Innovative Lightweight and Sustainable Composite Material for Building Applications

Abstract

In recent years, the application of sustainable cementitious materials has become of great importance to improve buildings efficiency and to achieve carbon neutrality. Main goal of this work to study and develop BIOAERMAC, an innovative construction material with low density, composed of synthetic anhydrous calcium sulfate obtained as by-product in the industrial production of hydrofluoric acid and an aerating agent composed of microorganisms and peroxides, with the addition of rubber from end-of-life tires (ELTs). A density from 600 to 950 kg/m³ with a compressive strength up to 6.0 MPa and a thermal conductivity from 0.15 to 0.3 W/mK are the key performance metrics of BIOAERMAC composites. Experimental results showed an improvement in technical and energy performance, combined with a reduction in natural resource consumption and the wide quantity of by-product reintroduced into the production process.

1. Introduction

In the European Union, the building sector is one of the major parties responsible for environmental and economic impacts because it requires a large amount of resources and global energy [1]. In particular, construction accounts for 50% of all extracted materials, 35% of greenhouse gas (GHG) emissions and 32% of waste flows [2]. The constant demand for raw materials and the ever-increasing scarcity of resources must make us understand the need to move towards a sustainable circular economy model [3,4]. In addition, the urgent need to reduce CO₂ emissions [5,6] has led researchers to focus on eco-efficient materials and environmentally friendly solutions [7] able to protect people’s health, mitigate the negative effects on the climate change and increase energy efficiency in buildings.

Starting from the idea of creating a product in line with these needs and in compliance with the European Directives, such as the European Green Deal challenge [8] and new Circular Economy Action Plan (CEAP) [9] (which provide for a drastic reduction in the use of materials from primary natural resources and an increase in those derived from recycled products and by-products), an innovative composite material suitable for the production of low-density panels and/or blocks for structural and non-structural applications was designed and created. This new lightweight product, called BIOAERMAC, is composed by synthetic anhydrous calcium sulfate obtained as a by-product in the industrial production of hydrofluoric acid and an aerating agent composed by microorganisms and peroxides. Moreover, the inclusion of rubber particles from end-of-life tires (ELTs) enhances its thermal performance. The recycling and disposal of end-of-life tires (ELTs) represent some of the main challenges in waste management due to its complex structure and composition. For this reason, one of the most reasonable and sustainable options is to use them into the formulation and manufacturing of new materials [10]. Recent trends in production of lightweight mortar materials for structural and non-structural applications include incorporation waste-derived materials [11,12,13,14,15,16,17], such as polystyrene, rubber, glass, recycled plastics or other polymer-based waste. These materials are increasingly used to improve sustainability, reduce construction waste and enhance lightweight properties in mortars.

Therefore, the use of industrial wastes and/or by-products [18] as substitutes for cement, additives and/or natural aggregates in cementitious composites allows us to address environmental issues and conceive more environmentally friendly and energy efficient solutions while also improving durability, fire resistance and thermal insulation properties [19].

This has become an attractive solution to address the challenges linked to a reduction in CO₂ emissions, exploitation of non-renewable natural resources, energy use in cement production [20] and waste management in the construction field [21]. Thanks to alternative building materials, the construction market can count on lightweight solutions and technologies [22] that are growing because they are accessible, affordable [23] and, furthermore, can be adapted and designed depending on the climate context and structural needs.

The main goal of this work is to explore the salient aspects of BIOAERMAC technological innovation, which concern the mix design and the aeration method adopted to reduce its density (bio-aeration). A lower density means a reduction in dead load in building structures, which, at the same time, allows for increasing material efficiency and decreasing envelope’s thermal conductivity. As a result, one of the most important parameters in lightweight material characterization, is material porosity [24]: BIOAERMAC presents a small size and a homogeneous distribution of the pores, a feature that contributes directly to mechanical and physical performance, thus leading to an innovative technological solution with excellent energy and structural performance characterized by clear advantages in terms of environmental and economic sustainability. The uniqueness of the study contributed to the development of the patent application WO 2023/152629 [25]. The aim of this work is the preparation and development of new material by testing its mechanical and thermal performance. Moreover, the high content of recycled material used in the production of the manufactured elements presents a new challenge in the field of lightweight construction materials, by contributing to the development of eco-friendly and energy-efficient technological solutions.

