to the microbially induced autogenous setting of non-hydraulic lime mortars or air lime mortars. It is also an object of the present invention to provide a non-hydraulic lime mortar comprising micro-organisms capable of accelerating carbonation and increasing the compressive strength of non-hydraulic lime mortars. Although the addition of bacteria to cementitious materials has been investigated by several authors (Ramachandran et al., 2001 ; Ramakrishnan et al., 2001 ; Ghosh et al., 2005; Jonkers et al. 2007 and 2008), micro-organisms have never been used before in lime mortars such as non-hydraulic lime mortars. It is well-known that lime is a disinfectant in which micro-organisms are unable to grow or to be active. Hence, it is surprising that micro-organisms can be used that are capable to induce autogenous setting of air lime via producing CO₂ and accelerating carbonation.

Said acceleration in carbonation results in an increase in compressive strength of at least 1.2 times (i.e. factor 1.2) higher compared to a reference non-hydraulic-lime mortar and is measured at a specific time point after completion of the fabrication of said mortars. Measuring carbonation and compressive strength is well known in the art and can -for example- be measured via determining carbonation depth via spraying phenolphthalein (carbonation depth), via performing strength tests according the European standard EN 1012-1 1 (compressive strength) and by means of ultrasonic waves as proposed by Cazalla et al. (1999) (evolution of setting or hardening of lime due to drying and/or carbonation). Said specific time point after completion of the fabrication of said mortars can be any day after said completion, for example: 3, 7, 14, 21 , 28, 35, 42, 49, 56 days post completion of fabrication of said mortars. Twenty eight (28) days post completion of fabrication of said mortars is a preferred specific time point. Said compressive strength is at least 1.2 times (i.e. factor 1.2) higher compared to a reference non-hydraulic-lime mortar. The terms ' at least 1.2 times higher compared to a reference non-hydraulic-lime mortar' means that when the compressive strength of a non-hydraulic lime mortar fabricated in the absence of micro-organisms (i.e. the reference mortar) is equal to 1 , 2 or 3..., that the compressive strength of the non-hydraulic lime mortar -which is fabricated in the same manner as the reference mortar but whereby micro-organisms are used according to the invention- is at least equal to 1.2, 2.4, 3.6...'At least 1.2 times higher' means 1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.1 , 2.2, 2.3, 2.4, 2.5, 2.6, 2.7, 2.8, 2.9, 3, 3.1 , 3.2, 3.3, 3.4, 3.5, 3.6, 3.7, 3.8, 3.9, 4, 4.1 , 4.2, 4.3, 4.4, 4.5, 4.6, 4.7, 4.8, 4.9, 5, 5.1 ,

5.2, 5.3, 5.4, 5.5 or higher times higher. Hence, the invention relates to the use of microorganisms for accelerating carbonation of non-hydraulic lime mortars resulting in an increase in the compressive strength wherein said compressive strength is at least 1.2 - 5.0 times (i.e. is between 1.2 and at least 5 times) increased compared to a reference non-hydraulic lime mortar.

The term 'micro-organism' refers to an organism that is microscopic and is usually too small to be seen by the naked eye. The term includes bacteria, archaea, fungi, yeast, protista or protozoa such as amoeba, microscopic plants such as green algae, plankton etc. The term 'micro-organism' preferably refers to micro-organisms which are capable to grow and/or to be active under alkaline and/or under anaerobic conditions. An example of an alkaline condition is a soda lake. The term 'micro-organism' more preferably relates to yeasts, alkaliphilic bacteria or ureolytic bacteria. Therefore, it is a preferred object of the present invention to provide the use of micro-organisms which are capable to grow and/or to be active under alkaline and/or under anaerobic conditions, and more preferably to provide the use of yeasts, alkaliphilic bacteria or ureolytic bacteria, for accelerating carbonation of non-hydraulic lime mortars resulting in an increase in the compressive strength of said non-hydraulic lime mortars of at least 1.2 times compared to a reference non-hydraulic lime mortar, and wherein said compressive strength is measured at a specific time point after completion of the fabrication of said mortars. Examples of the latter micro-organisms are Bacillus sphaeήcus, Sporosarcina pasteuήi, Bacillus alcalophilus, Alkalibacillus haloalkaliphilus, Bacillus cohnii and Saccharomyces cerevisiae. The choice of micro-organisms is mainly governed by their ability to resist alkaline conditions and/or their rate of CO₂ production. The micro-organisms of the present invention can be added to the mortar mixture as such (i.e. after the microbial culture solution is removed via centrifugation), as a liquid (i.e. as micro-organisms suspended in microbial culture solutions), or adsorbed on aggregates such as quarry limestone sand, river sand, expanded clay or any other aggregate. Expanded clay is a preferred aggregate and protects the micro-organisms against the adverse conditions of the lime mortar. In addition to aggregates, any material capable of immobilizing or sorbing of microorganisms (e.g. cellulose, silica sol-gel alginate, ceramic materials or any other organic or inorganic material) may be added to the mortar matrix (whether or not loaded with microorganisms). The micro-organisms may form biofilms on the aggregates. Hence, it is an object of the present invention to provide the usage of micro-organisms as stated above, wherein said micro-organisms are immobilized or sorbed on any material or carrier which is capable of immobilizing or sorbing said micro-organisms, wherein said material or carrier is preferably an aggregate, and wherein said aggregate is more preferably expanded clay. Alternatively, said micro-organisms can be firstly (i.e. before they are added to said mortar mixture) encapsulated by compounds such as polysaccharides which will (i.e. once the micro-organisms are within the mortar) be degraded in this alkaline environment. The micro-organisms of the present invention metabolize nutrients outside the mortar (i.e. when residing in microbial culture and before said culture containing the micro-organisms are added to the mortar mixtures) and/or inside the mortar in order to produce CO₂ which results in accelerated carbonation and increased strength of the mortar. In a preferred embodiment, the present invention relates to the use of micro-organisms as stated above, wherein said micro-organisms produce CO₂ within said non-hydraulic lime mortars.

