FIELD OF THE INVENTION

The present invention relates to an aqueous mixture or grout to be used in the preparation of a particular type of autoclaved aerated cement and to the relative process of preparation of said particular type of cement.

STATE OF THE ART

Autoclaved aerated cement (AAC) is a prefabricated cementitious material used in the building industry, consisting of a mixture of Portland cement, lime, water, silica sand powder and/or industrial waste (silica fumes, fly ash and slag), metallic aluminium powder. The proportions of the components vary according to the final density of the product.

The following table 1 summarizes the main characteristics and properties.

Table 1 - Properties of autoclaved aerated cement

Pln si eal para met rs Val ue I nit

( ^" om pressix e St rciigl li MPa

Density 300 ÷ 800 Kg/m3

Hew m a I Strengt h 0.5 ÷ 1.25 MPa

Sial ic l;is( icit Mod ul us 1.1 ÷ 3 KN/mm2

The AAC production process can be summarized as follows: the silica sand is finely ground and mixed with cement, lime and water, until a very fluid mixture is obtained, a semi-liquid "grout", to which a minimum quantity of expanding and leavening agent is added. Subsequently, the grout is poured into suitable moulds, where it expands at a controlled temperature. Hydrogen bubbles form inside the material, thus creating those micro porosities that constitute the salient characteristic of the cellular concrete. This process lasts a few hours and at the end the cellular concrete mass is sufficiently solid and stable to be deformed and cut into individual elements, subsequently sent to the autoclave for stabilization and maturation. Here the elements stay for twelve hours at a pressure of about 12 atmospheres and at a temperature of about 200°C. At the exit of the autoclave, the control operation on the product quality and integrity is carried out, in order to evaluate the correspondence to the preset quality standards.

Figure 1 summarizes the steps previously described.

At present, the aluminium metal powder (Al) is used as the only leavening agent in commercial products. By adding it to the cement grout, it determines a chemical reaction whose direct consequence is the development of molecular hydrogen (H ₂ ), responsible for the specific porous structure of AAC. In detail, the aluminium powder reacting with the calcium hydroxide in an alkaline environment (pH > 12) and in the presence of water, due to the oxidation of the metal, releases molecular hydrogen according to the following reaction:

2A1 + 3Ca(OH) ₂ + 6H ₂ 0 = 3CaOAl ₂ 0 ₃ -6H ₂ 0 + 3H ₂ †(g) (1)

The reaction forms a tri-calcium hexahydrate aluminate and molecular hydrogen, as a result of which bubbles form inside the mixture. The action of the aerating medium produces an increase in the volume of the material from 1.5 to 5 times higher than the original volume of the cement grout. The leavening process stops when the material reaches a solid consistency: the voids stabilize in the volume of material and the contained hydrogen is gradually released into the atmosphere, replaced by air. During the subsequent autoclaving, at a pressure of 10 bar in saturated steam conditions for a period of 8-12 hours, the lime, reacting with water and silica, forms a hydrated lime silicate (CSH),

Ca(OH) ₂ + Si0 ₂ + xH ₂ 0 = CaOSi0 ₂ -xH ₂ 0 (CSH) (2) called by Heddle in 1880 "Tobermorite", Ca5Si ₆ Oi6(OH) ₂ 4(H ₂ 0), a crystalline material that gives the product a better mechanical resistance and makes it more stable than that formed at room temperature.

The described process, developed at the beginning of the last century, as better specified later in the section dedicated to "relevant documents", has various problems, including the one relating to the use of aluminium powder as the raw material of the final product. In fact, aluminium powders can react quite easily with oxygen at room temperature: the smaller its particles (less than 100 μπι), the greater the risk of explosion or fire. Please note how in the AAC production process the leavening phase is favoured and controllable starting from aluminium particles having very small dimensions. Moreover, the larger particles are not less dangerous, since their combustion speed is greater. These powders are rarely uniform, since it is very likely to find aluminium particles of a size considered "safe" mixed with particles smaller than 100 μπι. As a result, a high risk of explosion is achieved together with a high combustion speed.

