Technical field

This application relates to fire-resistant core-shell alumina-silica nanoparticles , a method of producing the same and fire-resistant composite construction materials comprising the core-shell alumina-silica nanoparticles .

Background art

The applications of alumina nanoparticles are wide and diversi fied . One field that incorporates not only alumina nanoparticles (AI2O3 ) , but also silica ones ( Si02 ) is nano cement . The main goal of incorporating nanomaterials in cementitious materials is to improve their properties . Rashad ( 2013 ) and, more recently, Paul et al . ( 2018 ) published an extensive review of the literature on this application, including the incorporation of nano-alumina particles . They concluded that the existing papers approached mainly mechanical properties . Alonso-De la Garza et al . ( 2022 ) also studied the mechanical-physical properties of ceramic tiles with nano-alumina and nanosilica . Another important result was obtained by Nazari & Riahi ( 2011 ) , regarding the improved thermal properties of concrete containing alumina nanoparticles . To achieve a combined ef fect , nanoparticles composed of alumina and silica appear as a possibility . However, no study about the incorporation of core-shell alumina-silica particles in cement was found in the literature . Regarding alumina nanoparticles, they can exhibit different properties depending on the dominant phase - gamma (y) , alpha (a) or hydrated alumina. The latter can be very interesting in terms of flame retardancy, due to the dehydration process during heating. Many studies have been made covering different methods of synthesis of nanoparticles, such as mechanical methods, precipitation, vapor deposition, combustion, and sol-gel method. Tabesh et al. (2017) studied the use of citric acid and triethanolamine as chelating agents and ethylene glycol as gel agent in the sol-gel route. Aluminium nitrate nonahydrated was used as a precursor. Different reagent proportions were considered, and it was concluded that it influences directly on the size and form of the obtained particles. The synthesis with triethanolamine (TEA) alone leads to semi-spherical smaller particles of 80 nm, since the use of citric acid led to increased size, while ethylene glycol together with citric acid led to agglomeration. It is important to notice that, in this case, TEA also acts as a gel agent. Rajaeiyan & Bagheri-Mohagheghi (2013) also studied the sol-gel route, with the same reagents as Tabesh and coworkers but changing the gel agent to 1 , 4-butandiol and ethanol. This procedure involved more complex steps and was compared to a precipitation route of synthesis. They observed that the sol-gel particles had an elongated shape and uniform distribution, were more spherical and smaller than particles obtained by precipitation and had size distribution of 10- 20 nm. Mohammed et al. (2020) conducted a simple route with only aluminium precursor and ethanol, obtaining a wide size distribution of particles, with different shapes. Mirjalili et al. (2011) , on the other hand, tested the use of surfactants (sodium bis-2ethylhexyl sulfosuccinate - Na(AOT) , and sodium dodecylbenzen sulfonate - SDBS) in the synthesis and compared the use of aluminium nitrate nonahydrated and aluminium isopropoxide as precursors. They concluded that the presence of surfactants controls the size of the particles, their degree of aggregation, and their shape. The use of more surfactant leads to less aggregation and, consequently, particles of smaller size. A few studies have reported the coating of silica nanoparticles with alumina, as by Koroleva et al. (2017) for biomedical applications. There have also been some reports on the coating of alumina particles with silica. Busse et al. (2011) have synthesized this kind of particles to reduce friction in acrylnitril butadiene copolymers, using a sol-gel procedure with a silica liquid precursor polymer, hyperbranched polyalkoxysiloxane (BAGS) , in an elastomer mixture. On the other hand, Nithiyanantham et al. (2019) have studied the effect of adding silica-coated-alumina particles to the thermal conductivity of nanofluids, for thermal energy storage applications. The silica shell was incorporated in commercially available alumina nanoparticles using a wet chemical route. Wang et al. (2005) proposed a sol-gel route for coating ultrafine a-alumina particles but did not address a clear application. Siddiquey et al. (2011) also proposed a method based on microwave irradiation for coating alumina particles. The coating of other types of nanoparticles with a silica shell has also been attempted. For instance, Vaz-Ramos et al. (2020) proposed an effective sol-gel route for the coating of a superparamagnetic iron oxide core. Nevertheless, these studies propose uses and synthesis methods different from the present application. In addition, in all cases, the silica shell was not stated as mesoporous, which is a crucial point of the present innovation. Moreover, in the present invention the obtaining of aluminium oxy-hydroxide nanoparticles, instead of aluminium oxide , is favoured by the very low temperature and lack of the calcination stage . The aluminium hydroxides are known as very good heat absorbers , and they are even used as fire retardants due to that feature .