The present invention also envisages the usage of non-metabolizing or dead microorganisms, or a mixture of living and dead micro-organisms for accelerating carbonation of non-hydraulic lime mortars resulting in an increase in the compressive strength of said non-hydraulic lime mortars of at least 1.2 times compared to a reference non-hydraulic lime mortar, and wherein said compressive strength is measured at a specific time point after completion of the fabrication of said mortars. Dead micro-organisms or fragments thereof within said mortars form a frame or act as a nucleation site wherein carbonate crystals can be deposited which results in increased strength of said mortars. In addition, said micro-organisms might also be used as sporulating micro-organisms or as spores. For an increased production of carbon dioxide by the micro-organisms, nutrients might be, together with the micro-organisms, added to the mortar mixtures. A nutrient (also referred to as an 'additive') can be any chemical that said micro-organism requires to live and grow or a substance used in said micro-organism's metabolism which must be taken in from its environment. A non-limiting list of said nutrients includes: urea, yeast extract, NaHCO₃, Na₂CO₃, bacterial nutrient broth (i.e. peptone, meat- and yeast extract, and NaCI), glucose, calciumacetate, calciumlactate, calciumoxalate, calciumtartrate, natriumnitrate (as an electron acceptor), amino acids, sucrose,... It is thus an object of the present invention to provide for the usage of micro-organisms as indicated above, wherein nutrients or additives are added to said micro-organisms. A preferred set of nutrients are/is organic acids, glucose and/or urea. The latter nutrients correspond -with regard to the production of carbon dioxide- to three metabolic pathways: the hydrolysis of urea (eq.1 ), the oxidation of organic acids (eq. 2) and the oxidation of glucose under aerobic (eq. 3) and anaerobic conditions (eq. 4). CO(N H₂)₂ + 2 H₂O → 2 NH₄ ⁺ + CO₃ ^2" eq. 1 CH₂O + O₂ → CO₂ + H₂O eq. 2

C₆H₁₂O₆ + 6O₂ → 6 CO₂ + 6 H₂O eq. 3

C₆H₁₂O₆ → 2 CO₂ + 2 C₂H₅OH eq. 4

It is an object of the present invention to provide a non-hydraulic lime mortar comprising micro-organisms capable of accelerating carbonation and increasing the compressive strength of non-hydraulic lime mortars obtainable by using the micro-organisms as indicated above. Hence, it is an object of the present invention to provide a non-hydraulic lime mortar comprising micro-organisms capable of accelerating carbonation and increasing the compressive strength of non-hydraulic lime mortars and having a compressive strength of at least 1.2 times compared to a reference non-hydraulic lime mortar wherein said compressive strength is measured at a specific time point after completion of the fabrication of said mortars. It is a further object of the present invention to provide a non-hydraulic lime mortar comprising micro-organisms capable to grow and/or to be active under alkaline conditions such as yeasts, alkaliphilic bacteria or ureolytic bacteria, which are possibly adsorbed on a material capable of immobilizing or sorbing microorganisms such as the aggregate expanded clay and which are capable of accelerating carbonation and increasing the compressive strength of non-hydraulic lime mortars and having a compressive strength of at least 1.2 times, or between 1.2 and at least 5.0 times, compared to a reference non-hydraulic lime mortar wherein said compressive strength is measured at a specific time point after completion of the fabrication of said mortars. It is a further object of the present invention to provide the latter non-hydraulic lime mortars wherein nutrients such as organic acids, glucose and/or urea are added. It is a further object of the present invention to provide the latter non- hydraulic lime mortars wherein said micro-organisms produce CO₂ within said non- hydraulic lime mortars.