Furthermore, particular attention must be paid to the percentage of oxygen present in the production plant, which, if included between 10% and 15%, can favour the propagation of the fire, always in the presence of aluminium powder.

The search for an aerating agent that avoids all these problems is therefore of high relevance for the producers who, moreover, could also reduce the safety-related costs.

In the last 5 years, it has been thought to use hydrogen peroxide (H ₂ 0 ₂ ) instead of aluminium in association with foaming agents and metal salts, such as potassium permanganate and magnesium chloride.

Hydrogen peroxide is in fact a molecule characterized by the oxygen-oxygen bond, and for this reason it is a highly reactive and unstable molecule. H2O2 is available on the market in a low concentration water solution of 2-3% for sanitary use and of 30, 70 or 98% for industrial use.

CN 105503105 describes in fact a process for preparing an aerated cement formed from anhydrous materials and auxiliary materials, in which the anhydrous materials contain from 40 to 70% by weight of Yellow sand river, from 20 to 40% of Portland cement, from 1 to 7% of special cement and from 1 to 10% by weight of FGD gypsum, a synthetic product derived from exhaust gas desulphurisation systems at power stations, and the remaining part to 100 of waste materials; auxiliary materials include hydrogen peroxide, potassium permanganate, a foam stabilizer, a substance that can reduce water.

The advantage of this process would lie in the use of the cheap Yellow river sand instead of pure lime.

WO2011/1355083 describes a process for preparing aerated cement, which can also be autoclaved, comprising the following steps:

a. preparing a composition comprising at least water, a cementitious material, calcium oxide, a composite comprising reactive silica, a source of oxygen, gypsum and a mixture selected from sodium carbonate, sodium bicarbonate and sodium hydroxide,

b. pouring the mixture obtained in step a. in a mould and allowing the mixture to harden, forming a rigid body,

c. removing the rigid body from the mould,

d. possibly cutting or moulding the rigid body from step c.

e. maturing the cement.

Preferably, hydrogen peroxide, sodium percarbonate, sodium perborate, magnesium peroxide and relative mixtures are used as the source of oxygen.

The oxides and the salts of transition metals such as Mn, Fe, Cu, Co, Pd, as well as enzymes such as catalase are considered as catalysts.

SUMMARY OF THE INVENTION

The Applicant has found that, by employing a bio-organism containing catalase such as for example a commercial yeast, in particular Saccharomyces cerevisiae and hydrogen peroxide as aerating agents directly in the cement mixture, it is possible to obtain an autoclaved aerated cement, which does not require adding lime, gypsum, carbonates, alkaline or alkaline earth metal bicarbonates, alkali metal or alkaline earth metal hydroxides.

An object of the present invention is therefore an aqueous mixture or grout containing

a) cementitious material,

b) a source of silica such as quartz,

c) an aerating agent,

in which

i) said aerating agent c) consists of hydrogen peroxide and of a bio-organism containing the enzyme catalase

ii) said aqueous mixture does not require the addition of lime, of alkaline and alkaline earth hydroxides and of calcium sulphate, in hydrated or non-hydrated form.

The present invention also relates to:

• the use of said aqueous mixture in the preparation process of this particular type of autoclaved aerated cement, defined as bioaerated autoclaved cement or BAAC

• the preparation process of said autoclaved bioaerated cement, which comprises the following steps: A) preparing the aqueous mixture or grout, which is the object of the present invention;

B) placing the resulting grout inside a mould, where it remains for 2 to 4 hours, preferably 3 hours, at room temperature for the completion of the cement setting or consolidation phase;

C) removing the cement from the mould after the setting phase of step B);

D) placing the material from step C) in an autoclave at a temperature of less than or equal to 200°C, preferably at 190°C, at a pressure of between 10 and 14 atmospheres, preferably at 12 atmospheres, for a time of between 10 and 15 hours preferably 12 hours, to obtain the maturation of the bioaerated cement.