Summary

The present invention relates to fire-resistant core-shell alumina-silica nanoparticles comprising a mesoporous silica shell . These nanoparticles are suitable to be incorporated in fire-resistant composite construction materials and make them resistant to fire .

General description

The present application relates to the sol-gel synthesis of fire-resistant core-shell alumina-silica nanoparticles , with a mesoporous silica shell , that are suitable to be incorporated in composite construction materials ( such as , cementitious , metakaolin, gypsum-based construction materials ) , to enhance their thermal properties , so they can be used as fire protection . The innovation of the present work relies not only on the incorporation of the mesoporous silica shell in alumina nanoparticles , with a simple and scalable sol-gel route procedure , but also on the aim of applying them in composite construction materials to enhance their properties .

The sol-gel route is ideal to produce the presently disclosed nanoparticles because of its simplicity and ef fectiveness . The sol-gel route is based on using an aluminium salt as precursor, a chelating agent , and a solvent , in this case water . The precursor is the source of aluminium, while the chelating agent is used to reduce the crystal growth in solution since it forms chelates with the aluminium cation in the centre . Therefore , smaller particles can be obtained . A gel agent can also be used i f the formation of a gel is expected as an intermediate step . Usually, the hydrolysis and olation/oxolation reactions may occur at di f ferent pH levels and/or stages of temperature , and, at the end, calcination may be carried out to study the phase transition, for instance to obtain a-alumina at about 1200 ° C . The formation of the silica coating on the alumina particles is favoured by the presence of hydroxyl groups on the alumina phase , which condense with the silanol groups formed by hydrolysis of the silica precursors . In this case a surfactant can also be added to promote the formation of a porous shell .

Brief description of drawings

For easier understanding of this application, figures are attached in the annex that represent the preferred forms of implementation which nevertheless are not intended to limit the technique disclosed herein .

Figure 1 show scanning electron microscope ( SEM) images of the alumina nanoparticles dried at ambient pressure .

Figure 2 shows SEM images of the core-shell alumina-silica nanoparticles dried ( a ) at ambient pressure and (b ) by freeze drying .

Figure 3 shows the weight loss pro files for alumina and coreshell alumina-silica nanoparticles up to 1000 ° C .

Figure 4 shows the DRX patterns of the ( a ) hydrated alumina nanoparticles and (b ) core-shell alumina-silica nanoparticles . Detailed description of embodiments

The present application relates to the sol-gel synthesis of fire-resistant core-shell alumina-silica nanoparticles , with a mesoporous silica shell . The presently disclosed nanoparticles are suitable to be incorporated in composite construction materials in order to improve their properties , particularly against fire , since i ) the nano-silica and nanoalumina improve the physical properties materials , such as cementitious mortars , as a result of the high poz zolanic activity, filling and nucleating ef fects of nanoparticles ; as well as ii ) they allow to reduce the air percolation through the material ; and iii ) the dehydration of these particles during temperature increase acts as a heat sink .

The nanoparticles are constituted by an aluminium oxyhydroxide core and a mesoporous silica shell .

In one embodiment , the fire-resi stant core-shell alumina- silica nanoparticles have a si ze between 50 and 500 nm .

Optionally, the fire-resistant core-shell alumina-silica nanoparticles can have a dense silica layer of a si ze of at least 50nm between the alumina core and the mesoporous shell .

The alumina core of the core-shell alumina-silica nanoparticles has a cylindrical shape . The mesoporous silica shell is made of spherical nanoparticles with a si ze of less than 50 nm, that cover the alumina core nanoparticles .