It is further an object of the present invention to provide a process to fabricate the latter non-hydraulic mortars comprising mixing water with a material capable of immobilizing or sorbing microorganisms, preferably aggregates such as limestone sand, sand or expanded clay, a binder such as Ca(OH)₂, and micro-organisms according to the invention. The following non-limitative examples are given in order to illustrate the present invention Examples

Constituents and micro-organisms -Aggregates and binder

Table 2 gives an overview of the different types of aggregates used. The limestone sand originates from a quarry in Frasnes (Belgium) and was supplied by Carmeuse (Belgium). The sand was delivered with nominal size 0/4 mm. For mortar mixtures prepared with limestone sand with nominal size 0/2 mm, this sand had to be passed through a 2 mm sieve first. The siliceous river sand was provided by Kesteleyn (Belgium) with nominal size 0/5 mm, of which only the fraction 0/2 mm was used for the experiments. The expanded clay was provided by Argex (Belgium). The expanded clay was chosen as a replacement material for sand, because of its high porosity and water retention. Because of its high porosity, this material presents an ideal carrier for microorganisms. The apparent dry density or bulk density of the different types of sand was determined according the Belgian Standard NBN B1 1-206(1981 ).

Table 2 Characteristics of the aggregates

Type Code Shape Nominal size Apparent dry density

(mm/mm) (kg.m^"3)

Quarry limestone sand LS 0/2 Angular 0/2 1639

LS 0/4 Angular 0/4 1510

River sand RS 0/2 Round 0/2 1561

Expanded clay (Argex) AA 0/2 Angular 0/2 570

AR 0/2 Round 0/2 880

AR 0/4 Round 0/4 692

The binder used in this study was Supercalco 90 (Carmeuse, Belgium), i.e. hydrated lime powder. -Micro-organisms

Table 3 gives an overview of the micro-organisms. With exception of Saccharomyces cerevisae (Algist Bruggeman, Belgium), all microorganisms were obtained from the BCCM culture collection (BCCM, Belgium). Four types of alkaliphilic bacteria have been screened for the production of carbon dioxide resulting from the oxidation of organic acids. Among these bacteria, Bacillus alcalophilus and Alkalibacillus haloalkaliphilus were observed to have the highest degradation rate of acetate, and hence, were selected.

Table 3 Overview of the different microorganisms

Metabolic pathway Microorganism Acces. Growth media Growth

Nr.^* (g-L-¹) cond.

Hydrolysis of urea Bacillus LMG Urea: 20 T: 28°C sphaericus 22257 Yeast extract: 20

Sporosarcina LMG Urea: 20 T: 28°C pasteurii 7130 Yeast extract: 20

Oxidation of organic acids Bacillus LMG Nutrient broth: 13 T: 37°C alcalophilus 7120 NaHCO₃: 4.2 pH: 9.7

Na₂CO₃: 5.3

Alkalibacillus LMG Nutrient broth: 13 T: 28°C haloalkaliphilus 17943 NaCI: 50 pH: 9.7

NaHCO₃: 4.2

Na₂CO₃: 5.3

Oxidation of glucose Saccharomyces Glucose: 10 T: 28°C cerevisae

^*Accession number of the BCCM culture collection (Gent, Belgium)

Micro-organisms were added to the mortar mixtures in two different ways, i.e. in solution or adsorbed on the aggregates. The micro-organisms might form biofilms on the aggregates. In order to allow a good adsorption, porous aggregates, i.e. expanded clay, were used.