2. Materials and Methods

The experimental setup was designed in order to identify the best mix design in terms of density, thermal conductivity and compressive strength parameters (

Table 1. Components of BIOAERMAC mix design.

Ingredients	Characteristics
Anhydrite	Milled powder with 97.6 w/w% of anhydrous CaSO4
CSA	Pure calcium sulfoaluminate cement clinker
Superplasticizer	Melamine product based [26]
Water	Tap water
Aerating agent	Brewer’s yeast + hydrogen peroxide
ELT rubber particles	Particle size from 0.5 to 2.5 mm

).

As mentioned before, the product is a lightweight composite material based on calcium sulfate, synthetic anhydrous gypsum CaSO₄ (anhydrite) in particular, deriving from the hydrofluoric acid production cycle.

Calcium sulfate, from natural or synthetic origins, exists in different hydration phases, among which the most common are as follows: (1) the dihydrate CaSO₄∙2H₂O; (2) the hemihydrate CaSO₄∙0.5H₂O; (3) the anhydrite CaSO₄. All of these are characterized by their peculiar properties, which permit applications in several industrial sectors ranging from cement production to building materials and agriculture.

The production process is based on the endothermic reaction of natural acid-grade fluorspar (the fluorine mineral used as raw material for all the fluorochemical derivatives) and sulfuric acid, which are mixed in a rotary kiln heated externally by hot gases:

CaF₂ + H₂SO₄ → 2HF + CaSO₄

(1)

These two raw materials are fed together into the kiln making a sort of sticky paste until the reaction is almost completed, whereupon the HF is recovered in gas phase while the solid calcium sulfate is discharged by the end of the kiln at a temperature of 200–250 °C. This raw anhydrite maintains some impurities deriving from residual raw materials used, consisting of about 2% of CaF₂ and 0.5% H₂SO₄. The raw anhydrite is treated with lime in a dry process to neutralize the free acidity, and the result of this neutralization is still the production of calcium sulfate. The raw anhydrite is finally milled with a proper mill in order to obtain a homogeneous fine product that finds application as binder in several building materials.

Milled anhydrite can be hydrated to form the calcium sulfate dihydrate by mixing the material with the right amount of water, according to the following:

CaSO₄ + 2H₂O → CaSO₄∙2H₂O

(2)

The hydration process starts the setting phenomenon because it changes the crystal structures of anhydrite, producing thin needles of calcium sulfate dihydrate, less soluble than anhydrite, which form a dense weave and texture, bringing the material from a plastic phase to a hard solid one [27].

The anhydrite is a white powder composed by 97.6% of calcium sulfated with the presence of CaF₂, magnesium, iron and potassium oxide. The particles size distribution has normally a d50 in the range 15–25 µm. In

Table 2. Chemical composition of anhydrite.

Element	Quantity	Unit of Measure
CaSO4	97.6	[w/w%]
SO3	57.42	[w/w%]
CaF2	1.11	[w/w%]
SiO2	0.09	[w/w%]
K2O	0.006	[w/w%]
MgO	0.07	[w/w%]
Fe2O3	0.05	[w/w%]
Al2O3	0.15	[w/w%]
Ca(OH)2	0.85	[w/w%]
H2O	0.14	[w/w%]

, the chemical composition of anhydrite obtained by elemental X-ray fluorescence analysis (XRF) related to thermal gravimetric analysis and the alkalinity method for water and Ca (OH)₂ content, respectively, is reported. This component is the principal by-product of HF production provided by the Fluorsid plant in Cagliari [28].

The powder X-ray diffraction analysis was performed by using a Bruker D8 Advance instrument with a θ − θ, Bragg–Brentano geometry with Cu − Kα wavelength (1.554 A), in the angular range of 5–80° with a step of 0.01°. The elemental composition was determined by Wavelength Dispersive X-ray Fluorescence (WDXRF) analyses (Bruker S8 Tiger, Billerica, MA, USA). The powder sample was mixed and dispersed in a cellulose matrix and then prepared a pressed pellet for measurement.

The CSA powder is a sulfoaluminate cement that mainly consists of ye’elimite (C₄A₃SO₄; 4CaO∙3Al₂O₃∙SO₃) and belite (C₂S; 2CaO∙SiO₂); XRD analysis (

Table 3. X-ray diffraction analysis and X-ray fluorescence analysis of CSA.