DESCRIPTION OF THE FIGURES

Figure 1 schematically shows the production cycle of autoclaved aerated cement (extracted from www.Ytong.it).

Figure 2 is a representative photographic image of the formation of oxygen bubbles deriving from the combination of hydrogen peroxide and Saccharomyces cerevisiae yeast, in an alkaline environment and in the presence of cementitious materials in the preparation phase of the BAAC, following the operating conditions described in the Example 1.

Figure 3 is a photographic image of the specimen obtained from the composition of Figure 2 obtained from the grout prepared as described in Example 1, after setting in a mould and maturation thereof in an autoclave.

Figure 4 is a photographic image of a series of specimens made in plastic containers containing the same amount of the same grout but increasing percentages of hydrogen peroxide as described in Example 2.

Figure 5 shows the geometrical scheme of the mould and of the height increase of the cement mortar inside it, as described in Example 2.

Figure 6 shows in graph form the degree of aeration according to the amount by weight of hydrogen peroxide (at an initial volumetric concentration of 35%).

Figure 7 is an optical microscope (OLYMPUS 10X) image of macropores in a BAAC specimen prepared with an excess of H2O2 equal to 2.2% by weight of the grout at the initial volumetric concentration of 35% as described in Example 2.

Figure 8 is an image referring to the components, which are contained in the aqueous mixture besides water according to the present invention and are employed in Example 3.

Figure 9 is an image of the steel mould having standard dimensions ΙΟ χ lO x 10 cm, used for the manufacture of the specimens, with the method according to the present invention. The profile of the upper surface of the grout highlights the convex shape achieved as a result of the grout leavening process as described in Example 4.

Figure 10 is a schematic representation of the flow sheet of the process according to the present invention for obtaining the bioaerated autoclaved cement according to the present invention and according to the preferred operating conditions described in Example 4.

Figure 11 is a photographic image of the BAAC specimen after passing in autoclave (11a) with the process according to the present invention, following the operating conditions described in Example 4, in comparison with the photographic image of a commercial AAC specimen (l ib) with the same density, obtained by means of a conventional process.

Figure 12 is an electron microscope photographic image with 10X magnification of a commercial specimen AAC (12a), and of an experimental BAAC specimen (12b) prepared as described in Example 4. Figure 13 represents a graph of the breakage behaviour by compression of BAAC prepared as described in Example 4.

DETAILED DESCRIPTION OF THE INVENTION

For the purposes of the present invention, the expression "comprising/containing one or more components" does not exclude the presence of further components in addition to one or more explicitly listed. For the purposes of the present invention, the expression for which an object "consists or is formed or composed of one or more components" means that the presence of additional components in addition to the one or more listed components is excluded in the object.

For the purposes of the present invention, catalase-containing bio-organism means a single-cell living organism, such as for example yeast, preferably an Ascomycete fungus of the genus Saccharomyces, more preferably Saccharomyces cerevisiae. For the purposes of the present invention, bioaerated autoclaved cement means an autoclaved aerated cement in which the swelling of the cement occurs by means of a dismutation reaction of hydrogen peroxide, whose overall reaction is as follows

2H ₂ 0 ₂ →0 ₂ +2H ₂ 0 (I) (3)

that is the resulting reaction of the following intermediate oxidation-reduction reactions:

reduction: H ₂ 0 ₂ ^ [0]+H ₂ 0

oxidation: H ₂ 0 ₂ +[0] - 0 ₂ +H ₂ 0

In the case described in the present invention, the dismutation reaction of hydrogen peroxide is catalysed by enzymes present in the yeast Saccharomyces cerevisiae of the catalase type. Catalase is an enzyme synthesized by most living organ cells to control the harmful effects of oxidation of biomolecules due to aerobic cell metabolism or exposure to exogenous oxidizing species.