In case the micro-organisms were added in solution, the amount of tap water required for the production of a workable mortar was replaced by an identical volume of the respective microbial cultures. The microbial cultures were either directly added to the mortar mixture or subjected to a centrifugation procedure prior to application. In this way it was possible to discriminate between the hardening due to carbonate production in situ, resulting from the microbial activity in the mortar, and the ex situ carbonate production, resulting from microbial activity in the culture medium before addition to the mortar. For the centrifugation procedure, the liquid culture was transferred to 50 mL sterile centrifuge tubes (TPP, Switzerland). The tubes were subjected to centrifugation in a Sorvall RC5C Plus centrifuge (Sorvall, USA) for 10 minutes at 7000 rpm. The supernatant was removed and the pellet was resuspended in 50 ml. of physiological saline. Subsequently, the tubes were subjected to another 10 minutes of centrifugation after which the pellet was resuspended in 50 ml. of distilled water containing 2 g.L^"1 yeast extract. The resulting solution was then added to the mortar mixture. Additionally, mortar mixtures were prepared with different concentrations of centrifuged bacteria, in order to investigate the effect of the number of cells, and hence, amount of bacterial activity on the hardening process. Finally, mortar mixtures were also prepared with autoclaved (i.e. killed) centrifuged bacteria. The centrifugation procedure was similar as described above. Instead of physiological saline, however, the bacterial pellets were washed with demineralized water. Furthermore, the pellet obtained after the second centrifugation step was also resuspended in 50 ml. of demineralized water. Next, the contents of several centrifugation tubes (800 ml. in total) were transferred to a 1 L-Schott bottle (Schott Duran, USA) and autoclaved for 20 minutes at 120⁰C.

For the adsorption of microorganisms on sand, both porous (argex) and non-porous (limestone) aggregates have been evaluated. The aggregates were immersed for 24 hours in half a litre of an overnight grown culture of microorganisms. After this period, the culture medium was poured away and the aggregates were wept dry by means of a paper towel. Subsequently, these aggregates were added to the mixture.

- Nutrients (= additives)

Table 4 gives an overview of the nutrients. The nutrients consist of compounds that need to be degraded by micro-organisms first in order for carbonate production to occur.

In order to observe a substantial effect of the nutrient on the carbonation of the lime mortar, it was proposed that at about one fourth of the binder should be converted to calcium carbonate due to the action of the nutrient. For the calculation of the dosage of the nutrients, the following assumptions have been made: (1 ) all carbon (C) atoms of the nutrient are converted to CO3^2" and (2) all CO3^2" ions react with Ca²⁺ from the binder to form CaCO₃. Therefore, the dosage of nutrients was expressed as mol C/mol Ca.

Table 4 Characteristics of the spontaneously dissociating compounds and nutrients

Type of carbonate production Formula M.W. Dosage^a Solubility (g.mol^"1) (g) at

20⁰C

Microbial breakdown required Hydrolysis of urea* Urea (NH₂)₂CO 60.06 14.4^b 1080

43.2

86.4^C

Oxidation of organic acids

Calcium acetate CaC₄H₆O₄ 158.17 28.4 400

Calcium lactate CaCβH-ioOe 308.3 37.0 90 (25°C)

Calcium oxalate CaC₂O₄ 146.12 52.5 0.0067

Calcium tartrate CaC₄H₄O₆ 188.15 33.8 0.37 (0⁰C) Oxidation of glucose

Glucose CβHi₂θ6 180.16 21.6 470

^*a25, ^b8.3 and ^c50% (mol C/mol Ca) for a binder content of 213.1 g lime. ^TFor the enzymatic hydrolysis of urea, jack bean meal was added at a concentration of 20% (w/w) of urea.

The following paragraph illustrates the calculation for a dosage of calcium acetate of 25% on 213.1 g of lime. Given the molecular weight of lime, i.e. 74.09 g.mol^"1, 213.1 g of binder corresponds with 2.876 moles of binder. In order to convert 25% of the binder, 0.719 moles of CO3^2" are required. As one mole of calcium acetate theoretically produces four moles of CO₃ ^2" (Table 4), 0.180 moles of calcium acetate are required. Given the molecular weight of calcium acetate, i.e. 158.17 g.mol^"1, this corresponds with a mass of 28.43 g.

In order to compare microbially induced autogenous setting of lime mortars with chemically induced autogenous setting, ammonium carbamate was added to the mortar mixture in a concentration of 5% (w/w) of the mortar as proposed by Baglioni et al. (1997). This corresponds to a ratio of 48% (mol C/mol Ca) of the binder. Additionally, similar to the other series, ammonium carbamate has also been added to the mortar at a concentration of 25% (mol C/mol Ca) of the binder. The latter corresponds with a ratio of 2.6 % (w/w) of the mortar.

-Mortar composition All mortar mixtures were prepared according to a fixed binder to aggregate ratio (B:A) and a fixed water to binder ratio (W: B), both ratios on weight base. The ratios were derived from a reference mixture with quarry limestone sand 0/2. The B:A ratio was calculated as suggested by Hayen et al. (2001 ), i.e. the amount of binder was determined by means of the bulk density of the hydrated lime and the volumetric porosity of the limestone sand 0/2 skeleton (calculated from the measured bulk density and the absolute density as supplied by the manufacturer). This resulted in a B:A ratio of 0.13 (or 1 :6.69), corresponding to a ratio of 1 :1.54 by volume.

