Fly ash was obtained from traditional pulverized coal combustion. Portland cement, vermiculite and polypropylene fibers were also used.

We used type II Portland cement (PCII) with 32.5 MPa compressive strength according to EN 197-1 (9). We used PCII because it was the most economically efficient cement. Although its mechanical properties were below 42.5 MPa, according to a previous study (8), mechanical properties of materials with 32.5 MPa cement are appropriate.

Vermiculite is a hydrated silicate that contains iron, magnesium and aluminium and has very low specific density (0.15 g/cm³). Materials used to manufacture sound absorbing devices usually contain vermiculite (10, 11). The vermiculite used in this study was a commercial product (VERLITE) manufactured in Asturias, Spain, with 85 wt.% and particle size below 1.4 mm. Polypropylene fibers range from 20 to 50 µm in diameter and 40 mm in length. They were added to enhance flexural strength (12).

Table 1 shows the chemical composition, specific gravity and loss of ignition (LOI) of FA and PCII. The chemical composition was determined according to the ASTM D3682-13 standard test method (13). The main components in FA are SiO₂, Al₂O₃ and Fe₂O₃. FA can be classified as an F-type ash in accordance to ASTM C 618-17 (14) that has a low content of K₂O; MgO, Na₂O and CaO.

Specific gravity was determined according to the EN 1097-7 standard (15). The specific gravity of the fly ash (2.7) was lower than that of PCII (3.1).

We performed a leaching test of the fly ash in accordance with EN 12457-4 (16) to determine any risk to the environment or human health due to the leaching of heavy metals. We compared the results obtained (Table 1) with the European Landfill Directive (17). The Directive establishes three different categories: inert, hazardous or non-hazardous. As shown on Table 1, FA can be classified as non-hazardous waste, with molybdenum content slightly above the inert waste limit. Note that this leaching test reproduces unfavorable conditions but does not adequately represent the behavior of the final construction material.

Figure 1 shows the particle size distribution of FA, where most of the particles had a size ranging between 0.2 and 100 μm, although the prevalent particle size ranged between 5 and 30 μm. Fly ash had a slightly lower particle size than PCII.

Previous studies (8, 11) have determined the optimal fly ash/cement/vermiculite ratio to develop a material with high sound absorption - by using an impedance tube - and acceptable compressive strength using the highest possible amount of fly ash. In this study, we built a semi-industrial scale prototype based on the best dosage previously determined in (8) and tested it in an accredited laboratory. We also characterized other acoustic and non-acoustic parameters in much greater detail.

We concluded that 60 wt.% of fly ash is appropriate for developing an adequate sound absorbing porous material. We also used 14.5 wt.% expanded vermiculite to increase open porosity. We added a dosage of 0.5 wt.% polypropylene fibers to increase the mechanical properties based on a previous study (18). The composition of the barrier is presented in wt.% in Table 2. However, the same values are very different when they are expressed in v/v% because of the specific densities of their components. Vermiculite (% v/v) = 76.1; FA (% v/v) = 17.2; PCII (% v/v) = 6%. The purpose of showing the values in wt.% rather than v/v% in Table 2 was to show the mixing dosage and allow researchers to reproduce these results.

The solid components (i.e., fly ash, cement, vermiculite and fibers) were added in a vertical mixer and were rotated for 5 minutes. Next, water was poured into the mixer and rotated for another 15 minutes.

Due to the different particle size distribution of cement, fly ash and vermiculite, the mixture could be considered a mortar. This mortar was used to cast test specimens of different sizes and shapes for the different acoustic, mechanical and fire tests. They were demolded 24 hours later and cured for 27 days (relative humidity: 45%, temperature: 20 ºC). The intention was to manufacture the panels in a similar way as they are made in situ on the road. Higher relative humidity (e.g., submerging the samples in a water bath) could yield better mechanical properties but did not influence the acoustic properties of the barrier.

The barrier characterization (non-acoustic and acoustic) was conducted in compliance with the European standards for road traffic noise reducing devices. The legislation framework comprises EN 1793-1 (19) for acoustic characterization, and EN 1794-1 (20), EN 1794-2 (21) and EN 1794-3 (22) for non-acoustic properties. EN 14388 (23) shows the different requirements for these tests. The results were also compared to the properties of other construction materials commonly used in the same applications.

The sound absorption coefficient complied with EN ISO 354 (24) in the reverberation room. The test was performed in the INERCO-ACÚSTICA laboratory at 14 ºC, in a room with a volume of 150 m³, with a relative humidity of 32% and a static pressure of 1 atm. The standard requires a minimum surface area of 10 m². This requirement was adjusted to 3.08 m² due to the limitations of the research methodology. Small samples and/or reverberation chambers are not new at all and there is great interest in them, despite the fact that they are not standardized (25, 26). Nevertheless, the room volume provides a diffuse sound field. Additionally, a previous study (25) showed a coefficient of determination of essays with limitations similar to those of our study versus standardized size, showing a coefficient of determination very close to 1 at frequencies higher than 400 Hz. Consequently, it was determined that the results could be used for comparison purposes.

