Acoustic Performance of Porous Mortar and Potential Use for Traffic Noise Mitigation: A Review

Porous mortar (PM) is a porous building material used to reduce noise levels in economic emerging cities to achieve acoustic comfort. This research paper provides a comprehensive review of PM’s sound absorption performance as reported in selected published works. The selection criteria are limited to experiments conducted on specimens with a thickness ranging from 20 to 75 mm, a range suitable for application as a sound-absorbing layer on noise barriers or building walls. This paper explains the underlying principles of sound absorption in PM and outlining methods for assessing sound absorption. This review paper includes the performance of conventional or typical PM with modified PM, considering elements such as mix design and significant factors that influence sound absorption, notably material density, and pore size. Subsequently, this paper reveals on the evaluation of PM’s suitability as a sound-absorbing material, encompassing an assessment of its mechanical properties. In conclusion, the paper identifies the potential of PM as an efficient sound absorber, particularly in the context of mitigating traffic-generated noise.


Introduction
Proper consideration in urban planning and development is crucial to mitigate excessive and prolonged noise pollution exposure contributed by intensified road traffic in urban areas, especially in noisesensitive areas such as residential, educational, and healthcare areas [1].About 79% of noise generated on roads, creates stress in the nearby community and affects health conditions in terms of physical, physiological, and psychological effects as it was considered a disturbance to their well-being [2][3].It was projected that more than 389.09 million motor vehicles will be added to roads by 2025 [4].The intensified traffic noise pollution was generally comprised of 64% of passenger cars, 24% of motor vehicles and 12% of heavy vehicles, which commonly generate noise levels from 70 to 80 dBA in urban areas of especially among upper middle-income countries (MICs) [5].This range of noise levels has surpassed permissible noise level guidelines regulated by World Health Organization.It was recommended that average noise exposure (Lden) in noise-sensitive areas be below 53 dB while 45 dB at nighttime (Lnight) to ensure minimum adverse effects on health associated with noise exposure [6].To solve this issue, previous researchers innovate various types of noise barriers, including concrete noise barriers to reduce noise levels in economic emerging cities to achieve acoustic comfort [7].
A significant advancement in noise control technology has turned potential success towards porous concrete noise barriers replacing conventional concrete barriers along the bustling roads.These potentials in terms of engineering and acoustical properties are integrated to address the new development of noise control technology induced through pore network structure modification in the concrete mixture.The pore network is affected by the specific gravity aggregate which causes the density and porosity to change and affects the SAC [8].Previous researchers have been experimenting by substituting lightweight aggregates in concrete mix noise barriers to produce more porous noise barriers, consequently, enhancing the acoustic performance of noise barriers, such as seashells [8], bottom ash slag [9], recycled plastic [10], silica aerogel [11][12], rubber waste tires [13], arlite [14] and vermiculite [14] in the concrete mix design.
Porous Mortar (PM) is a porous building material made using binder and aggregate measuring less than 5mm [15].PM can also be made with a binder with pores created using the foam technique [12].This research paper reviews the sound absorption performance of PM in selected published works.The selected work is limited to the experimentation of specimens with a thickness of 20-75 mm as this thickness range can be applied as a sound absorbing layer on a noise barrier or on the wall of a building.The study begins with the principle of PM sound absorption and the method of assessing sound absorption.The performance of conventional / typical PM and modified PM is discussed which involves mix design and important factors affecting sound absorption including material density and pore size.The following discussion on the evaluation of suitable PM for sound absorbing materials covers the ability of its mechanical properties.Finally, the potential of PM as a sound absorber for traffic is identified.