Table 5 Overview of the different reference mixtures, prepared with a B:A of 0.13 and a W:B of 2.

Name Aggregates Binder Water

(g) (g) (g)

Sand Argex^*

Limestone sand 0/2 1639.0 213.1 426.2

80% Limestone sand 0/2 and 20% AR 131 1.4 175.9 193.4 386.8

0/2

80% Limestone sand 0/2 and 20% AA 131 1.4 1 14.4 185.4 370.8

0/2

Limestone sand 0/4 1510.0 196.3 392.6

80% Limestone sand 0/4 and 20% AR 1208.0 138.3 175.0 350.0

0/4

Silica sand 0/2 1560.6 202.9 405.8

^* Replacement of sand by Argex calculated on volume base

The amount of water was determined with the workability of the mortar as criterion. For the mixtures without nutrients, a good workable paste was obtained with a W:B of 2. This corresponded with a flow of about 150 mm. For the mixtures with nutrients, a W:B of 2 was observed to give too fluid mixtures. Therefore, these mixtures were prepared with a W:B of 1.5.

Table 5 gives an overview of the different references mixtures corresponding to different aggregates. For the mixtures prepared with aggregates that have been immersed in microbial cultures, the amount of tap water to be added to the mixture was decreased with the amount of water taken up by the aggregates. Process and characterization -Casting and curing

Fabrication of the mortars occurred in an air conditioned room at 20⁰C. Mortar mixtures were mixed by means of a N50 Hobart mixer (Hobart, USA). For the preparation of the mortar, about 50% of the water was poured in the bowl, followed by half of the aggregates, the binder and nutrients, and then the other half of the aggregates. Mixing was done according to the procedure suggested by Hendrickx et al. (2008), i.e. one and a half minutes of mixing, half a minute of scraping and homogenizing and finally another two minutes of mixing. All mixing was done at low speed (136 rpm). During the first mixing period, the other half of the water was gradually added to the bowl. Subsequently, mortar mixtures were cast in steel moulds containing three compartments with the following dimensions: 40 x 40 x 160 mm³. The mixture was compacted by means of a jolting apparatus according to EN 196-1 :1994. After casting, the mortar prisms were stored at 20⁰C and 65% R. H. until determination of the bending tensile and compressive strength. Mortar prisms were de-moulded after seven days. One series of the Limestone 0/2 sand was subjected to accelerated carbonation. After seven days of storage at 20°C and 65% R. H., the prisms were de-moulded and transferred to a CC>₂-chamber (20⁰C, 65% R. H. and 10% CO₂). -Testing

Flow

Immediately after mixing, the consistency of the fresh mortar was evaluated by means of a flow table according to the European standard EN 495-2. The flow of the mixture (mm) was calculated as the mean of two diameters of the mortar, measured in two directions at right angles to one another. For each mixture, two measurements of the flow were performed.

Ultrasonic measurements

The hardening of lime mortars with time was measured by means of ultrasonic waves, as proposed by Cazalla et al. (1999). According to these authors, the carbonation process is characterized by an increase in the velocity of the longitudinal ultrasonic waves in positive correlation to the degree of compaction.

In this research, the velocity of propagation of ultrasonic pulses was measured and analyzed by means of the FreshCon apparatus and FreshCon2 software, both developed at the University of Stuttgart, Germany (Rheinhardt et al., 2001 ). For each series under investigation, measurements were performed on three prisms. On each specimen, analyses have been performed at two locations, i.e. at one end (location A) and at the middle of the prisms (location B) (-Figure 2). The direction of the measurements corresponds with the direction to which the prisms were afterwards subjected to the load in the compressive strength tests. Strategies used for the autogenous setting of lime mortars were expected to result in a more homogeneous carbonation along the prism, and hence, similar wave velocities at these two locations. Reference prisms, on the other hand, were expected to show a faster carbonation - and hence, higher wave velocities - at the end of the prisms compared to the middle due to the ingress of CO₂ from the side. Therefore, we proposed the ratio of the velocities of the ultrasonic waves at these two locations (V_aA/_b) as a parameter to give an indication of the possible occurrence of autogenous setting. To ensure a good contact between the sensors (2 cm diameter) of the Freshcon apparatus and the mortar prisms, the sensors were moistened with glycerine. Measurements have been performed at an age of the prisms of 14, 21 and 28 days. The reported values for the velocities measured at the end and the middle of the prism, i.e. V_a and Vb represent the average of 15 measurements (three prisms and five successive measurements for each location).

Bending tensile and compressive strength

Strength tests were performed at an age of 28 or 56 days according to the European standard EN 1012-11. The compressive strength given in the figures represents the mean from six measurements.