The mass ratio of aluminium and silicon in the core-shell alumina-silica nanoparticles vary between 0 . 20 and 3 . 85 . The present application also relates to a method of producing the core-shell alumina-silica nanoparticles comprising the following steps:

Dissolving aluminium nitrate nonahydrate between 0.005 and 0.015 mol in between 25 and 75 mL of water under stirring at a temperature between 60 and 90°C;

Adding between 2.0 and 4.0 g of triethanolamine or citric acid;

Stirring the mixture in a covered flask between 1 and 4 hours at a temperature between 60 and 90°C;

Centrifuging the mixture for a time between 15 to 45 minutes, discarding the liquid phase and resuspending the solid phase in ultrapure water;

Repeating the previous step between one and five times;

Drying the solution obtained between 15 and 24 hours at a temperature between 60 and 80°C to obtain hydrated alumina nanoparticles ;

Dispersing between 10 and 15 mg of dried alumina nanoparticles, obtained in the previous step, in between 10 and 15 mL of water or directly diluting between 0.7 and 0.9 mL of the alumina nanoparticles suspension, obtained before the drying step, in between 10 and 15 mL of water;

Stirring and sonicating the previous suspension for a time between 3 and 30 minutes and transferring it to a flask;

Preparing a hexadecyl-trimethylammonium bromide (CTAB) solution of concentration between 0.1 and 0.2 M at a temperature between 30 and 50°C;

Adding between 3 and 5 mL of the CTAB solution to the flask comprising the alumina nanoparticles;

Adding to the previous solution between 0.2 and 0.3 mL of a sodium hydroxide solution with concentration between 1.5 and 2 M and between 25 and 30 mL of water; Heating the previous mixture in an oil bath until reaching a temperature between 50 and 70 ° C ;

Adding between 0 . 5 and 1 mL of tetraethyl orthosilicate ( TEOS ) and between 2 and 3 mL of ethyl acetate to the mixture ;

Carrying out the reaction for a time between 2 and 4 hours under magnetic stirring and reflux conditions ;

Cooling the solution obtained at a temperature between 25 and 30 ° C ;

Centri fuging the mixture for a time between 15 to 45 minutes , discarding the liquid phase and resuspending the solid phase in ethanol ;

Repeating the previous step between one and five times ;

Adding the previously obtained suspension to a solution of ammonium nitrate in ethanol , with a concentration between 0 . 05 and 0 . 1 M to remove CTAB from the nanoparticles ;

Stirring the previous mixture between 20 and 45 minutes at a temperature between 50 and 70 ° C to obtain the coreshell alumina-silica nanoparticles ;

Centri fuging the mixture for a time between 15 to 45 minutes , discarding the liquid phase and resuspending the solid phase in ethanol ;

Repeating the previous step between one and five times with the last dispersion being in water ;

Drying the suspension obtained between 15 and 24 hours at a temperature between 60 and 80 ° C, or by freeze drying between 60 to 72 hours to obtain the core-shell alumina- silica nanoparticles .

In one embodiment , a dense layer o f silica can be introduced between the alumina core and the mesoporous silica shell with an optional step, prior to the reaction with TEOS and CTAB, TEOS and ammonium hydroxide are added to the alumina nanoparticles suspension, with ethanol as solvent . The present invention also relates to a fire-resistant composite construction material comprising the fire- resistant core-shell alumina-silica nanoparticles herein described .

In one embodiment, the fire-resistant composite construction material is selected from mortar or concrete made from cement, or gypsum or calcium silicate or metakaolin or similar binders, or their mixtures.

In one embodiment, the fire-resistant composite construction material comprises between 0.5 and 3% (w/w) of fire-resistant core-shell alumina-silica nanoparticles.

Examples

1. Materials

For the synthesis of the core-shell nanoparticles, aluminium nitrate nonahydrate (Al (NO3) 3.9H2O, 99+%, Acros Organics) , triethanolamine (TEA, >= 97%, Fluka Chemika) , ethanol (C2H6O, 99%, Jose Manuel Gomes dos Santos, LDA) , hexadecyltrimethylammonium bromide (CTAB, CH3 (CH2) 15N (Br) (CHsH, >99%, Acros Organics) , sodium hydroxide (NaOH, 98.7%, Fluka Analytical) , ethyl acetate (CH3COOC2H5, >= 99.8%, Fisher Chemical) , tetraethyl orthosilicate (TEOS, Si (OC2Hs)4, 98%, Acros Organics) , and ammonium nitrate (NH4NO3, > 99%, PanReac AppliChem ITW Reagents) were used as purchased, without additional purification. Ultrapure and distilled water was used throughout the syntheses.