Mechanism of sound absorption in porous mortar
Surface of the element of porous mortar containing open pores where some of the pores are closed and some are connected to the inside as shown in figure 1(a).These pores contain air to aid in the exchange of sound energy for heat energy.The mechanism of sound waves traveling through porous materials has three mechanisms as illustrated in figure 1(b).First, when the sound wave approaches / comes to the surface of the material, the sound energy is transferred and dissipated into heat energy by the air molecules in the porous material and changes the acoustic properties.Second, mechanism occurs when longitudinal sound waves undergo compression and rarefaction as they enter and exit the pore causing energy to be lost during energy transfer.Last mechanism is the conversion of sound waves into mechanical and thermal energy causes the phenomenon of resonance in the pore layer [16].[16][17] The ratio of the total pore volume to the volume of the specimen is called porosity.Porosity affects sound absorption in ways; first the addition of fine aggregate results in an increase in porosity and SAC values at high frequencies [18].For example, smaller aggregates with an aggregate size range of 0.5-1 mm produce smaller and more uniform pores and improve the acoustic performance of concrete [19].
Second, the high pore volume of the material can be taken into account at least 10% of the total porosity of the concrete matrix for better acoustic performance and compressive strength [20].Third; sound absorption performance as an inconsistent trend on porosity when incorporating aggregates in the concrete matrix.Further studies are recommended by scholars to study acoustic performance in concrete mix proportions.Fourth; sound absorption performance is significantly affected by the pore size and pore distribution in the matrix but is insignificantly related to the effective porosity of the porous matrix [21].However, Xiong et.al. conducted a correlation study on other geometric pore parameters such as effective pore size, pore diameter, critical pore neck size and nominal opening size to porosity and circumference of porous structure [22].

Identification of sound absorption properties
The sound absorption properties of materials can be identified using laboratory tests on the ability of materials to absorb sound.Measurements are made to obtain the sound absorption coefficient SAC or denoted as α, which is α=1-Reflection coefficient.The sound absorption coefficient of the material can be measured at low (< 2000 Hz), medium (2000-4000 Hz) and high (> 4000 Hz) frequency ranges.The SAC value sought depends on the equipment used for the measurement.If the measurement uses tube impedance (IT) according to ISO 10534-2 and ASTM E1050-12 standards, then the selection can be done twice, i.e. < 2000 Hz using a 100 mm diameter IT and over 2000 Hz using a smaller tube that is 30 mm [23].The results of these two tests can be combined to get the overall frequency.However, researchers can make selections to make only at low frequencies because they think traffic noise is dominant in this range [24].
Apart from IT, the SAC coefficient can be found using reverberation room (RR), where the sound source arriving at the sample comes from various directions, therefore requiring a larger and slab-shaped sample.The sample is placed on the floor in the RR room which has sufficient volume to allow sufficient absorption and reflection that can imitate the actual conditions in the field.This test follows ISO 354 and ASTM C423-22 standards [25].IT testing exhibits more accuracy due to less sound diffraction effect on samples [26].The results of these two tests can determine the nature of sound absorption, that is by examining the SAC curve but the noise reduction coefficient (NRC) which is the average value of sound frequencies at 250 Hz, 500 Hz, 1000 Hz and 2000 Hz [27], can determine the quality of an efficient sound absorber if the value is 0.45 [28].SAA, which is the average at 125 Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz and 4000 Hz determines the ability of the material with a value above 0.2 as a good material as a sound absorber.There are also researchers stating that materials with SAC > 0.2 have interconnected pores and can be called sound absorption material [29].[23,26] 7th International Conference on Noise, Vibration and Comfort (NVC 2023)