XRD Analysis		XRF Analysis
Phase	w/w%	Oxide	w/w%
O-ye’elimite	59.4	CaO	42.55
C-ye’elimite	-	Al2O3	34.48
a-Ca2SiO4	-	SO3	8.43
b-Ca2SiO4	26.1	SiO2	7.72
C3A	4.3	MgO	0.07
CaSO4 (anhydrite)	0.7	F	1.67
Na2SiO5	-	Fe2O3	0.46
MgO	2.3	K2O	1.54
CaTiO3	1.8	TiO2	0.94
Ca2(Al,Fe)O5	5.4	MgO	1.41

) and pattern (Figure 1) shows the phases of used binder and XRF analysis (Table 3) shows the oxide composition.

The superplasticizer is a free-flowing spray dried powder of a sulphonated polycondensation product based on melamine usually used for cement and calcium-sulfate-based materials [26] to reduce the water content of mix design. Mixture workability is evaluated according to flowability or fluidity criteria, which are influenced by the characteristics of each cement binder and the used superplasticizer. Superplasticizers (SP) are generally divided into the two following classes: (1) polycondensate type (including polynaphthalene and polymelamine-sulfonate polymers); (2) polycarboxylate type, in which the methyl methacrylate main chain is replaced with a side chain containing polyethylene derivatives [29].

The brewer’s yeast used in the mix design is a freeze-dried commercial product without the addition of flours or other ingredients. This ingredient is used as a source of the enzyme catalase (Saccharomyces cerevisiae). The hydrogen peroxide is reagent-grade water solution of 130 volumes.

The rubber particles are produced from an end-of-life tires (ELT) reduction process. The particles size distribution is reported in Figure 2 provided by size separation using ASTM sieves.

The preparation method includes the following steps: (1) solid ingredients, such as anhydrite, CSA, superplasticizer and ELT particles (if any) are placed in a tank and dry mixed for 1 min; (2) yeast is dissolved in water (1/7 of total amount) at a temperature of 25 °C and mixed for about 2 min; (3) the remaining part of water (6/7) is gradually added to the solid mixture and the grout is mixed by hand for 1 min and then by an electric mixer; (4) water + yeast solution is added to the grout and the hydrogen peroxide is gradually added in the resulting mixture; (5) after 1 min of mixing the grout is placed into the mold, where it remains for 24 h in order to obtain the volume increase and the completion of the setting phase.

The H₂O₂ permutation reaction is completed within 25–30 min of its addition: this results in an increase in the volume of the mixture of approximately 80% compared to the initial one [30,31].

For this purpose, three different steel cubic formworks are used (Figure 3): 100 × 100 × 100 mm and 150 × 150 × 150 mm for the mechanical test and 200 × 200 × 200 mm for the thermal conductivity test.

After 24 h, the resulting material was removed from the formwork and placed in a suitable environment with a temperature of 20–25 °C and a humidity of 60–70% (conditions under which all tests were conducted). After 28 days of curing, cubic (100 × 100 × 100 mm) and panel (200 × 200 × 40 mm) specimens were obtained by cutting the preformed sample with a band saw. For a correct testing procedure, the surfaces must not exhibit deviations in flatness greater than 0.1 mm, so they have been rectified as needed. Subsequently, the specimens were stored at a temperature of 50 °C until a constant dry mass was achieved and were left under laboratory conditions for two hours before starting the tests.

BIOAERMAC is a new material and for this reason there are not detailed reference standards. For this work, in order to evaluate composite performance, the produced specimens are tested in according to the following UNI standards:

• Dimensions (UNI EN 772-16: 2011) [32];
• Density (UNI EN 678: 1994) [33];
• Compressive strength (UNI EN 679: 2005) [34];
• Thermal conductivity (UNI EN ISO 12664: 2002) [35] suitable for samples with a thermal resistance not less than 0.1 m²K/W and a thermal conductivity not greater than 2.0 W/mK; UNI EN 1745: 2020 [36] useful for the determination of the dry thermal and design values of thermal conductivity of masonry and masonry products).