2. Synthesis of alumina nanoparticles

Aluminium nitrate nonahydrate, triethanolamine (TEA) and water were used as starting materials. Initially, 0.005 mol of aluminium nitrate nonahydrate was dissolved in water under magnetic stirring. The solution was heated in a hot plate until the reaction temperature was reached, and when the solution was completely transparent, TEA was added slowly. After that, the beaker was kept closed to avoid solvent evaporation. The reaction was carried for 2 hours, at the selected reaction temperature and under magnetic stirring. The quantities of water and TEA used, as well as the reaction temperature were varied between the experiments, according to a design of experiments procedure. The suspension obtained was partially diluted with ultrapure water and centrifuged for 30 minutes at 8500 rpm. After that, the liquid phase was discarded, and the solid phase was redispersed in ultrapure water, for washing. The suspension was centrifuged for 30 min at 8500 rpm and the liquid was discarded. This washing procedure was repeated three times. After the last centrifugation, the solid phase was redispersed in ultrapure water, aiming a high concentration. For Scanning Electron Microscopy coupled with Energy Dispersive X-ray Spectroscopy (SEM-EDS) , Simultaneous Differential Thermal (SDT) and X-ray Diffraction (XRD) analysis, the high-concentration suspension was dried at the oven for 17 hours, at 60 °C and the dried particles were ground.

2.1 Design of Experiments (DOE)

The design of experiments analysis was performed with three factors: volume of water, mass of TEA and reaction temperature. A full factorial design with two levels, being + the highest and - the lowest, and one central point was planned. The central point was synthetized in triplicate, in order to evaluate the experimental variability. Table 1 shows the values of the factors chosen for each level and the central point, based on previous literature research and experiments. Therefore, the experiment consisted of 11 runs: 2 ³ runs for the extreme levels and three runs for the central point .

Table 1. Factors and central point for DOE.

- 0 +

A Water / mL 25 50 75

B TEA / g 2.78 3.08 3.38

C Reaction temperature / °C 70 80 90

The particle size and stability were chosen as responses to be analysed in the DOE. For the average particle diameter, Dynamic Light Scattering (DLS) was used, while zeta potential was measured for stability evaluation. Both responses were assessed using the equipment Zetasizer Nano ZS from Malvern Instruments. Before measuring particle size, the samples were sonicated for 3 minutes. The DOE results were analysed with JMP Pro software.

3. Particle coating with a mesoporous silica shell by sol- gel reaction

The alumina nanoparticles used for the core-shell nanoparticles were synthetized with the optimized conditions obtained from the DOE. An equivalent of 12 mg of dried alumina nanoparticles were dispersed in 15 mL of water. The suspension was stirred and sonicated for 5 minutes. It was then transferred to an adequate flask. An aqueous solution of CTAB (0.11 M) was prepared at 35 °C, and 5 mL of this solution were transferred to the flask with the suspension. Under magnetic stirring, 0.3 mL of 2 M NaOH aqueous solution and 30 mL of water were added. The mixture was heated in an oil bath until 60 °C. When this temperature was reached, 1.0 mL of TECS and 3 mL of ethyl acetate were added to the mixture. The reaction was carried for 2 hours, under magnetic stirring and reflux conditions. After cooling to a temperature between 25 and 30°C, the suspension was centrifugated for 20 min at 8500 rpm. In order to wash the obtained particles, they were redispersed in ethanol and centrifugated again in the same conditions. This washing procedure was repeated three times. After the last centrifugation, the particles were redispersed in 30 mL of ethanol. In order to remove the surfactant, the suspension was added to a NH4NO3 aqueous solution, prepared by dissolving 160 mg of NH4NO3 in 30 mL of ethanol. The mixture was left under magnetic stirring for 30 minutes at 60 °C. The obtained particles were again centrifugated and washed with ethanol, following the same procedure as before. After the last centrifugation, the particles were redispersed in water, for DLS and zeta potential measurements. Two different drying procedures were used: freeze drying and ambient pressure drying. In the first procedure, the particles were frozen at -80 °C and them left under vacuum for 72h to dry. In the second case the particles were dried in the oven during the day at 80 °C and remained at 60 °C over the night. The dried particles were characterized with SEM-EDS and SDT analysis. Before characterization with XRD, the particles were heated at a rate of 10 °C/min, up to 800 °C, and remained at this temperature for 1 hour.