Typical porous mortar
To produce a mortar with an adequate sound absorption coefficient, the porosity of the mortar is very important.For that purpose, researchers generally use water/cement and cement/aggregate ratios that are usually used for the fabrication of porous concrete in ACI 522R.Mix design value for water-cement ratio between 0.26-0.40,cementitious materials 18-20% and aggregate 80-82%, however some research use values slightly outside this range.For example, the selected water-cement ratio is between 0.33-0.7 while cement is 18-33.33%and fine aggregate is between 66.67-82.14%[15].Table 1 lists the mix design variations in the development of porous mortar using fine aggregate consisting of river sand, limestone, and dune.The design of this mixture affects the sound absorption coefficient, especially the selection of aggregate size, w/c ratio and cement content.
The sound absorption coefficient of a material is affected by porosity and pore size distribution [17], but not all research determines these parameters.From the data in table 1, Neithalath et.al. has studied the PM (PM1) with porosity and its average pore size of 22% and 2.17 mm, respectively.The porosity value meeting the porosity and pore diameter requirements as stated for porous concrete.The design mix produced a high connectivity factor to produce maximum sound absorption of SAC max 0.62 [30].For PM2 and PM3, it can be seen that with the same w/c ratio and thickness, the specimen with a large fine aggregate size (PM2) produces a higher NRC compared to finer aggregate (PM3).However, when the w/c and fine aggregate decrease and the cement content increases (PM4), the NRC decreases.A study by Rodrigues et.al. produced similar results when the cement content increased causing the pore size to decrease and the porosity to decrease, resulting in narrow channels and unable to carry sound energy to be converted into heat energy through the pore walls [30].
The shape of the aggregate slightly affects the NRC as happened in the PM6 specimen made using dune sand which has a round shape and smaller size (0.05-0.6mm) compared to the PM5 specimen made using river sand which is more irregular in shape (0.05-5mm).thus, dune sand has a larger specific surface which requires an additional amount of water for wetting the grains [10].According to a study by Kim & Lee, it was found that the same aggregate size, but different shape did not show a significant change in the properties of SAC [31].However, in this case, the size of the aggregate is not the same, so it is possible that the shape of the size of the dune sand produces a pore size that can match the size of the pore produced by the larger size of river sand.This is evidenced by the PM5 density differing only 2.19% from the PM6 density value, making the NRC not significantly different.

Porous mortar with waste materials as aggregate
As shown in the table 2, PM is made from replacing natural fine aggregate with waste material to produce a good sound absorbing material.For example, the use of bottom ash and ACFBF with a size of 1.25-5 mm produces a higher NRC compared to conventional PM using the same size and thickness (PM1) [33].This is due to the fact that this aggregate is porous with a specific gravity of 2.16-2.12compared to limestone aggregate with a specific gravity of 2.83 g/cm 3 , resulting in high porosity and higher sound absorption.Specific gravity aggregate affects the subsequent density of porosity and NRC.This can be seen with PM using scallop and mussel, with the same design mix but due to the higher specific gravity compared to ACFBS, the density is higher and the porosity decreases resulting in a low NRC.
For PM specimens with ceramic waste content (PM ceramic), although ceramic waste has a high density, the aggregate size range is larger and produces high porosity and NRC that is almost equivalent to BA and ACBFS which have a smaller aggregate size range but are highly porous due to the porous nature of the aggregate.For PM ceramic 1 (2-4.5 mm) and PMBA specimen (1.25-5 mm), the NRC is almost the same even though the porosity is slightly different, because the density of PM ceramic waste is lower due to the lower specific gravity of ceramic (2.15 g/cm 3 ) and the size larger aggregate creates larger pores compared to bottom ash even though it has a porous structure but has a higher specific gravity coupled with a range of stone sizes containing small sizes causing it to be denser [34][35].For PM RP consisting of a mixture of 75% aggregate plastic instead of dune sand, porosity is said to be very low because plastic has a very low specific density (0.531-0.91 g/cm 3 ) and the opposite produces a low density compared to PM containing 100% dune sand.The fairly good sound absorption by PM 75% plastic + 25% dune sand even with a thin thickness due to its surface structure but cannot be explained in detail by the researchers [10].Likewise with PM using rubber tire (PM TYRE) as part of sand replacement, although it has high porosity, but the NRC value is low.