In nature there are different forms of catalase that differ in structure, sequence, composition of the catalytic centre, but all have in common the fact that they catalyse the aforementioned oxidation-reductive dismutation reaction of hydrogen peroxide. Most of the catalases are composed of a protein tetramer coordinated by as many iron-containing porphyrin heme groups, which constitute the catalytic reaction centre capable of reacting with hydrogen peroxide.

In detail, the two steps of the dismutation reaction promoted by the heme centre are: H ₂ 0 ₂ + Fe(II)-E→ H ₂ 0 + 0=Fe(III)-E

H2O2 + 0=Fe(III)-E→ H ₂ 0 + Fe(II)-E + 0 ₂

wherein Fe-E represents the ferrous centre of the heme group of the enzyme. An oxygen atom binds an iron atom of the ferrous heme core and releases a water molecule. In the next stage of the reaction, the ferric iron binds another peroxide molecule, thus releasing another molecule of water and molecular oxygen.

The reaction is strongly exoenergetic and shows an enthalpy of decomposition of hydrogen peroxide at 25°C equal to:

(ΔΗ° = -47 kcal/mol; AS° = +34 kcal/mol K; AG° = -57 kcal/mol)

Catalase enzymatic activity allows decomposing millions of hydrogen peroxide molecules per second and the optimal pH range varies from 4 to 1 1 depending on the species (Switala J, Loewen PC. Diversity of properties among catalases. Arch Biochem Biophys. 2002 May 15;401(2): 145-54). The aqueous mixture object of the present invention exploits precisely the catalase's ability to speed up the dismutation reaction of the hydrogen peroxide molecule in water and molecular oxygen.

Among the various existing catalases, one is present in yeast cells and in particular in the species Saccharomyces cerevisiae (the common brewer's yeast), widely used in bread making due to its high enzymatic activity. The genus Saccharomyces, belonging to the mushroom kingdom, is a well-known species of yeast from the Saccharomycetaceae family that reproduces by budding and is in fact used in most of the industrial fermentations for the production of wine, bread, and beer. In yeast cells there are two forms of the enzyme catalase (Traczyk Aleksandra, Bilinski Tomasz, Litwinska Jadwiga, Skoneczny Marek and Rytka, Joanna, Catalase T deficient mutants of Saccharomyces cerevisiae, Acta Microbiologica Polonica, February 1985, vol. 34, no. 3-4, pp. 231-241), designated as catalase A, confined within the peroxisomes, and catalase T, cytoplasmic (Seah Tony CM, Bhatti A and Kaplan Gordin J. Novel catalytic proteins of baker's yeast, catalase, Canadian Journal of Biochemistry, November 1973, Volume 51, No. 11, pp. 1551-1555.), encoded by two different genes, CTA1 and CTT1, respectively. These two catalases have a different molecular weight equal to 170-190 kD for the catalase A and 225-250 kD for the catalase T. Other classes of enzymatic molecules in yeast cells contribute to the degradation of hydrogen peroxide: among these, cytochrome c peroxidase, which is localized in the mitochondria. In the case of the catalysts of Saccharomyces cerevisiae the optimal pH with which the highest activity is obtained ranges from 6.0 to 8.0 and has a specific activity among the highest among all the species of catalase: 116100 (Switala J, Loewen PC: Diversity of properties among catalases, Arch Biochem Biophys, 2002 May 15, 401 (2): 145-54). Saccharomyces cerevisiae cells are readily available on the market as a fresh product in the form of pressed cakes or as dry yeast (shelf life of about 1 year).

The Applicant has also found that the bioaerated autoclaved cement obtained by the process of the invention and which uses as starting material the aqueous mixture or grout of the present invention is characterized by a lower final product density than that of an autoclaved aerated cement obtained by the known art, which uses metallic aluminium powders as an aerating agent, with the same ratio water/solids 0.7. As an indication, the technical characteristics (average compressive strength and dry thermal conductivity) of the BAAC produced in the laboratory are completely in line with those of AAC commercial products with density values similar to the one of BAAC.