4. Incorporation of the particles in mortars

4.1 Product composition

The mortar used as passive fire protection consists of gypsum powder, expanded perlite with dimensions between 0.025 to 0.40 mm, core-shell alumina-silica nanoparticles and water. The proportion of these materials by volume is in the following percentages: 40% gypsum powder, 60% expanded perlite. The percentage of core-shell alumina-silica nanoparticles was 1.0% of gypsum powder (in weight) . The amount of water used in the production of this mortar corresponds to a water/gypsum powder ratio equal to 1.21 (in weight) .

4.2 Manufacturing procedure and application

The materials used in the preparation of a mortar were kept in a dry place with controlled temperature (25 °C) and a relative humidity (RH) of 55%. Manufacturing procedure required the orderly realization of the following steps:

1. The raw materials were weighed and placed inside a mixer container;

2. A vertical shaft mixer was put into operation for 5 min at a slow speed (136 rotations per minute) . At the same time, the corresponding amount of water was added, with a constant flow rate (8-10 ml/s) to ensure the homogeneous addition of water in the whole mortar. The added water contained the diluted core-shell alumina-silica nanoparticles, with a concentration of 0.116(6) g/ml;

3. After this procedure, portions of the mortar that were on the walls of the container were removed and thus the mixture was homogenized, putting them back in the mixer for another 2 minutes;

4. At the end of this procedure, the mortar was ready to be used .

In one example of use, in the case of metallic members (stainless-steel, carbon steel and aluminium columns and beams) with constant cross-section, the guarantee that the mortar thickness (10-40mm) applied along the metallic members was constant and uniform was given by using a modular formwork with spacers that allowed the vertical concreting of the metallic members. 5. Experimental results

Fig. 1 shows the SEM image of the alumina nanoparticles (central point from the DOE) . They have a cylindrical shape and a uniform distribution of sizes, with an equivalent spherical diameter varying between 200 and 500 nm. The weight percentages determined with EDS varied between 51.0-56.6 % of oxygen and 43.4-49.0 % of aluminium. The correspondent molar proportion indicates the presence of aluminium oxyhydroxide in the sample, besides aluminium oxide. In the DOE experiments, the hydrodynamic average diameter of the alumina nanoparticles varied between 220-360 nm. Regarding stability, all samples presented positive surface charge and were considered stable in water, with a zeta potential between 30-50 mV. With JMP Pro, a standard least squares fit was used to obtain a model. Since the zeta potential presented desirable values for every experiment, only 25% of relevance was designated for this response, while for size it was attributed 75% of importance. The goal is to minimize the size and, at the same time, maximize the zeta potential.

It was concluded that the reaction temperature is the factor that has the greater influence in both responses. Although the method performs well across the disclosed parameter ranges, the preferred synthesis conditions determined were 75 mL of water, 2.95 g of TEA and reaction temperature of 90 °C.

Regarding the core-shell alumina-silica nanoparticles, the average hydrodynamic diameter measured with DLS was equal to 450 nm. The zeta potential was equal to -32.1 mV. In Fig. 2 it is possible to visualize the influence of the drying procedure in the particles. It can be observed that the mesoporous silica shell consists of spheres , smaller than 50 nm, that cover the aluminium particles . They form aggregates that resemble bigger particles , of 50-500 nm . These aggregates are the ones identi fied in the DLS analysis as particles , although they are each formed by several nanoparticles . The presence of the silica nanoparticles is clearer in the sample dried by freeze drying and the silica shell is also more porous . In the sample dried at ambient pressure , the nanoparticles are more condensed, since they are subj ected to higher temperatures and therefore more drastic drying conditions . The evaporation of the solvent also develops capillary stresses that tend to agglomerate the nanoparticles , providing conditions for their chemical bonding through silanol groups condensation .

