Silica Aerogel-Incorporated Cement and Lime Plasters for Building Insulation: An Experimental Study

Abstract

Silica aerogel has remarkable properties, particularly its translucence/transparency, extremely low thermal conductivity and density. Due to these properties, it can be used for the thermal insulation of buildings for energy saving, cost saving, and enhanced comfort. In this context, aerogel products such as aerogel blankets have already started to demonstrate their effectiveness in retrofitting projects and the development and adoption of aerogel glazing systems and aerogel-enhanced renders is promising. Other products, for example, through the incorporation of silica aerogel granules in cement and lime renders were obtained, with high thermal insulation properties, to achieve energy efficiency on buildings facades. This research aims to come up with new aerogel particle composition insulation plasters at cost-effective rates for application in building insulation. Their physical apparent mass density, mechanical–flexural and compressive strengths, thermal conductivity, and properties were investigated. As an experimental study, the thermal conductivities of six sets of cement and lime plasters with aerogel particles (0.1–4.0 mm) were investigated and it was concluded that the thermal conductivity of cement and lime plasters with 80% aerogel was 0.2287 W·m⁻¹·K⁻¹, about 3.4 times smaller than the respective value of traditional lightweight plasters of 0.76 W·m⁻¹·K⁻¹, while the cement and lime plasters with less than 40% aerogel showed a thermal conductivity value as low as 0.3172 W·m⁻¹·K⁻¹. It was confirmed that the end product plasters’ mechanical qualities included low apparent mass densities, no apparent shrinkage, and mechanical strength values that matched those of the prepared compositions. This suggests that the obtained plasters are suitable for use in both new constructions and renovation projects.

1. Introduction

At the global level, the increasing warming of the environment creates more and more complex problems. Also, the episodes of energy crises motivated the development of innovative materials that can be used as thermal insulation materials for buildings with nearly zero energy buildings (nZEB) requirements [1,2]. The Energy Performance of Buildings Directive (EPBE) requires all new residential and non-residential buildings to be zero-emission buildings as of 1 January 2028 for buildings owned by public bodies and 1 January 2030 for all other new buildings. According to this directive, a zero-emission building has no on-site carbon emissions from fossil fuels and a very high energy performance. Improving the thermal properties of a building envelope can be considered a solution to reduce a building’s energy consumption [3].

Aerogels represent a state-of-the-art thermal insulation material and may be the one with the highest potential at that moment. In buildings, they are usually applied as aerogel glazing and blankets but can also be employed in building structures, inside bricks or incorporated in cement plaster composites [4].

Aerogel-based renders represent one of the most promising solutions to reduce energy losses through envelopes [5,6]. The main ingredients responsible for their low thermal conductivity are SiO₂ aerogel granules with a total porosity of more than 90% and their nanoporous structure. Due to this structure, air molecules are trapped in pores smaller than their free mean path length, which reduces their contribution to the heat transfer by conduction [7].

Regarding their special properties, many insulating products with silica aerogel used for buildings have emerged in the market [4,8,9,10,11] and aerogel-based renders have been available since 2013 [12]. Granular aerogel-based translucent insulation materials and transparent monolithic aerogel are the most used [13]. Generally, if monolithic aerogels possess superior thermal and optical properties, are expensive to be produced, need long processing times, and need to be protected from tension and moisture, granular aerogels dominate the commercial market and even have low performance, but they are most robust, cheaper, and easier to be produced on the commercial scale [14,15]. These aerogels are used for roofs, façades, walls, or insulation bricks (aerobricks) [16] and windows, but also for sound insulation, fire retardation, and air purification. For old buildings, an aerogel can be used in the restoration and reconstruction of historical buildings such as museums and art galleries [11]. It can also be used for interior and exterior insulated plasters for breathable building envelopes and façades [17]. However, these products are still expensive for many countries where the costs of constructing a building with aerogel-incorporated plaster insulations is much higher than the costs of the energy consumption necessary for the operation of a building, as it is in the case of Romania.

Considering the above, the goal of this work is to create novel, inexpensive aerogel-based plaster compositions that can be enhanced and applied to the insulation of buildings. Comparing this work to other published research, it is innovative due to the fact it focuses on laboratory-produced plaster compositions with cheaper cement and lime, as well as low aerogel content—a significant consideration given that aerogel is still an expensive material. While many researchers discuss more about the performances of plasters with high compositions of aerogel due to their low thermal conductivities and other specific properties [18,19,20], this paper discusses about the performances of own plasters with low compositions of aerogel (40% vol.), with cheap hydrated lime used for mortars and plasters and masonry cement where the balance between the obtained thermal conductivities and mechanical properties are similar to the experimental values and quasi-similar to the properties of other aerogel-based plasters that have been studied and presented so far. The following sections present the details of obtaining and testing new compositions of aerogel-based plasters that can be used as insulation materials for buildings, and preliminary conclusions reached at this stage of the research.