Determination of carbonation depth

Freshly broken surfaces obtained from mortar specimens previously subjected to compressive strength testing were sprayed with phenolphthalein for the determination of the carbonation depth. Phenolphthalein changes from colourless to purple in case the pH increases from 8.3 to 10. Therefore, phenolphthalein solutions have been used to differentiate the carbonated zone (pH of about 7-8) from the non carbonated zone (pH of about 12.5). The carbonation depth was measured at nine locations of the broken surface. The side opposite to the troweled face was not included for the calculation of the average carbonation depth, as it showed in general a lower carbonation depth. The latter could be attributed to the lower contact with the surrounding air during curing, as specimens were in contact with the support by means of this side. As four broken surfaces were evaluated per series, the values represent the mean of 36 measurements. Results

-Accelerated carbonation

An increase of the compressive strength was observed with an increased duration of curing in the climatized room at 20⁰C and 65% R. H. (-Figure 3). The 28 day compressive strength of specimens that had been cured in the CC>₂-chamber was about 3.3 times higher than that of specimens that had been cured in the climatized room. From phenolphthalein measurements, it could be observed that the complete matrix was carbonated for the limestone prisms that had been cured in the CO₂-chamber.

- Microbial induced autogenous setting

All mixtures containing microorganisms were prepared with a W:B ratio of 1.5. Similarly, the reference series presented in the graphs represent a mixture made with a W:B of 1.5.

- Microbial hydrolysis of urea

Urease producing microorganisms have been added to lime mortar mixtures to stimulate the autogenous setting of lime by means of the microbial hydrolysis of urea. The addition of the Bacillus sphaericus culture medium (BS Med.) resulted in a large increase of the flow compared to the reference mixture (-Figure 4). Furthermore, the addition of the culture medium resulted both in an increase of the compressive strength and carbonation depth. Higher increases in compressive strength, however, were observed in case Bacillus sphaericus had been added to the mixture after centrifugation. The highest strength was observed for a concentration of Bacillus sphaericus of 10⁶ cells. ml.^"1 (i.e. 1.6 times higher than the reference), with a decrease in strength at higher concentrations. No differences could be observed between the compressive strength of mortar prisms treated with different types of ureolytic microorganisms. The mixtures prepared with autoclaved centrifuged bacteria and urea (BS A.C. 10⁸ + 8.3% Urea) showed a lower workability and compressive strength compared to the respective mixtures prepared with centrifuged bacteria and urea (BS C. 10⁸ + 8.3% Urea). This finding indicates the involvement of living and/or intact cells in the autogenous setting of lime mortars. The adsorption of the microorganisms on sand prior to the fabrication of the mortar mixture resulted in prisms with a higher compressive strength compared to the reference prisms (-Figure 5). For mortar prisms containing bacteria adsorbed on sand, the addition of 25% urea resulted in an unchanged or decreased (for prisms containing Bacillus sphaericus) compressive strength. The addition of urea at a concentration of 25% (mol C/mol Ca) on the binder resulted in efflorescence at the outer surface of the prisms. The highest amount of efflorescence was observed at the troweled face of prisms containing 25% urea and 1/5 jack bean meal. Prisms containing urea to which bacteria have been added showed significantly less efflorescence compared to the prisms that contained jack bean meal. The smallest amount of deposits was found on the prisms to which bacteria have been added in culture medium. No or only limited efflorescence was observed on the prisms to which 8.3% urea had been added. -Oxidation of organic acids