Table 2 shows the characteristics of the materials used in the aqueous mixture object of the present invention for the preparation of the BAAC with a final density of 500 kg/m ³ .

Table 2

By way of comparison, Table 3 below shows the characteristics of the components used for the laboratory preparation of a "traditional" AAC specimen having the same water/solids ratio of 0.7, but with a final density of 600 kg/m ³ . Table 3

The function of gypsum in commercial autoclaved aerated cement is twofold: a) supporting the dough during leavening so that it does not collapse under its weight; b) giving the product a greater resistance to compression.

In the case of the BAAC obtained with the aqueous mixture and with the process object of the present invention, the two manufacturing conditions (with and without gypsum) have been experimentally verified with the same final density of 500 kg/m ³ . In particular: a) no phenomenon of collapse of the cement mixture was observed during the leavening of the gaseous-free BAAC; b) the compressive strength tests of the gypsum-free specimens gave the same and in some cases better results than those related to the specimens containing gypsum.

In this regard, it can therefore be argued that the use of gypsum in the cement mixture of BAAC is not necessary for the purpose for which it is instead used in commercial autoclaved aerated cement.

The Applicant has furthermore found that the BAAC cement does not even require the presence of alkaline or alkaline earth metal carbonates, nor even of alkaline or alkaline earth metal hydroxides, so it cannot be traced back to WO2011/1355083. The cementitious material used in the aqueous mixture object of the present invention can be selected from:

al : the following cement classes according to the UNI EN 197-1 2001 :

• Portland cement type I: pure Portland cement consisting of a percentage of Portland cement clinker of at least 95%.

• Portland cement type II: mixed Portland cement, since the constituent present in a greater percentage is represented by the Portland cement clinker combined with one or more mineral additions.

• High-grade cement type III: Portland cement clinker and granulated blast furnace slag added in percentages above 35%, divided into three subtypes, each identified by the letter A, B or C (immediately after the Roman numeral) which identifies the percentage by which the slag is present in the included cement, respectively, in the ranges 36-65%, 66-80% or 81-95%.

• Pozzolanic cement type IV: obtained by mixing the Portland cement clinker with micro silica, natural and natural calcined Pozzolan and fly ash, divided into two subtypes that differ in the percentage of pozzolanic additions, which are variable in the 11-35 %> and 36-55%> ranges, respectively identified respectively by the letters A or B (after the Roman numeral IV).

• Composite cement type V: consisting of a mixture of Portland cement clinker, blast furnace slag, and Pozzolan and/or silica fly ash, articulated in the two subtypes in which the percentage of the slag, on the one hand, and of the Pozzolans and the ash, on the other hand, must be included in the range 18-30%> (type V/A) or 31-50% (type V/B) or:

a-2: special hydraulic binders based on sulphoaluminate clinker with up to 40% by weight of cementitious material. The sand used for making the BAAC, and more commonly also for making the AAC, contains at least 80% of silica (S1O2), more preferably 90%, and in particularly preferred embodiments at least 95% of silica. For example, the use of 99.2% by weight of quartz, finely ground, finds its justification in improving the performance of the final product as well as in increasing the crystallinity of calcium silicate hydrate (CSH), which forms in an autoclave. Although in principle other polymorphous crystalline forms can be used in the aqueous mixture object of the present invention, such as for example natural and/or synthetic zeolites for the preparation of autoclaved aerated cements, the relative products have turned out to be tendentially worse in terms of mechanical performance, (ref : C. Karakurt, H. Kurama, IB Topcu, Utilization of natural zeolite in aerated cement production - Cement & Cement Composites 32 (2010) - M. Albayrak, A. Yorukoglu, S. Karahan, S. Atlian, HY Aruntas, I. Gurgin Influence of zeolite additive on properties of autoclaved aerated cement- Building and Environment 412 (2007) pp. 3161-3165) Furthermore, even if the use of a sand less rich in silica, such as sand from fly ash or amorphous silica (e.g. fumed silica or precipitated silica) allows obtaining breaking strength values that are even higher than those relative to quartz-containing specimen, its practical use is made very difficult due to the high shrinkage during maturing (ref.: B. Straube, H. Walther, AAC low thermal conductivity httpsJ/www.xella om/^ /Straube B.,

