The European Green Deal [1] sets the goal of making the EU the world's first climate-neutral zone by 2050 by consolidating a new awareness of environmental issues caused by climate change, drawing attention to the urgent need to reduce CO₂ emissions [2]. Consequently, the European Commission has adopted a series of proposals to transform EU climate, energy, transport and taxation policies and to reduce net greenhouse gas emissions by at least 55% by 2030, compared to 1990 levels.

It is well known how the global construction sector heavily impacts on the environment given the great carbon footprint of traditional materials such as cement, clay, steel, etc. The data show that the construction sector contributes 5 - 12% of the total greenhouse gas emissions, considering the entire supply chain from the production processes of building materials to the construction of new buildings and/or renovation of old ones and their operation (heating and cooling are responsible for consuming about 40% of the EU's energy consumption) and finally the management of waste related to the construction sector. Therefore, to follow up on policies to mitigate the climate impact due to the industrial construction sector, it is essential to act on each of the production segments in order to increase their degree of sustainability.

 

In this context, autoclaved aerated concrete (AAC) is an interesting solution that is already widely used in building construction, due to its main intrinsic characteristic of having a low density and the ability to guarantee excellent thermal and acoustic insulation that contribute to improving the building's environmental qualities.

AAC was first developed in Sweden in the 1920s by architect Axel Eriksson who patented a process for producing a mixture of cement, calcined shale and aluminium powder that was subsequently treated at high temperature and pressure in an autoclave. A few years ago, the centenary of the invention of this building material was celebrated. Its most obvious characteristic is its high and homogenous porosity, which results in a lower density and better thermal conductivity compared to conventional concrete.

 

AAC is a structural material, belonging to the cellular concrete family, with raw materials consisting mainly of sand, lime, cement, water and an aerating agent, the most common and widely used of which is aluminium powder/paste. Over time, there have been variations in the original mix design of AAC in order to improve its sustainability, especially with respect to the source of materials such as limestone or silica. An example of this is the use of industrial by-products such as blast furnace slag or fly ash from incinerators to partially replace Portland cement. With respect to the aerating agent, however, there have been no significant innovations from an environmental point of view, as aluminium powder has remained the only component used to activate the aeration of cement. Different methods of aeration concern in particular non-autoclaved cellular concretes, in which the formation of air voids is achieved through the introduction of a volumetric fraction of preformed foam within the cement paste.

Further experiments [3] were carried out using H₂O₂ as an aerating agent for a concrete obtained from a geopolymer fly ash matrix, demonstrating the formation of microporosities similar to the material aerated with aluminium powder with the same density and mechanical performance.

 

Totally innovative, however, was the methodological approach that led ENEA to the development of a new aerating agent, which is the result of combining the expertise gained in technological research of innovative materials for energy efficiency with that of biological processes. The new aerated concrete is based on the simplification of conventional constituent materials and the use of microorganisms as a source of enzymes capable of breaking down hydrogen peroxide into oxygen and water. The results of the experimentation led to a final product that is very similar to the commercial product in terms of density, mechanical strength, thermal and acoustic conductivity and that holds great potential for environmental sustainability.

 

Experimental approach  

 

As is well known, aluminium powder reacting with calcium hydroxide in an alkaline environment (pH ≥ 12) and in the presence of free water, produces, due to the oxidation of the metal, molecular hydrogen according to the following reaction:

 

 2Al + 3Ca(OH)₂ + 6H₂O = 3CaO∙Al₂O₃∙6H₂O + 3H₂↑(g)

 

 

The development of H₂ results in a porous structure within the mix matrix, which once hardened will have a volume 1.5 to 5 times that of the original cement slurry. The rising process stops when the material reaches a solid consistency: the voids stabilise in the material matrix and the hydrogen content is gradually released into the atmosphere and replaced by air.

 

Bio-aeration, which produces Bio-Aerated Autoclaved Concrete (BAAC), is the result of the dismutation reaction of hydrogen peroxide (H₂O₂) activated by organic catalase. Catalase is an enzyme synthesised by most cells of living organisms to control the harmful effects of oxidation of biomolecules due to aerobic metabolism of cells or accidental exposure to oxidising chemical species of exogenous origin. In nature, there are various forms of catalase, differing in structure, sequence, and composition of the catalytic centre, all of which, however, have in common that they can catalyse the oxidation-reduction dismutation reaction of H₂O₂.

 

The reactions are as follows:

 

 

2H₂O₂ → O2+2H₂O

reduction: H₂O₂ → [O]+H₂O

oxidation: H₂O₂+[O] → O₂+H₂O

 

The enzymatic activity of catalases is thus capable of breaking down millions of molecules of H₂O₂ per second into water and molecular oxygen with an optimum pH range of 4 to 11, depending on the species [4]. The hydrogen peroxide dismutation reaction is characterised by a strongly exothermic decomposition enthalpy even at room temperature (approx. 25°C), so no additional heat supply is required.

This ability of catalase is exploited in BAAC. As the reaction takes place within the cement slurry, the oxygen released will form porosities that increase the volume of the mass and consequently reduce the final density of the composite.

Among the various biological organisms capable of supplying the catalase enzyme to the system, ENEA researchers have identified yeast cells and in particular the Saccharomyces cerevisiae strain, the common brewer's yeast, widely used in activities that require leavening and fermentation, particularly in the production of wine, bread, and beer.

 

The first experimental tests on the feasibility of the idea determined the reactive capacity and quantity of gas produced by first mixing just the yeast, dissolved in water at a concentration of 10 g/l, with hydrogen peroxide (titrated to a 35% concentration in water) (Fig. 1) and then adding just the commercial cement, without the addition of aggregates and lime (Fig. 2).