The compressive strength is determined by an Instron 3400 double column universal machine (Norwood, MA, USA) with a load cell of 50 kN and a constant downward speed of 1.5 mm/min; it is equal to the ratio between the breaking load in axial compression and the cross-sectional area of the test sample perpendicular to the direction of casting. The maximum load is recorded when specimen breaking occurs. The apparatus used for thermal conductivity measurements, suitably calibrated as required by the manufacturer, is a heat flow meter Netzsch HFM436/3 (Selb, Germany) placed in a laboratory with the conditions required by the standard UNI EN 12664: 2002 [35]. The optical microscope (Leica Microsystem model DVM6, Wetzlar, Germany) has been used for the morphological characterization of all the samples.

Mix Design

In this work two different mix design are implemented to produce the final material (

Table 4. Mix design of proposed A-type and B-type materials.

A-Type Material		B-Type Material
Anhydrite	73.9%	Anhydrite	66.5%
CSA	24.6%	CSA	22.2%
ELT particles	-	ELT particles	10%
Superplasticizer	1%	Superplasticizer	0.89%
Brewer’s yeast	0.50%	Brewer’s yeast	0.49%
Water/solid ratio	0.31	Water/solid ratio	0.28
H2O2/yeast ratio	1.3–2.6	H2O2/yeast ratio	1.3–2.6

). The A-type mix design material includes anhydrite, CSA, superplasticizer and brewer’s yeast in amounts of 73.9%, 24.6%, 1% and 0.5%, respectively. The amount of water is calculated as water/solid ratio and the quantity of hydrogen peroxide varying from 1.3 to 2.6 as H₂O₂/yeast ratio. The ELT particles amount in the B-type material (10 w/w%) is a partial substitution of CSA + anhydrite powders which keeps the mass ratio between the two components unchanged (Figure 4). The choice of using a by-product such as ELT particles is due to two reasons: the first one is to improve the thermal insulation capacity of the material and the second one is to study the potential of the mix design to incorporate various types of organic/inorganic materials for future performance improvements.

As previously described [37,38], the enzyme catalase (present in the common brewer’s yeast) allows a breakdown reaction of hydrogen peroxide with the production of water and molecular oxygen. The production of gas inside the semi-liquid grout will be responsible for the increase in volume of the final product, as well as for its reduced density. The preparation process of the bio-aerated composites is described in greater detail in patent WO 2023/152629 [25].

3. Results

3.1. Influence of Anhydrite and CSA

One of the main objectives of this experimental work is to investigate a new building material with similar characteristics to those commonly used in construction industry. Furthermore, maximizing the amount of anhydrite aims to create a more sustainable material given that anhydrous calcium sulfate represents a by-product of the hydrofluoric acid supply chain and the current applications of this powder present a low added value [39,40]. According to UNI EN 13454-1 [41] a content of calcium sulfate (CaSO₄) ≥ 50% and <85% by mass determines a calcium sulfate composite binder (CAC). The content of calcium sulfate is determined as specified in EN 13454-2 [42]. Moreover, calcium sulfate composite binders have a class A1 fire rating, without testing, according to Commission Decision 96/603/EC [43] and subsequent amendments, if they contain less than 1% by mass or volume of organic material, which is the most onerous. The studied mix design permits classification of BIOAERMAC as a calcium sulfate composite binder material with a Class A1 fire rating.

The use of CSA is important for two reasons: the first one is its function as a binder because, mixed with water, it helps to combine the composite materials; the second one is the ability of the CSA to obtain a rapid setting [44,45] compatible with the dismutation reaction of the hydrogen peroxide, thus ensuring the desired reaction times and setting of the material. In the preliminary part of the work, different amounts of CSA were tested: 5%, 10%, 15%, 20% and 25%. Only with a concentration of calcium sulfoaluminate cements greater than 20% it was possible to obtain the following:

• Setting times compatible with the increase in volume of the material;
• Good workability of the mixture (10 min max);
• Possibility of removing the specimen from the formwork in approximately 2–3 h;
• Good compressive strength.

3.2. Influence of Hydrogen Peroxide

In the table below (

Table 5. Mix design ingredients and dry density of BIOAERMAC materials (A-type and B-type).

Ingredients	Amount [g]	Density [kg/m3]	Mix Design A-Type	Amount [g]	Density [kg/m3]	Mix Design B-Type
Anhydrous CaSO4	1312.5			1181.25
CSA	437.5			393.75
Superplasticizer	17.5			15.75
ELT particles	-			175
Brewer’s yeast	8.75			8.75
H2O2 [mL]	4	950 (±30)	A1	4	900 (±30)	B1
	5	810 (±11)	A2	5	800 (±12)	B2
	6	750 (±30)	A3	6	710 (±30)	B3
	8	650 (±35)	A4	8	610 (±35)	B4

), the optimal amount of ingredients of the BIOAERMAC material mix design is reported.