2. Materials and Methods

2.1. Materials

The obtained cement and lime plasters are composed of Z100 cement, a Romanian plaster cement, with a standard strength of 12.5 MPa after 28 days [21], supplied by Carpatcement HIEDELBERG Group-RO (Bucharest, Romania) (15.05 EUR/100 kg), Supercalco M hydrated lime for construction CL80S, in accordance with EN 459-1 [22], which requires less mixing water, supplied by Carmeuse Holding-RO (26.9 EUR/100 kg), sand in accordance with EN 12620+A1 [23], supplied by Noua Tei Company–RO (25.2 EUR/m³), and P100 aerogel particles, supplied by CABOT Corporation, Boston, MA, USA (95 EUR/1 kg) [17].

The used sand has an apparent volumetric mass of 1655 kg·m⁻³, an apparent density of 2700 kg·m⁻³, and a maximum grain size of 4.0 mm. The grain size curve was the same as the one used for the determination of the standard cement class [24].

P100 aerogel particles with a size range between 0.1 and 4.0 mm were used (Figure 1a) and their thermal conductivity performances are show in Figure 1b. Water with a pH between 6.4 and 6.8 was used.

Some properties of the employed silica aerogel, lime, and cement, provided by the manufacturers and obtained according to European and USA norms, are provided in

Table 1. Materials’ compositions and properties: cement, lime, and aerogel particles.

Material	Property	Value	According to EU Norm
Cement Z100	Portland clinker High-purity limestone LL Min. compressive strength/7 days Standard compressive strength/28 days Air content in fresh plaster Water retention SiO2 Al2O3 Fe2O3 CaO MgO K2O Na2O Cl− SO3 Apparent density	Min. 40% Max. 60% Min. 7 MPa Min. 12.5 MPa Min. 8% 80% 20.32% 4.12% 3.87% 63.02% 2.43 0.6% 0.3% 0.08% 2.91% 1650 kg/m3	SR EN 413-1:2011 [21]
Hydrated lime Supercalco M	CaO MgO CO2 H2O CaCO3 Ca(OH)2 Insoluble residue Apparent density	91.1% 1.7% 0.9% 1.2% 2.0% 4.8% 5.1% 580 kg/m3	SR EN 459-1:2015 [22]
P100 Cabot aerogel particles	Particle size range Pore diameter Porosity Particle density Bulk density	0.1–4.0 mm ~20 nm >90% 120–180 kg/m3 80–100 kg/m3	-

.

2.2. Composition Designs and Samples

Six mixtures of silica aerogel-incorporated cement–building lime plasters were prepared. The reference plaster (S₀) was made in the proportion of 1:0.22:0.11 (sand/cement/ lime), in volume, with a mixed ratio of water/cement and lime of 0.6:0.4 [21,25]. From the reference mixture (S₀), different volume contents of the sand were replaced with P100 aerogel particles by 40%, 50%, 60%, 70%, and 80%, respectively (the values of percentages of the volume of the sand and aerogel were changed to mass, specifically, to the weight of the mass of sand and the mass of the aerogel). For 1 set of 3 prismatic moulds (160 mm × 40 mm × 40 mm) filled with aerogel-incorporated cement–building lime plaster, 3000 g of sand was used. As a calculus example, for the plaster sample (S₁) (40% of aerogel from the total volume of sand of 1111.1 cm³), the volume of sand is 666.7 cm³ and the volume of aerogel is 444.4 cm³ (1111.1 cm³ × 0.40). These values multiply with the apparent density of sand (666.7 cm³ × 2.7 g/cm³) and with the particle density of the aerogel (444.4 cm³ × 0.15 g/cm³), resulting in the mass of sand (≈1800 g) and the mass of the aerogel (≈67 g). Thus, the added aerogel represents a part of the “skeleton” of the plaster on which the cement–lime binding matrix is strengthened. The masses of cement and lime remain the same as in the reference sample (S₀). By replacing sand with silica aerogel, the plaster workability was changed. To confirm the mixtures’ reproducibility, three different sets for each composition were successfully produced. The used water/binder ratio was in accordance with the standard recommendation of plasters obtained, using the spreading table method [25]. For 1 set of 3 prismatic moulds (160 mm × 40 mm × 40 mm) filled with aerogel-incorporated cement and lime plaster, 3 kg of sand was used.