The acoustic absorption assessment index (DL_α) was determined according to EN 1793-1 (19) from the sound absorption coefficient.

The transmission loss index (R) was calculated using the airborne sound insulation test, as defined in EN 10140-2 (27) by the INASEL authorized laboratory. The data were the following: air temperature 17^αC, static pressure 1 atm, relative humidity 36%, barrier surface 3.08 m², receiver room volume 66.2 m³ and transmitter room volume 57.4 m³. The transmission loss assessment index (DL_R) was determined according to EN 1793-2 (28).

The mechanical characteristics were determined experimentally. Compressive tests were carried out according to (29) using 50 × 50 × 50 mm-sized cubes. All the cubes were made of the same matrix to avoid any differences in the properties of the material. The load was progressively increased at a rate of approximately 0.5 MPa/s until the specimen was completely ruptured. Flexural tests were carried out according to (30) using 40 x 40 x 160 mm samples. Impact strength tests were carried out according to (31) and superficial hardness tests were carried out according to (32), both on panels of 160 x160 x 40 mm.

The open void ratio (VR) was determined since it is strongly related to the acoustic behavior of materials. A vacuum water saturation method was followed (33). Samples were previously dried at 105 ºC in a furnace. Next, they were weighed (S1) and submerged in water in a vacuum vessel until saturation. Subsequently, they were lifted out of the water and weighed (S2). The open void ratio was determined as VR (%) = WV/SV·100, where WV is the water volume and SV is the total volume of the sample. Water volume can be calculated as WV=(S2-S1)/ρ_w, wherein ρ_w is the water density. The density was determined by weight (S1) and volume (dimensions) of the samples. The pH was measured as defined in EN 12859 (32). Five grams of barrier were extracted and left in contact with water in a mass ratio of 10/1 (water/solid). The pH was measured after 5 minutes. The moisture content (MC) was determined according to EN 12859 (32). The mass of the barrier at ambient temperature (C1) and after heating at 40 ºC until reaching a constant mass (C2) were determined. The MC value is MC=100·(C2-C1)/C1.

Four different samples were tested for physical and mechanical characterization.

According to the environmental and general safety regulations defined in EN 1794-2 (22), the resistance of a noise reducing device must be measured in the case of a fire in the brushwood nearby. Hence, a fire test was carried out according to EN 1363-1 (34). For the fire test, small panels (270 × 320 mm and 20 and 40 mm thick) were prepared. These two thicknesses were chosen (40 mm was the maximum) because the size of the furnace for the fire test did not make it possible to test panels of 120 mm in thickness.

As regards environmental safety, Table 2 shows that fly ash can be classified as non-hazardous waste and therefore that its use in acoustic barriers is not harmful to people or the environment Nevertheless, the final material was subjected to the NEN 7345 tank leaching test (35). This is a monolithic leaching test that makes it possible to assess the leaching of heavy metals from the final material and not only from the raw materials separately. This is important when wastes are mixed with other solid materials like cement, because a solidification/stabilization process occurs. The main leaching factor in construction materials is rain, which mainly affects the outer surface of the construction material. One of the advantages of this test is that it simulates rain conditions.

Physical properties are shown in Table 3. As observed, open porosity was high, although porous concretes, used as sound absorbing materials (composed only of cement and coarse aggregates) can reach 40% (36).

As observed in Figure 2, the porosity of the FA barrier presented a flaky structure due to the effect of the vermiculite and its pore size between 1 and 3 mm, while porous concretes show values between 3 and 10 mm of pore size according to a previous study that used X-ray computed tomography (37). Open porosity is strongly dependent on particle size and increases when a larger aggregate is used. Typical porous concrete (with coarse aggregates between 5-10 mm) has predominant pore sizes above 7 mm with a total porosity of 46%, while FA barriers (with vermiculite particle size lower than 1.14 mm) have pore sizes ranging mainly between 2 and 4 mm and a total porosity of 40%.

According to EN 12859, the barrier was classified as medium density (between 800 and 1100 kg/m³). The density of the FA barrier was lower than that of a porous concrete commonly used to build road traffic noise reducing devices (900-1100 kg/m³) (36). Despite this, the specific density of the fly ash and vermiculite was lower than that of cement and coarse aggregates (2.7 g/cm³), resulting in a material with lower density. According to EN 12859, the barrier was classified as having normal pH (in the range between 6.5 and 10.5), which was lower than that of other barriers composed mainly of wastes (36). Moisture (M) was higher than in gypsum barriers (18) or other porous concretes (36). This was due to the fact that the water was located in the flaky structure of the FA barrier, while the larger pores of porous concrete cannot store much water.

Table 3 presents the mechanical properties of the barrier. Compressive and flexural strengths were low because of the porous and flaky nature of the barrier, which reduced the cohesion between the fly ash, cement and vermiculite. Other porous concretes with wastes have a compressive strength between 5 and 10 MPa (37). Flexural strength was higher than 0.6 MPa, the minimum level established for gypsum barriers (32), and higher than that of other porous concretes containing wastes (5, 36). The addition of fibers increased flexural strength due to its bridge effect, but this did not have a significant influence on compressive strength (18).