Walther H.: AAC with low thermal conductivity, page 399- 404, Bydgoszcz 2011. The hydrogen peroxide titrated at 35% is added to water to prepare the aqueous mixture of the present invention in concentrations ranging from 30% to 35% by volume. The aqueous mixture object of the present invention is preferably prepared by means of a process comprising the following steps:

A. l) the silica and the cementitious material are placed in an appropriate container and dry mixed for 1 minute;

A.2) hot water is added to the cementitious material in water/solid weight ratios comprised between 0.5 and 0.8, preferably 0.7, at a temperature comprised between 35°C and 45°C, preferably at 40°C.

The resulting mixture is mixed first for 3 minutes by hand and then for 5 minutes with an electric stirrer;

A.3) the yeast is dissolved in the included water;

A.4) 35% hydrogen peroxide is added, in amounts ranging from 1 to 7% by weight of water, preferably 2.1% by weight based on the total weight of the grout water.

Preferably, in step A-2, the total water is added in water/solid weight ratios comprised between 0.6 and 0.8, preferably 0.7.

The following examples of embodiments of the present inventions are given for illustrative but not limitative purposes.

EXAMPLE 1

A fundamental and characterizing aspect of the BAAC invention is the replacement of the conventional aerating medium (aluminium powder) with the enzyme catalase of organic origin (present in almost all the cells and in particular amounts in the cells of the yeast Saccharomyces cerevisiae) and the hydrogen peroxide. The catalase enzyme catalyses a dismutation reaction of hydrogen peroxide, as a result of which molecular oxygen and water reaction products are obtained. If the reaction is carried out inside the cement mortar, the released oxygen will form the porosities that will increase the volume of the cementitious mass with the consequent reduction of the final density of the product. Figure 3 shows the stereoscopic microscope image of the porosity obtained in a preliminary feasibility test. The test was carried out using only commercial cement (without adding aggregates and lime) to which the fresh yeast dissolved in water at a concentration of 10 g/L was mixed, and the hydrogen peroxide was subsequently added (titrated at 35%), in a ratio of 1 : 15 if compared to solution water. The entire compound was then further mixed to allow a uniform distribution of the parts. Already during the mixing phase, it was possible to observe the formation of bubbles (Figure 2) with a consequent leavening of the mixture. Once verified the feasibility, further experimental tests have been carried out to optimize the physical parameters of density, homogeneity of distribution and pore size of the BAAC object of the present invention.

EXAMPLE 2

Leavening: the leavening was evaluated by measuring the increase in height of specimens prepared with the same formulation of the materials composing the grout (cement, silica, water and yeast) to which different percentages of hydrogen peroxide weight were added.

200 grams of grout containing the same quantity of yeast equal to 0.58 g were poured into the cylindrical specimen containers with a volume of 500 ml (Fig. 4), then variable amounts of hydrogen peroxide titrated at 35% were mixed to the mixture. The height of the grout inside the container has been determined both in the contact surface with the container and in the central surface, before the addition of hydrogen peroxide and at the end of the leavening process, as shown in Figure 5, in which:

• initial height (HI) on the wall before the addition of H2O2;

• final height (H2) on the wall and at the end of the leavening;

· final height (H3), evaluated at the centre and at the end of the reaction. The results are summarized in the graph of Figure 6, where each point (blue triangle the wall values, red circle the values at the centre) represents the arithmetic mean of three experimental tests.