Before being combined in a mechanical mortar mixer, the components—sand, cement, lime, and the aerogel—were weighed, placed in a polyethylene plastic bag, and thoroughly stirred until the low-density aerogel was well incorporated. After that, the powder mixture was transferred to a recipient and manually mixed with water until it exhibited tactile and visual signs of adequate mixing. Afterwards, the mixture was moved into a mechanical mortar mixer, with a slow rotation, for 2 to 3 min. Figure 2 shows its appearance at the mixing stage.

The curing process consisted of the following:

(i)

Allowing the moulds for lime plaster and cement to wet cure for seven days inside a polyethylene bag.

(ii)

Removing moulds from the bag and unmoulding.

(iii)

Curing in the chamber with controlled temperature and humidity conditions, 20 ± 2 °C and 65 ± 5% relative humidity, until 28 days, in accordance with EN 1015-11 [24].

2.3. Experimental Methods

The mechanical tests and the apparent mass density test were performed after 28 days of curing, following EN 1015-11 [26] and EN 1015-10 [27] on prismatic samples.

Similar prismatic samples were subjected to the tests for their compressive and flexural strengths by implementing static methods using a Hegevald & Pesche universal testing machine. The compressive strength tests were conducted with a 30 kN load cell, and the flexural strength test was undertaken with a 10 kN load cell. The same specimens were utilized in both tests. The compressive tests were performed after the flexural tests on the halved prismatic samples.

The tests associated with thermal conductivities followed the specification of EN 1745 [28] and were carried out using a Heat Transfer Service Unit for Building and Insulating Materials, type H111N, on samples of a 28-day age and temperature of 19.5 °C.

3. Results

3.1. Apparent Mass Density

In Figure 3, the variations in the apparent mass density of the samples with different percentages of aerogel particles are presented. It can be observed that compared with the reference sample (S₀), all compositions with aerogel particles (S₁–S₅) have lower densities, and the densities decrease upon increasing the percent of aerogel, as expected, due to the replacement of parts of sand with aerogel particles. Also, what can be observed is a very low density at high percentages of aerogel 70–80% (S₄, S₅ samples) where the content of sand is smaller.

3.2. Mechanical Strengths

The heads of the prismatic samples produced after the flexural tests were used for the compressive tests and are shown in Figure 4.

By replacing parts of sand with aerogel particles, decreasing mechanical strengths are observed (Figure 5a,b) possibly due to the insufficient amount of a binder (higher aerogel/binder ratio), which leads to an increase in the volume of voids and creates a large network of capillaries with a relatively high amount of air content.

These low performances, related to the compressive and flexural strengths, upon adding aerogel particles, have also been presented in other studies [19,29,30,31,32].

The results above show that decreases in the flexural and compressive strengths were noted as the aerogel content increased. At 40% of aerogel, vol. %, in the analyzed samples, the highest flexural and compressive strengths were achieved as 0.73 MPa, 2.62 MPa, and 2.69 MPa, respectively. This trend was the same for compressive strength. When 80 vol. % of aerogel was added into the sample, substantial reductions in the flexural and compressive strengths were observed at 0.13 MPa, 0.97 MPa and 1.19 MPa, respectively, which can be attributed to the higher volumes of air and pores. Compared with the reference sample without aerogel particles (S₀), the mechanical strengths of the samples with aerogel particles (S₁–S₅) decrease by averages between 47% and 66%, respectively.

3.3. Thermal Conductivity

Considering the very high porosity, nanoporous structure, and very low thermal conductivity of aerogel particles (Table 1), the obtained plasters have low thermal conductivities. Based on the measured values, there are noticeable decreases in them between 0% and 40% volume percentages of aerogel in the plasters and then more progressive decreases between 40% and 80% volume percentages of aerogel.

After an evaluation of the experimental values presented in Figure 6, it can be noticed that there was a drop of 5.53 times of the thermal conductivity coefficient compared with the limit values between 0% vol. and 80% vol., respectively, aerogel contents in plasters.

This has implications in terms of building materials using a lower insulation material thickness with the same thermal performances. The thermal performance of a building envelope can drop by more than six times when a layer of plaster without aerogel content is applied. Thus, using insulation materials containing aerogel particles has a direct impact on how energy-efficient buildings are.