The goal of this experiment was to obtain the autogenous setting of lime mortars by means of carbonate production resulting from the oxidation of organic acids. The addition of alkaliphilic bacteria and calcium acetate resulted in a higher flow compared to the reference mixture. Both the addition of alkaliphilic bacteria in their culture medium and the addition after centrifugation resulted in a significant increase of the strength as can be observed from -Figure 6. The highest increase in strength, however, can be observed for the mortar prisms containing calcium acetate. For these prisms, a compressive strength of about 4.5 N. mm^"2 was observed. This is about one and a half times the compressive strength of mortar specimens that have been cured in the CO₂-chamber and 4.5 times that of the reference mixture cured under normal atmospheric Cθ₂-conditions. No additional strength was observed for the combination of bacteria and calcium acetate compared to prisms with calcium acetate alone. On the contrary, for the combination of calcium acetate with bacteria in their culture medium, a decrease of the strength could be observed. The mixtures prepared with autoclaved centrifuged bacteria (AB A.C. 10⁸ and AB A.C. 10⁸ + 25% Acetate) showed a lower workability and compressive strength compared to the mixtures prepared with centrifuged bacteria (AB C. 10⁸ and AB C. 10⁸ + 25% Acetate, respectively). Mortar prisms containing a combination of alkaliphilic bacteria and calcium acetate showed a higher carbonation depth compared to prisms containing either bacteria or calcium acetate alone, the effect being more pronounced with an increased concentration of centrifuged bacteria (-Figure 6). Furthermore, the carbonation depth observed for mixtures prepared with centrifuged bacteria and acetate was much higher compared to the carbonation depth observed for mixtures prepared with autoclaved centrifuged bacteria and acetate. This finding indicates the involvement of living and/or intact cells in the autogenous setting of lime mortars. The use of sand as a carrier of alkaliphilic bacteria resulted in an increased strength compared to the reference mixture (- Figure 7). An additional increase of strength could be observed, in case calcium acetate had also been added to the mixture. The increase of strength, however, was low compared to prisms that only contained calcium acetate (-Figure 6). Because of the fact that an intense efflorescence was observed on the prisms that contained calcium acetate, calcium salts from other organic acids were evaluated regarding their performance in relation to the autogenous setting of lime mortars. With exception of calcium oxalate, all salts resulted in an increased flow (-Figure 8). The use of calcium lactate and calcium oxalate resulted in an increased strength compared to the reference. The increase was, however, low compared to the use of calcium acetate. The use of calcium tartrate, on the other hand, resulted in a decrease of the strength. The combination of Bacillus alkalophilus with calcium oxalate or tartrate did not result in an increased strength compared to the mixtures with only microorganisms. The addition of calcium acetate at concentrations of 25% (mol C/mol Ca) of the binder resulted in the occurrence of crystalline deposits at the outer surface of the prisms. The highest amount of efflorescence was observed for the prisms containing centrifuged bacteria and/or acetate. Limited or no efflorescence was observed on the prisms containing acetate to which bacteria had been added in culture medium or adsorbed on limestone sand. Prisms containing acetate and bacteria adsorbed on expanded clay, on the other hand, showed significant more efflorescence compared to prisms containing bacteria adsorbed on limestone sand. The addition of calcium lactate resulted also in the occurrence of white crystalline deposits on the surface. These crystals, however, appeared to be more firmly attached to the surface compared to the calcium acetate crystals which could be easily removed by wiping. No efflorescence was observed on prisms containing calcium tartrate or calcium oxalate. -Oxidation of glucose

The aim of this experiment was to obtain the autogenous setting of lime mortars by means of carbonate production resulting from the microbial oxidation of glucose. The addition of glucose to the mortar mixture resulted in an increased flow as can be observed in -Figure 9. The addition of yeast cells (SC), on the other hand, did not affect the workability of the mortar mixture. Both the addition of glucose and yeast cells resulted in an increase of the compressive strength. Mortar mixtures prepared with autoclaved centrifuged yeast cells showed a smaller compressive strength compared to mixtures prepared with centrifuged yeast cells. This finding indicates the involvement of living and/or intact cells in the autogenous setting of lime mortars. The use of sand as a carrier for yeast cells resulted in an increased strength compared to the reference mixture (-Figure 10). -Ultrasonic measurements

From -Figure 11 , it can be seen that the addition of microorganisms and/or nutrients resulted in significantly higher wave velocities of ultrasonic pulses compared to the velocities measured for the reference prisms. The highest wave velocities were observed for mortars to which calcium acetate has been added (-Figure 11 C). Most series showed an increased wave velocity with time. The highest increase was observed for the specimens containing Bacillus alcalophilus in their culture medium together with 25% acetate between 14 and 21 days of curing.

The prisms containing Bacillus sphaericus and 8.3% urea (-Figure 11 B), 25% acetate (- Figure 1 1 D) or 25% glucose (-Figure 1 1 F) showed significant lower values of V_aA/b compared to the reference prisms. For these series, the V_aA/_b values were below one during the first 21 days. At an age of 28 days, however, values below 1 were only observed for the prisms containing Bacillus sphaericus (BS) or BS and 8.3% urea.

-Taken together, the addition of micro-organisms, either separately or in combination with nutrients, resulted in an increased workability and strength of non-hydraulic lime mortars: Regarding the addition of micro-organisms, the following observations have been made:

• The compressive strengths of mortar prisms to which bacteria in culture medium have been added were observed to be 1.2-1.8 times higher than those of the reference prisms (i.e. 1.0 N. mm^"2). Prisms containing yeasts, alkaliphilic or ureolytic bacteria in culture medium showed a compressive strength of about 1.4,

1.8 and 1.2 N. mm^"2, respectively. The increase in strength was clearly accompanied by an increase of the carbonation depth, indicating the accelerated conversion of portlandite to calcite.