The percentage increase in wall height is calculated with the formula (H2- H1)/H1 * 100, whereas the one in the centre is defined as (H3-H1)/H1 * 100 and is shown in the ordinate. The percentage by weight of H2O2 at 35% is shown in the abscissa compared to the total weight of the used components.

As the value of hydrogen peroxide increases, it is observed an increase in the height of the specimen, then in the porosity, and a decrease in the final density.

The maximum value of the curve is obtained for 2.2% by weight of hydrogen peroxide corresponding to a 115% increase in height, that is to a final double volume compared to the initial one.

However, this high concentration of hydrogen peroxide resulted in a non- homogeneous pore distribution within the specimens, with the presence of randomly distributed macro cavities, as shown in Figure 7 obtained by means of an optical microscopy.

EXAMPLE 2-A

200 g of grout were poured into cylindrical containers having a volume of 500 ml

(cement component selected among the following classes: I, III, IV, V or sulphoaluminates, replacing up to 40% of the cement component containing the same amount of yeast used in Example 2 equal to 0.58 g, and using a quantity of hydrogen peroxide equal to 2.2% calculated as in Example 2.

The results obtained are summarized in the following Table 4.

The tests show small percentage variations in the height increase of the considered mixture by varying the used cement component. Table 4

EXAMPLE 3

The aqueous mixture or grout for the preparation of the BAAC requires the presence of water, cementitious material, quartz sand, yeast and hydrogen peroxide as shown in Figure 8.

Unlike the aqueous mixture used in the production of AAC, in the one developed for the BAAC it was not necessary to use lime to give the product the same density value. In fact, as shown in the reaction (1), the lime added in the commercial AAC is necessary for the production of molecular hydrogen, essential for the leavening of the grout. Otherwise, in the new product, the process of aeration depends on the reaction of dismutation of hydrogen peroxide by means of the enzyme catalase, following which molecular oxygen is produced, regardless of the presence of lime. The absence of the lime added in the aqueous mixture object of the invention gives the BAAC economic advantages and a lower environmental impact.

EXAMPLE 4

The particularly preferred operating conditions for carrying out the process according to the present invention are shown below, using the aqueous mixture object of the present invention to obtain the BAAC. The process developed in the laboratory for the preparation of the BAAC was carried out as follows: 120 parts by weight of sand, 80 of cement, 140 of water and 3 of hydrogen peroxide at 35% for a part by weight of yeast). This process, carried out for all the specimens, which were then subjected to subsequent characterizations, comprises the following steps:

1. sand and cement are placed in an appropriate container and dry mixed for 1 minute;

2. 90% of the total amount of water required for the mixture at a temperature of 40°C is added to the solids and slowly mixed by hand;

3. the yeast dissolved in the remaining 10% of water required for the mixture is added and further mixed for about one minute;

4. hydrogen peroxide is added and mixed by hand for 30 seconds;

5. the resulting grout is placed inside the mould, where it remains for 3 hours at room temperature for leavening and completion of the setting phase. In this passage, cubic geometry formwork was used, such as those shown in Figure 9, containing 0.5 kg of dry weight of the solid materials composing the grout.

6. The permutation reaction of the H2O2 present in the grout takes place at room temperature and is completed within 15-20 minutes after the addition of H2O2 with a consequent volume increase of the manufactured article of about 80% if compared to the initial one.

7. Once the cement-setting phase has taken place, the specimen assumes a consistency sufficient to be removed from the mould and is placed in an autoclave where it is brought to a temperature of 190°C at 12 atmospheres in saturated steam conditions for a total time of 12 hours. 8. At the end of this step, the specimen is dried in an oven at 60°C until completely dried.

The passage in the oven at 60°C is not strictly necessary for the process, but only for accelerating the drying times for the subsequent characterization of the physical and mechanical parameters of the specimens.