Since the thermal conductivity value drops by around 0.7 times, from 0.3172 W·m⁻¹·K⁻¹ to 0.2287 W·m⁻¹·K⁻¹, upon increasing the aerogel content in cement and lime plasters from 40% vol. to 80% vol., even going to a lower percentage of the aerogel, the effects on reducing energy consumption are significant. If these results are compared with values of thermal conductivities obtained by other researchers such as 0.45 W·m⁻¹·K⁻¹ [18], 0.4 W·m⁻¹·K⁻¹, and ≈0.2 W·m⁻¹·K⁻¹, respectively, for aerogel-incorporated plasters with 60% vol. and 70% vol., respectively, of silica aerogel [33,34] and 0.2365 W·m⁻¹·K⁻¹ for plaster with 30% vol. aerogel and 0.1488 W·m⁻¹·K⁻¹, respectively, for plaster with 36% vol. aerogel [35], it can be said that our own economic compositions of aerogel-incorporated plasters are promising and can be considered suitable for the façade application of buildings. Regarding the economic cost and considering the values of the thermal conductivities of the obtained silica aerogel-incorporated cement and lime plasters, the optimum aerogel content is 40% vol. in contrast to the results obtained by other researchers.

To improve the thermal conductivities of the obtained plasters with aerogel, other materials with low thermal conductivity should also be incorporated (for example, calcined clays) and one should improve the binding force between the aerogel and cement–lime matrix by adding surfactant materials. However, the amount of calcined clay that can replace the cement should be in a low percentage to not have a negative influence on the mechanical behaviours of plasters, especially in the case of clay rich in kaolin [36]. Another way to improve the thermal conductivities of plasters is the addition, together with aerogel particles, of other insulating aggregates [37], such as expanded polystyrene, and vermiculite, and by testing different storing and curing conditions of aerogel-incorporated cement–lime plasters.

3.4. Morphological Structure

Scanning electron microscopy (SEM) was used to examine the obtained samples of the cement–lime plasters with aerogel particles at the cement/lime/aerogel interface. This analysis also helps to identify any shrinkage around the aerogel particles or any degradations of the particles themselves. The used instrument was a Quanta Inspect F scanning electron microscope for a simultaneous morphological and topological analysis of the interfacial transition zone between the cement–lime plaster matrix and aerogel particles.

Sample S₃ which contains 60% aerogel (vol.% compared with sand reference sample S₀) in the cement–building lime plaster was used for the analysis and the obtained results are presented in Figure 7.

From the obtained images, it can be observed that there was an interfacial transition zone between the cement–lime plaster matrix and aerogel particles (Figure 7b), suggesting that the silica from the aerogel particles (Figure 7a) reacted with the solution of the cement–lime plaster pores and formed a calcium silicate hydrate. Additionally, aerogel particles are hydrophobic [18]. This can possibly be due to the mechanical mixing of the plaster where the aerogel particles interact with the cement–lime and water and where they are partially dissolved in the alkaline environment provided by the hydration of the clinker [38]. Additionally, a discontinuity between the cement–lime hydrates and high porosity of the sample (Figure 7b) was observed. This may be explained due to the formation of gap spacing between the binder matrix and aerogel particles (Figure 7c) where the amount of Si is large but the Ca/Si ratio from the pore solution is low.

4. Discussion

The incorporation of aerogel particles into the cement–lime matrix significantly impacts the overall properties of the resulting plasters. When the aerogel is added, many air-filled pores within the matrix disrupt the continuous phase of the cement–lime matrix, leading to a reduction in thermal conductivity by minimizing the pathways available for heat transfer. The nanoporous structure of aerogel particles effectively traps air, which is a poor conductor of heat, thereby enhancing the insulation properties.

Despite the thermal benefits, the inclusion of aerogel particles adversely affects the mechanical strengths of the cement plasters. The compressive and flexural strengths decrease with the increasing aerogel content, primarily due to the following reasons.

An insufficient amount of the binder. The replacement of sand with aerogel particles reduces the overall binder content available in the mixture. Since the aerogel does not contribute to the mechanical integrity in the same way as sand, this substitution leads to weaker bonds within the matrix.

Increased porosity. The high porosity introduced by aerogel particles results in a larger volume of voids within the plaster. These voids act as stress concentrators and weak points, which can lead to failure under mechanical loads. The inter-connected network of capillaries and voids filled with air increases the material’s susceptibility to cracking and deformation.

Inadequate particle bonding. The hydrophobic nature of aerogel particles can hinder their interaction with the hydrophilic cement–lime matrix. This lack of adequate bonding between the aerogel and cement particles further weakens the composite’s structure, contributing to its reduced mechanical strength. For this reason, the introduction of surfactants facilitates the improvement in aerogel particle bonding with the cement–lime matrix.

As highlighted by the experimental results, the flexural strength decreased from 0.73 MPa to 0.13 MPa as the aerogel content increased from 40% to 80%. Similarly, the compressive strength dropped from 2.69 MPa to 1.19 MPa over the same range of the aerogel content. These figures underline the significant differences in mechanical properties with higher aerogel incorporations. Additionally, the apparent mass density of the samples showed a clear decreasing trend upon increasing the aerogel content.