The results obtained from the EDS analysis are shown in Table 2 . It can be observed that there i s a variability regarding the ratio of silicon and aluminium within the samples , as well as comparing samples from di f ferent drying procedures . For the same sample , the percentages vary due to the di f ferent areas selected for each spectrum, which can be a region with more thick coating or not and/or small or larger alumina particles incorporated in the core . A variability is also observed comparing the di f ferent samples , which can be attributed once again to variations in the incorporation of the silica shell in the alumina particles , even with the same synthesis conditions . The above-mentioned variability was also confirmed in the final mass obtained of the coreshell alumina-si lica nanoparticles , varying between 10 and 50 mg, which can also be attributed to the amount o f silica coating that is achieved in each synthesis . Table 2. EDS weight percentages of the core-shell alumina- silica nanoparticles dried at ambient pressure and by freeze drying, for different spectra.

Weight (%)

0 Al Si

52.0 30.3 17.7

60.2 31.6 8.2

Ambient pressure drying

53.7 29.9 16.5

50.9 37.1 11.9

49.1 8.3 42.6

. 49.0 14.9 36.1

Freeze drying

49.6 31.2 19.2

50.2 22.1 27.7

The results for the SDT analysis are shown in Table 3 and Fig. 3. Both samples, hydrated alumina nanoparticles and alumina-silica nanoparticles, presented two main endothermic events. The first thermal event corresponds to the loss of moisture adsorbed in the samples, while the second one can be attributed to the loss of bonded hydroxyl groups and phase transition of aluminium oxyhydroxides. The total weight loss for the alumina nanoparticles was equal to 36.01 % up to 1000 °C, while for the core-shell nanoparticles it was equal to 28.57 %. The weight loss is lower for the core-shell particles, since the sample also contains silica, decreasing the percentage of alumina in the system. From the second step of mass loss in the alumina sample, corresponding to 15.8 %, it is possible to conclude that the existing alumina phase is mainly A10(0H) , in accordance with the conclusion drawn from the EDS analysis. This mass loss step corresponds to the following thermal event: Table 3. Temperature and weight loss for each thermal event with SDT analysis, for the hydrated alumina nanoparticles and core-shell alumina-silica nanoparticles.

Tonset Tend Weight loss

(°C) (°C) (%)

44.8 81.2 14.1

Alumina

277.7 412.3 15.8

Core-shell alumina 45.9 84.3 10.8 s ^ilica 314.0 442.1 10.0

The XRD patterns for the (a) hydrated alumina nanoparticles and (b) alumina-silica core-shell nanoparticles are shown in Fig. 4. The former (a) confirms the presence of boehmite (y- A10(0H) ) in the sample, since the obtained pattern matches the peaks expected for this phase. The crystallite size was determined with Scherrer Equation for the most intense peak, resulting in 2.0 nm. This small size indicates that the cylinders observed in the SEM image (Fig. 1) can be, in fact, aggregates of smaller particles.

In order to simulate the extreme fire conditions that the final construction material will be subjected, the coreshell nanoparticles were calcinated at 800 °C. The XRD pattern obtained for these particles shows the presence of an alumina phase (y-A12O3) , which confirms the dehydration of boehmite, and a lifting effect of the pattern in the smaller angles due to the presence of the amorphous silica (Fig. 4 (b) ) . However, the indexation with such large peaks is only indicative. The crystallite size was equal to 0.74 nm considering the first wide peak. These crystallites belong to the primary silica nanoparticles that form the secondary silica nanoparticles observed in the SEM images. Thermal conductivity and specific heat are the most important thermal properties in the development of mortars with good thermal performance. In turn, these three thermal parameters are strongly influenced by the density and porosity of the mortars. The results obtained by mercury intrusion porosimetry test (ISO/DIS 15901-1, 2014) indicated that this new composite material has a porosity of 56% and a density of 1080.4 kg/m ³ . Regarding the thermal properties, the results obtained using The Transient Plane Heat method indicated that: at ambient temperature, the thermal conductivity was 0.296 W/mK and the specific heat 1.00 J/kgK; and at 250 °C, it was 0.289 W/mK and 1.03 J/kgK, respectively .

This description is of course not in any way restricted to the forms of implementation presented herein and any person with an average knowledge of the area can provide many possibilities for modification thereof without departing from the general idea as defined by the claims. The preferred forms of implementation described above can obviously be combined with each other. The following claims further define the preferred forms of implementation.