Figure 11 shows how the porosity of the BAAC specimen obtained from the reaction between organic catalase and H2O2 (l ib) is completely comparable to the one of the commercial specimen (1 la) in terms of distribution and size of porosities.

Figure 12, obtained by means of an optical microscope with 10X magnification, shows a visual comparison of the porosities of the commercial AAC specimen (12a), respectively, and of the experimental BAAC specimen (12b).

The following table shows the values of the measurements of the physical and mechanical parameters of the BAAC product compared to those of the AAC product. Table 5 - Physical parameters of the BAAC product vs the AAC product

The data show that the compressive strength of the two types of specimen is the same, but with the same water/solids ratio of 0.7, the densities are different: lower for BAAC and higher for AAC. This means that the BAAC with the same mechanical performance (compressive strength) has a lower density and a greater insulating power (thermal conductivity λ) and therefore, in general, is more advantageous from a commercial point of view.

The following Figure 13 shows an exemplificative diagram of compressive stress vs deformation, which is qualitatively the same for all the tested BAAC specimens. The trend of the curve is typical of a material that reaches the maximum breaking load without plastic deformations.

The following Table 5. a instead shows the physical parameters related to density, compressive strength and dry thermal conductivity obtained from the technical data sheets of some AAC commercial products.

The simple comparison with previous BAAC data with a density of 500 kg/m ³ , but obtained in the laboratory, shows its technical validity.

Table 5a - physical parameters of some AAC commercial products

The BAAC as described above can be used advantageously in the building construction fields as an alternative to AAC products currently on the market.

Its strengths can be summarized as follows:

■ Safety: • The modest concentration of H2O2 in use and the complete absence of pathogenic elements of the yeast suggest a safe use in terms of bio-safety both in working environments and for end users.

• The production of AAC is based on the use of aluminium powder as an aerating agent, whose storage and handling require stringent safety rules to contain the risk factors due to its high reactivity with oxygen even at room temperature. As known, in fact, aluminium dust, when exposed to the air, can form the AI2O3 oxide (alumina) with a strongly exothermic reaction that, if not controlled, can cause fires with potentially explosive effects. All operations concerning the storage, handling and manipulation of the material must be carried out in compliance with the safety conditions set out in the ATEX guidelines (ATmospheres EXploisible) with consequent increase in management costs related to safety.

■ Physical properties

• The tests carried out in the laboratory have shown that, with the same ratio of water-solids of 0.7, BAAC has a density lower than that of AAC and absolute values of mechanical resistance substantially similar in numerical terms but relatively better in BAAC if considering the lower density.

· The lower density obtained in the BAAC cement specimens compared to the AAC ones determines better performance in terms of thermal insulation, the creation of lighter products as required in the earthquake-resistant building and finally, lower transport costs per volume unit.

• Another advantage in terms of costs and sustainability of the production process attributable to BAAC concerns the leavening phase that occurs at room temperature in contrast to the similar process that takes place in commercial AAC where it is necessary to maintain the grout at a controlled temperature variable between 40°C and 80°C for the whole leavening process.

■ Environmental sustainability

· The elimination of aluminium powder from the BAAC manufacturing process and its replacement with H2O2 and a microorganism is highly advantageous in terms of environmental impact.

• The production of yeast in the Saccharomyces cerevisiae species occurs in bioreactors fed mainly with waste materials from the food processing or agro- forestry industry (in general biomass with high carbohydrate content) and then inserted into the reuse cycle of the raw materials used in other industrial processes, mainly those based on fermentation. Furthermore, as regards production that does not require a food grade, yeast production and final product control processes can still be reduced considerably.

■ Economic aspect

A final consideration must be made in relation to the economic aspect, i.e. the production costs resulting from the cost of raw materials used in the manufacture of AAC and BAAC. Given the market prices of the components and their relative amounts necessary for the manufacture of AAC and BAAC, the total production costs considered gross of the production energy give BAAC an economic advantage.