EN 1794-2 (21) sets the requirement of the strength of noise reducing devices to withstand the impact if stones thrown from the road. It establishes that barriers must resist the impact of stones. The impact resistance of the FA barrier was higher than that of other recycled porous barriers, in which the diameter of the ball mark was around 20 mm (36).

Superficial hardness was low compared to the values of other barriers made with similar dosages of wastes (>60 Shore C) and was below the limits established for other types of barriers (non-acoustic applications: >55 Shore C for densities between 800 and 1000 kg/m³) (37). The low superficial hardness was due to the effect of the expanded vermiculite, which is a highly elastic material and therefore reduces superficial hardness (38).

Figure 3 shows the sound absorption coefficient from the reverberation room test. The absorption coefficient was low for frequency bands below 200 Hz and reached 0.7 at 4000 Hz. This trend is comparable to the results shown by other researchers in noise reducing devices made from other wastes and with similar physical properties (24, 37).

The regulation establishes that DL_α must be calculated according to (19), according to which the barrier was classified in Category A (see Table 4). DL_α was 4.1 dB, leading to the classification of the material as A2. Porous concretes have a slightly higher DL_α, but they are classified in the same category (39, 40). Sound absorption is generally linked to the energy losses produced by the friction of sound waves in the wall holes. Thus, materials with a low open void ratio usually have a low sound absorption coefficient at different frequencies. However, the flaky structure of the FA barrier enables it to retain small air particles that absorb noise and reduce the reverberation time in a wide range of frequencies, increasing total absorption despite its low open porosity (41).

In addition, R was determined by adapting EN 1793-2 (28). Figure 3 presents the transmission loss of the FA barrier at different frequencies. From the transmission loss index, DL_R can be obtained in accordance with the standard established in (28); this led to the classification of the product in Category B (see Table 4). The DL_R of the fly ash-based product was 29 dB, so the product was classified in Category B3. Commercial concrete barriers usually consist of a combination of two layers: one made of porous concrete (50-100 mm of thickness), which absorbs the noise, and a hard-backing layer of non-porous concrete (50-150 mm of thickness), which increases the transmission loss index. In this case, the FA barrier is composed of a single layer, which makes its manufacture easier. Nonetheless, the FA barrier belongs to the same category as porous concrete noise barriers (40, 41).

Figure 4 shows the change of temperature versus time on the non-exposed surface of the barrier when the reverse is bearing the fire. Fire resistance can be defined as the time necessary to reach 180 ºC on the surface that is not exposed to the fire. This value was 22.4 and 34.2 min for barriers 20 and 40 mm thick, respectively. The duration of the evaporation plateau is the main factor that determines fire resistance. It is the time during which the temperature of the surface not exposed to fire remains constant at a temperature slightly below 100 °C. This is because porous construction materials have a high water retention capacity (7, 42). Fire resistance was lower than in other barriers using gypsum as binder, due to the high chemically bound water included in the gypsum (43), but such barriers cannot be used in outdoor environments because they have durability problems. However, it was higher than in other typical porous concretes used in sound absorbing barriers (34), because the flaky structure contains more water than the coarse aggregates of traditional porous concretes. An increase of the thickness led to a higher fire resistance. Hence, a barrier 120 mm thick would have about 110 min of fire resistance (44). In addition, no smoke was emitted from the barrier at any time during the fire test.

After the fire test, we analyzed some physical and mechanical properties of the FA barrier (Table 5). The mechanical properties after the fire test decreased as a result of mass loss during the fire. This loss is related to the free (humidity) and chemically bound water of the mix, which increases porosity and reduces mechanical strength. The decrease of superficial hardness was lower on the non-exposed surface, because the temperatures were lower than those on the exposed surface.

The introduction section of the EN 1794-2 (21) reads as follows: “While performing their primary function, road traffic noise reducing devices should not pose hazards to road users or other people in the vicinity or to the environment at large”. It also states: “They (the noise reducing devices) should be made from materials which do not emit noxious fumes or leachates as the result of natural or industrial processes or as the result of fire”.

For fly ash, the major environmental concern should be the emission of heavy metals (particularly Mo) through leaching into the groundwater, according to the results presented in Table 1. Despite this, neither leaching tests nor maximum values to limit their use can be found in the specific literature on road traffic noise reducing devices.

The Dutch Decree of Soil Quality (DSQ) (45) is considered the leaching standard reference. The DSQ contains test methods and limits for any waste in any construction material in order to prevent the pollution of the surface water and soil. NEN 7345 is the test established by DSQ for monolithic construction materials that contain wastes. The results of the leaching test of the fly-ash barrier compared to the DSQ limits are shown on Table 6. The use of a product composed essentially of fly ash does not pose any leaching problem according to the DSQ. The Mo concentration was slightly below the DSQ limit due to the stabilization of Mo produced by the cement in the matrix of the barrier.