To produce a 150 × 150 mm base dimension A-type specimen, 1312.5 g of anhydrite, 437.5 g of CSA clinker, 17.5 g of superplasticizer and 8.75 g of yeast are used. In B-Type material, the ELT particles are added without modifying the anhydrite/CSA ratio (equal to 3/1) but only by reducing their total quantity. The hydrogen peroxide contents in the mix design vary from 4 to 8 mL per mixture. This variation produces A-type materials with different height and density (determined in dry conditions [33] after 28 days of aging) from 650 kg/m³ to 950 kg/m³ (with 8 mL and 4 mL, respectively). Meanwhile, the quantity of yeast is constant in both mix designs, and the increasing amount of hydrogen peroxide produces a lightweight material with the same density class [46]. The higher peroxide concentration allows an enhancement in oxygen bubble production and the result is an increase in the distribution and size of the concavities, in which the air is trapped, with a consequent decrease in the density of the material. The use of ELT particles in the mix design results in a reduction in the final density of about 5%. As a blank experiment, a specimen with the same A-type mix design without using aerating agent (yeast + H₂O₂) is performed; the resulting non-aerated material shows a density of 1820 kg/m³.

3.3. Influence of Superplasticizer

The use of superplasticizer inside the grout is necessary to reduce the amount of water used in the process and to maintain good workability of the system. In the mix design, the superplasticizer content is 1 w/w% as calculated based on anhydrite + CSA amount. In previous tests, 0.5%, 0.75%, 1% and 2% of superplasticizer have been also studied. The best choice for the proposed mix design is 1% since, with the same amount of water, adequate workability is not achieved for lower quantities, while for higher quantities it is not possible to maintain the volume of the final material without observing a collapse of the specimen due to the not reached setting time of the CSA + anhydrite mixture.

3.4. Mechanical and Thermal Behaviour of BIOAERMAC Materials

Table 6. Result of compression and thermal test listed for same density classes of A-type and B-type.

Specimen	Density [kg/m3]	Compression Strength [MPa]	Thermal Conductivity [W/mK]	Mechanical Variation [%]	Thermal Variation [%]
A1	950	6.0 (±0.06)	0.305	34.7	8.5
B1	900	3.92 (±0.26)	0.279
A2	810	3.15 (±0.28)	0.258	16.5	12.0
B2	800	2.63 (±0.32)	0.227
A3	750	2.3 (±0.12)	0.213	24.3	10.3
B3	720	1.74 (±0.14)	0.191
A4	650	1.7 (±0.12)	0.172	13.5	9.9
B4	610	1.47 (±0.02)	0.155

and Figure 5 show the mechanical and thermal behavior of BIOAERMAC materials. The specimens named A1, A2, A3 and A4 represent the A-type materials with the use of 4, 5, 6 and 8 mL of hydrogen peroxide in the mixture, respectively. Likewise B1, B2, B3 and B4 are the same materials with the use of ELT particles in the grout mix. For each formulation, at least three specimens were tested.

The compression strength is expressed in MPa and standard deviation of result are also shown. The thermal conductivity is calculated as λ₁₀, in according to UNI EN 12664 [35], and expressed in W/mK. The environmental conditions during the tests were a room temperature of 23 ± 2 °C and a relative humidity of approximately 60%. The variation between A-type and B-type material, calculated as (Ai − Bi)/Ai × 100, is also shown.