The result highlighted another important characteristic of bio-aeration, namely that of its functioning in complete independence from the materials used in the cementitious mix composition. This circumstance made certain components of conventional AAC mix design superfluous, in particular calcium hydroxide, which has the predominant task to react with the aluminium powder, forming hydrogen gas molecules.

 

 

 

The experimentation for the validation of BAAC then continued with the fine-tuning of the components of the mix design, consisting mainly of cement, sand, yeast, water and hydrogen peroxide. The modulation of the quantities of aerating agent (yeast and H₂O₂) corresponded to different values of the final density. The increase was assessed in percentage terms in relation to the height of the specimen made without the aerating agent using a formwork with dimensions of 10x10 cm.

 

The results are summarised in Fig. 3, in which each point (blue values obtained on the side of the test specimens, orange values obtained in the centre) represents the arithmetic mean of three individual measurements.

The results obtained in this experimental phase together with their repeatability made it possible to establish certain fundamental aspects: 

 

1.     The rising process takes place at different speeds depending on the amount of hydrogen peroxide added to the dough.

 

2.     The growth process ends within 15 - 20 minutes after the addition of H₂O₂ and the consequent start of the dismutation reaction, at the end of which the setting process begins.

 

3.     The trend of the change in height of the specimen can be approximated to a linear function of the quantity of hydrogen peroxide added to the mixture. The aeration reaction is independent from the presence of other chemical components in the mixture and only associated with the presence of hydrogen peroxide and yeast. It is therefore possible to identify other sources of hydrogen peroxide, e.g. sodium percarbonate, a product of sodium carbonate and hydrogen peroxide that releases H₂O₂ in water, making it available for bio-aeration. The solid nature of sodium percarbonate allows the preparation of premixes where all the components of the BAAC mix design are present in dry form, where bio-aeration can be activated with just the addition of water, thus simplifying the production process.

 

 

Constituent materials and mix design

 

For all the reasons described above, the basic mix design of BAAC is considerably simplified compared to that of traditional AAC. The essential components are therefore cement (Portland CEM II A-LL 42.5R), sand (Silverbond SA600), water (at room temperature) and the aerating system consisting of peroxide (in liquid or solid form) and yeast. In the production phase, the last ingredient to be added to the mixture is peroxide.

 

The final density can be scaled up mainly by varying the amount of H₂O₂ added to the mixture.

The following table summarises the average weight percentages of the BAAC components for a density of 500 kg/m³.

 

Table 1: BAAC mix design

 

Component	%
Silica sand	35
Portland cement	23
Water	40
Yeast	0,5
H2O2/yeast ratio	2,3

The laboratory mixing phase of the components has a duration of approximately 10 minutes and is concluded by the addition of the peroxide followed by an additional final mixing. Once the setting phase of the cement is completed, the specimen assumes a consistency that enables its removal from the formwork, following which it is processed and placed in an autoclave where it is heated to 190°C under saturated steam conditions for a total of 12 hours.

 

 

 

 

Test results

 

Three sets of BAAC specimens, each comprising three replicas, were produced in the laboratory, respectively with final densities of 350, 500 and 850 kg/m³. Table 2 shows the results of the tests carried out according to the technical reference standard, to determine the mechanical resistance to compression [5] and the λ value of the thermal conductivity [6].

 

 

Table 2: BAAC test results.

 

Test series	Density [kg/m3]	Compressive strength [MPa]	Thermal conductivity [W/mK]
A	350	1.08	0.090
B	500	3.10	0.115
C	850	9.20	0.316

 

 

Conclusions

 

The research carried out by ENEA researchers and technicians has made it possible to patent an innovative autoclaved aerated concrete, called BAAC (Bio-aerated autoclaved concrete) [7]. This was achieved by:

·       demonstrating the possibility of using an aerating agent other than aluminium powder/paste in autoclaved aerated concrete,

·       establishing that the reaction between the components of the aerating agent, as a result of which gaseous oxygen is formed, is independent of the presence of other materials/components, 

·       developing the mix design of BAAC, and

·       verifying the laboratory repeatability of the production process and test results, in terms of physical and mechanical characteristics.

Further studies involved the use of fibres, alternative materials to cement, and food industry waste as a source of yeast.

 

 

Acknowledgements 

 

The research was funded by ENEA as part of its Proof of Concept programme, which aims to narrow the gap between research and industrial application and support innovation in the manufacturing sector.

 

References

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[1] European Commission, Proposal for a Directive of the European Parliament and of the Council on energy efficiency, Brussels, 2021.

 

[2] ASTM C1693-11(2017) Standard Specification for Autoclaved Aerated Concrete (AAC) - Book of Standards: Volume: 04.05 Developed by Subcommittee: C27.60 DOI: 10.1520/C1693-11R17 ICS Code: 91.100.30.

 

[3] Ducman V,; Korat L.: Characterization of geopolymer fly-ash based foams obtained with the addition of Al powder or H2O2 as foaming agents. Mater Char 2016

 

[4] Switala J, Loewen PC. Diversity of properties among catalases. Arch Biochem Biophys. 2002 May 15; 401 (pp. 145-54).

 

[5] UNI EN 679:2005 Determination of the compressive strength of autoclaved aerated concrete.

 

[6] ISO 8301:1991 Thermal insulation — Determination of steady-state thermal resistance and related properties — Heat flow meter apparatus.

 

[7] P. De Fazio, G.Leter, G. F. Lista, C. Sposato, M.B. Alba, Patent WO/2019/049005: Process for preparing bioaerated autoclaved concrete, 2019.