The thermal conductivity of cement–lime plasters is significantly improved with the incorporation of aerogel particles. The extremely low thermal conductivity of the aerogel, coupled with its high porosity, makes it an excellent insulating material. The experimental results indicate a substantial reduction in thermal conductivity upon increasing the aerogel content. Specifically, the thermal conductivity of the plaster with 80% aerogel content was found to be 0.2287 W·m⁻¹·K⁻¹, approximately 3.4 times lower than the thermal conductivity of traditional lightweight plasters, which is 0.76 W·m⁻¹·K⁻¹. This improvement is related to the following.

Enhanced air entrapment. The aerogel’s nanoporous structure effectively entraps air, reducing the heat transfer through conduction. Its small pore sizes limit the movement of air molecules, further diminishing its thermal conductivity.

Reduced solid phase connectivity. By introducing the aerogel, the connectivity of the solid phase within the plaster is disrupted. This reduction in solid-phase connectivity minimizes the pathways for heat transfer, leading to lower thermal conductivity.

Synergistic effects. The combination of the aerogel’s inherent thermal properties with the cement–lime matrix results in a synergistic effect, further enhancing the insulation capabilities of the plaster.

The experimental investigations suggest that the aerogel-incorporated cement–lime plasters are highly effective for applications requiring superior thermal insulation, making them suitable for use in energy-efficient building envelopes. The tendency shows that even with a lower percentage of the aerogel, significant thermal conductivity reduction can be achieved, as evidenced by the thermal conductivity dropping to 0.3172 W·m⁻¹·K⁻¹ at 40% aerogel content.

The scanning electron microscopy (SEM) analysis provided additional insights into the microstructural changes induced by aerogel incorporation. The images revealed an interfacial transition zone between the cement–lime plaster matrix and the aerogel particles, indicating that the silica from the aerogel reacted with the alkaline environment provided by the cement hydration process. This reaction forms calcium silicate hydrate (C-S-H) phases, which are crucial for the mechanical strengths of the plasters. However, due to the hydrophobic nature of the aerogel, the bonding at the interface was not uniform, leading to areas with high porosity and discontinuities. These morphological characteristics explain the observed reductions in the mechanical strengths.

The practical implications of these investigations are significant for the construction industry, particularly in the development of energy-efficient building materials. The use of aerogel-incorporated plasters can reduce the thickness of insulation layers needed for achieving desired thermal performances, thus saving space and potentially lowering construction costs. However, the challenge remains to balance the thermal performances with mechanical integrity.

5. Conclusions

Silica aerogel is one of the most promising components that can be used for the development of thermal insulation materials for energy-efficient buildings. In this study, a new thermal plaster incorporated with aerogel particles as lightweight aggregates were obtained and investigated. The results of the investigations confirm that is possible to obtain plasters with low economic costs which contain silica aerogel particles, 40% vol., and cheap cement–lime plaster with a low thermal conductivity of 0.3172 W·m⁻¹·K⁻¹, quasi-similar to the thermal conductivity of plasters with noneconomic compositions (0.2287 W·m⁻¹·K⁻¹). However, these plasters have thermal conductivities much lower than the thermal conductivity of traditional lightweight plasters, which is 0.76 W·m⁻¹·K⁻¹.

Regarding the microstructural analysis, it was possible to identify an interfacial transition zone between the cement–lime plaster matrix and aerogel particles due to the partial dissolution of the aerogel particles in the alkaline environment provided by the hydration of the clinker in the cement–lime plaster. High porosity and gap spacing between the binder matrix and aerogel particles were observed, which explained the physical–mechanical properties of the obtained plasters.

Concerning the mechanical properties, the obtained plasters have low compressive and flexural strengths, correlated with the aerogel/sand compositions, which can be improved by further mechanical protection. A positive aspect is that plaster shrinkage does not occur, indicating its ability to move with the support, and this is an essential characteristic if it is to be used for a building envelope.

As a future research endeavour, the obtained plasters with silica aerogel will be studied in a multilayer system for façade applications of buildings with thermal insulations in a climate environment that is constantly changing and with large temperature variations. The optimization of compositions to achieve an optimal balance between thermal performances and mechanical strengths will be investigated. Additionally, analysing alternative processing techniques and exploring the integration of additives could enhance the overall performance and applicability of aerogel-based plasters in diverse building applications. By addressing these challenges, it will be possible to fully enhance the benefits of aerogels as sustainable and efficient building materials.