• The addition of centrifuged bacteria resulted in compressive strengths of about 1.3-1.6 times that of the reference series. The highest compressive strength was observed for a concentration of 10⁶ cells. ml_^"1, i.e. 1.6 N. mm^"2. Prisms containing centrifuged alkaliphilic bacteria or yeasts showed compressive strengths of about 1.5 and 1.3 N. mm^"2. • For yeast cells, alkaliphilic and ureolytic bacteria adsorbed on aggregates, compressive strengths of about 1.2, 1.5 and 1.4 N. mm^"2, respectively, have been observed.

• It should be noted that an increase of the strength by a factor 1.3 after 28 days corresponds with a compressive strength of the reference mortar at an age of 56 days.

• The increase in strength observed for the prisms to which bacteria have been added is in the same range of the strength increase observed for the prisms to which 5% (w/w) ammonium carbamate has been added. This compound had been proposed by Baglioni et al. (1997) for the autogenous setting of lime mortars.

The combination of bacteria and nutrients/additives resulted generally in higher compressive strengths compared to prisms only containing microorganisms. The highest compressive strengths were observed for the combinations of alkaliphilic bacteria and acetate (1.6-4.8 N. mm^"2), followed by the combinations of yeasts and glucose (1.4-1.8 N. mm^"2). The combinations of ureolytic bacteria and urea, on the other hand, resulted in prisms with a compressive strength in the range of 0.8-1.8 N. mm^"2. The addition of glucose or calcium acetate at a concentration of 25% (mol C/mol Ca) of the binder resulted in prisms with a compressive strength of about 2 and 4.6 and N. mm^"2, respectively. The latter being about 1.5 times the strength of prisms that have been subjected to accelerated carbonation in a CO₂-chamber.

The addition of urea and calcium acetate at concentrations of 25% (mol C/ mol Ca) of the binder resulted in the presence of efflorescence on the prisms. Urea concentrations of 8.3%, on the other hand, did not produce efflorescence. Prisms that showed the highest amount of efflorescence were observed to show the highest compressive strength. Glucose, on the other hand, did not produce any deposits on the surface, but resulted in a green discoloration of the surface.

Ultrasonic measurements indicated that the combinations of urea and centrifuged bacteria -where a high compressive strength was observed- show a homogeneous carbonation along the prisms at an age of 14, 21 and 28 days. The combinations of centrifuged bacteria and acetate showed a high carbonation depth. References

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ANOVA interaction analysis and ultrasonic testing to evaluate air lime mortars used in cultural heritage conservation projects. Cement and Concrete Research, 29(11 ), 1749-

1752. -Elert, K.,Rodriguez-Navarro, C.,Pardo, E. S..Hansen, E. and Cazalla, O., 2002. Lime

Mortars for the Conservation of Historic Buildings. Studies in Conservation, 47(1 ), 62-75.

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Brick and Block Conference, Sydney, Australia.

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Claims

1. Use of micro-organisms for accelerating carbonation of non-hydraulic lime mortars resulting in an increase in the compressive strength of said non-hydraulic lime mortars of at least 1.2 times compared to a reference non-hydraulic lime mortar, and wherein said compressive strength is measured at a specific time point after completion of the fabrication of said mortars.

2. Use according to claim 1 wherein said compressive strength is at least 1.2 - 5.0 times increased compared to a reference non-hydraulic lime mortar.

3. Use according to claims 1 or 2, wherein said micro-organisms are capable to grow and/or to be active under alkaline conditions.

4. Use according to claims 1 , 2 or 3, wherein said micro-organisms are yeasts, alkaliphilic bacteria or ureolytic bacteria.

5. Use according to claims 1 to 4, wherein said micro-organisms are adsorbed on a material capable of immobilizing or sorbing said microorganisms.

6. Use according to claim 5, wherein said material is an aggregate.

7. Use according to claim 6 wherein said aggregate is expanded clay.

8. Use according to claims 1 to 7, wherein nutrients are added to said micro-organisms.

9. Use according to claim 8, wherein said nutrient(s) are/is organic acids, glucose and/or urea.

10. Use according to claim 1-9, wherein said micro-organisms produce CO₂ within said non-hydraulic lime mortars.

1 1. A non-hydraulic lime mortar comprising micro-organisms capable of accelerating carbonation and increasing the compressive strength of non-hydraulic lime mortars according to claim 1-10.

12. A process to fabricate a non-hydraulic mortar according to claim 1 1 comprising mixing water with:

- a material capable of immobilizing or sorbing microorganisms such as limestone sand, sand or expanded clay, -Ca(OH)₂, and -micro-organisms.