The A-type materials show increasing densities with decreasing hydrogen peroxide concentration equal to 950, 810, 750 and 650 kg/m³ with, respectively, a mechanical strength of 6, 3.5, 2.3 and 1.7 MPa in compression. In the same way, the materials synthesized with the particles of end-of-life tires have increasing densities as the concentration of the aerating agent decreases with values equal to 900, 800, 710 and 610 kg/m³ with, respectively, a compressive strength of 3.92, 2.63, 1.74 and 1.47 MPa. The presence of ELT particles shows a decrease in mechanical strength of about 13–24% compared to A-type material up to a density of 800 kg/m³. This gap increases considerably for densities higher than 900 kg/m³ (up to 34.7%). The tire particles can be incorporated in the CSA/anhydrite mixture and they are distributed uniformly within it; to confirm this behavior, mechanical tests were performed on various specimens coming from the same mixture (the mother specimen is divided into four parts), showing mechanical strength with a standard deviation less than 3%. On the other hand, the addition of rubber particles as partial replacement of other solids has an adverse effect on the compressive strength of the material; this is probably due to the weak interfacial bond between the tire rubber particles and the cement matrix [47,48]. In Figure 6, a typical stress–strain behavior of BIOAERMAC is shown. This is just qualitative and it is characteristic for all material tested; we can appreciate a brittle behavior of the material since the tested specimens fracture when subjected to the maximum allowable stress, with little elastic deformation and without significant plastic deformation.

In Figure 7, the results of thermal performance are shown: the A-type materials show a decrease in thermal conductivity with the lowering of densities equal to 0.305, 0.258, 0.213 and 0.172 W/mK with, respectively, 950, 810, 750 and 650 kg/m³. Contrary to the mechanical behavior of the material, the presence of ELT particles shows an improvement in thermal performance up to 12%. This behavior of the material is also linked to the fact that the decrease in density is closely correlated with a greater distribution of the bubbles/cavities in which the presence of air guarantees a decrease in the thermal conductivity of the material.

The X-ray diffraction analysis performed on the resulting powder of the anhydrite-based aerated material (Figure 8) exhibit the crystallographic structure of BIOAERMAC. The chemical composition shows the presence of anhydrite, ettringite, b-Ca₂SiO₄ and Al(OH)₃, respectively, on 51.3%, 42.6%, 4.6% and 1.5%. The presence of aluminum hydroxide confirms that the ettringite production occurs in accordance with the following reaction:

4CaO·3Al₂O₃·SO₃ + 2CaSO₄ + 38H₂O → 6CaO·Al₂O₃·3SO₃·32H₂O + 2Al(OH)₃

(3)

The rapid hardening and dimensional stability characteristics of CSA cements are due to the formation of ettringite produced according to Reaction (3), in the absence of lime, that does not have expansive behavior and is able to promote high mechanical resistance even after short maturations [49].

The microscope images (Figure 9) show the morphology of the synthesized materials. The marker used is the same for all photos (0.5 mm). The spheroidal air cavities are uniformly distributed over the surface with an almost constant size range. In the case of the A1 material, with a density of 950 kg/m³, the average pore size is about 0.15 mm ± 0.05 mm; with the decreasing in density, the bubbles dimensions increase with 0.5 mm (average ±0.02 mm) for the A3 sample.

4. Discussion

In the construction sector, interest in the development and use of lightweight composite materials for load-bearing structures, partitions and filling applications, which can improve the structural, energy and sustainability performance of buildings, is increasingly emerging. Two of the most common commercial lightweight products are gypsum-based materials and cellular concrete [25].

Gypsum covers a wide range of applications as an airborne binder or as a component of composite mortars. Among the most used gypsum-based building materials are plasterboard and perforated blocks. Plasterboard is composed of a slab of gypsum coated with a special cardboard, usually laid on a load-bearing structure composed of galvanized sheet metal profiles. Applications for plasterboard are mainly internal partitions with a density range from 900 to 1100 kg/m³ and corresponding thermal conductivity equal to 0.2 and 0.32 W/mK. Blocks of gypsum represent construction solutions used for internal walls, with a thickness of 6–10 cm, characterized by both thermal and acoustic insulating properties; thanks to their chemical nature, they are also able to guarantee high fire resistance.

Cellular concrete refers to those concretes that have a porous structure: autoclaved aerated concrete (AAC) is undoubtedly the most common and it is available in blocks of various sizes. Depending on density, which generally ranges between 300 and 800 kg/m³, mechanical compressive strength from 2 to 8 MPa and thermal conductivity from 0.09 to 0.3 W/mK can be achieved. These products have also really good fire resistance and excellent acoustic performance.

Table 7. Comparison between BIOAERMAC and commercial products used in construction field for walls, partitions and false ceilings.

Material Typology	Density [kg/m3]	Compression Strength [MPa]	Thermal Conductivity [W/mK]
BIOAERMAC—Gypsum-based A-type	950 810 750 650	6 3.1 2.3 1.70	0.305 0.258 0.213 0.172
Vibrocompressed panel obtained from a mixture based on expanded clay, lapillus and cement [50]	1400	2.5–3	0.4
Vibrocompressed concrete obtained from a mixture based on expanded clay, cement and water [51]	900	4–4.5	0.289
Lightened concrete with expanded clay [52]	1000	3.5	0.208
Expanded clay concrete and expanded glass [53]	750	2.5	0.144
Autoclaved aerated concrete [54]	630	≥5	0.16
Plaster block [55]	950	>6	0.7
Thermally insulated plaster block [55]	820	3.5	0.23
Brick with vertical holes [56]	680	-	0.134
Gypsum core reinforced with mineral fibers [57]	1000	-	0.25
Perforated gypsum block [58]	900	1.18	-

summarizes the main lightweight composite materials currently on the market, with their respective mechanical and physical characteristics [25].

5. Conclusions

In this research, the technical feasibility, the standardization of the results and the reproducibility of the implementation method of a new material were tested. BIOAERMAC is a bio-aerated gypsum-based composite material, the physical, mechanical and density characteristics of which were found to be completely competitive with those of commercial lightweight products, thanks to the following considerations.

The laboratory tests have highlighted how, with the same density as commercial gypsum-based or cement-based lightweight products, the results of BIOAERMAC referring to the physical (thermal insulation) and mechanical (compressive resistance) parameters, are aligned with and, in some cases, better than potential competitive products (as highlighted in Table 7).

In the case of BIOAERMAC + ELTs, the addition of the tire rubber allows an improvement in the thermal insulation properties with the same final density as the sample without ELTs but, unlike the latter, it presents evident plastic behavior with respect to mechanical stresses compression. This, while considering a decrease in mechanical resistance values, allows the use of BIAOERMAC to be extended to a greater number of applications.

The bio-aeration system as an aerating agent that can be used for a wide range of mix designs, as it is independent of the type of material mix in which it acts.

BIOAERMAC highlights the potential of the mix design to incorporate into the grout waste materials of different nature (e.g., end-of-life tire rubber, fibers, etc.).

Taking into consideration the results obtained from this research, the new developed composite material can be considered as a valid alternative to produce more sustainable prefabricated gypsum bricks and panels for non-structural applications. The low specific weight of composites, together with their high thermal performance and good mechanical behavior, results into a very interesting material for use in industrialized construction. Moreover, the use of organic fillers derived from waste products such as end-of-use tire rubber (ELT) and inorganic ones such as basalt fiber needs to be studied in deep to achieve further performance improvements and an expansion of BIOAERMAC applications.

As regards to environmental issues, some other important advantages emerge regarding the use of synthetic anhydrite, such as its nature as a by-product or second raw material from the production of gaseous hydrofluoric acid, that allows a reduction in the CO₂ produced by the extraction activity of natural gypsum and a lower industrial impact on the land resources and guarantee energy savings, by avoiding mechanical (extraction and grinding) and thermal (calcination) processes required in conventional production practice for gypsum production.

Moreover, it is important to underline that, if we make a comparison with AAC, the BIOAERMAC process has a lower ecological impact due to a reduction in CO₂ emissions and of energy consumption, resulting on one hand from the complete replacement of Portland clinker with a cement material comprising cement and sulfoaluminate clinker, and, on the other hand, from the elimination of the autoclave curing process (which is highly energy-intensive).

The reduction in raw materials and energy resources, thanks to the presence of an industrial by-product and the reuse of waste in the mix design, could allow low construction costs and make this innovative material a more environmentally friendly option, in line with the criteria of circular economy foreseen by the European Green Deal legislation presented by the European Commission [8,9], but in the future development of the research it will be essential to perform a Life Cycle Assessment (LCA) and/or Life Cycle Costing (LCC) analysis to better understand the impact of all the activities.

Further explorations will focus on the implementation and optimization of the proposed material for onsite applications, including hygrothermal properties evaluation (such as water absorption coefficient), dimensional stability under humidity, long-term performance assessment (shrinkage, freeze–thaw resistance), integration with other sustainable technologies and upscaling strategies to support industrial production.

6. Patents

De Fazio, P.; Sposato, C.; Alba, M.B.; Leter, G.; Feo, A. Process for preparing bioaerated composite materials, 2023, Patent WO2023/152629 A1.