Porous mortar with lightweight aggregate
PM can be made by mixing light aggregate such as arlite which is porous or vermiculite in the form of flaky flat lamellar which has high water absorption and has a low specific gravity (0.64-0.69 g/cm 3 ) (PM Ar, PM vers).This results in low density and high porosity to produce a good NRC (> 0.2) with a pore diameter of 1-4 mm [14].The situation changes when PM is made of sand and expanded clay (PM EC) which is porous as an aggregate.Expanded clay has a low specific gravity (0.32 g/cm 3 ) with a low relative amount compared to sand to produce a high-density material because sand has a high specific gravity, and low porosity and NRC.The same happens with mix design using very light perlite (PM Per) with aggregate specific gravity of 0.15 g/cm 3 , resulting in high density and low porosity [36].studied geopolymer PM made from metakaolin and blast furnace slag powder with a rate of 70% and 30% respectively mixed with alkali equivalent AE of 18% with a water-binder ratio of 0.6.They succeeded in making a mortar with a density of 400 kg/m 3 with preformed air bubbles that were used as much as 43.6 kg/m 3 to produce PM with excellent acoustic properties.Specimens with a thickness of 60 mm had an NRC of 0.55 and were found to have pores ranging from 0.1 to 6 mm, with an average of 2 mm [17].PM geopolymer can be designed using fly ash, activator, foaming agent and the addition of aerogel according to the design mix shown in the table.Chen et.al. used 4 types of aerogels IC3100, IC3110, IC3120 and LA1000 with sizes of 2-40 um, 100-700 um, 100-1200 um and 700-4000 um, each as shown in figure 3 (a).The density of foam mortar as in figure 3 (b) with the inclusion of aerogel decreases compared to concrete foam to 770 kg/m 3 , while the porosity increases from 62.3 to 66.90%.The pore condition of foam mortar due to aerogel depicted in figure 3 (c) increased SAA from 0.12 to 0.42.Pore size formed from the range of 0.01-0.7 mm with an increase of 41.21% at 0.7 mm size with the use of gel as illustrated in figure 4. The use of geopolymer reduces the carbon footprint due to the use of cement.

Discussion and potential use in industry
PM can be made using a typical mixture of cementitious, finely ground stone and water, (typical PM), using a mixture of cementitious materials and replacing fine aggregate with waste material or lightweight aggregate, and using geopolymer that is using fly ash / blast furnace activated by silicate and using foaming agent as well as the use of aerogel.From the studies that have been conducted on sound absorption by PM, density and porosity are elements that are focused on.This may be due to laboratory tests being more common for a construction material.The density of PM is affected by the specific gravity of the aggregate where the porous aggregate has a low specific gravity and produces a low density.Low density results in high porosity and high sound absorption properties.The microstructure of pore-related materials, such as diameter and pore distribution, is an important factor for sound absorption.But only a few researchers studied the pore diameter [14,17,30,33,37].Studies involving PM pore sizes up to now show that ACBFS porous aggregate sizes less than 5 mm can produce pore sizes up to 6 mm in diameter, 1-4 mm for light aggregate, while up to 0.7 mm for PC geopolymer.Pore size and distribution are very important for predicting sound absorption properties of materials [14].
From the above study, it was found that even if the porosity exceeds 15%, the sound absorption test shows that the porosity does not show an effective factor for SAC.For those whose NRC is less than 0.2, the material can be considered a reflective material.This has been studied by Arenas et.al., showing a very high reflection coefficient value, showing that it is the opposite of SAC.The SAC curves for the selected PM were depicted as in figure 5.A PM that has a high NRC alone is not sufficient because it is an indicator for the purpose of evaluating the material's ability up to 2000 Hz but the ability up to a medium frequency of 4000 Hz is also referred to assess the efficiency of a good material.Some PM is categorized as good because SAA exceeds 0.2 and has the potential to make it as one of the components in materials for sound absorbers [34][35].Sufficient mechanical strength properties are also required which is a value of 2.8 N/mm 2 recommended by ACI 522R for porous concrete, but according to Arenas et.al. the recommended strength as a sound absorbing layer is 3.1 N/mm 2 .In addition, the selection is based on high sound absorption at 600-1250 Hz frequency because traffic noise is dominant in this range.Therefore, only PM BA and PM ceramic with aggregate 2.5-4.5 mm.These sound absorption properties also change higher when the thickness increases, as studied by researchers, the thickness of 120 mm results in NRC increasing to 0.59 and 0.61 from 80 mm and 120 mm.PM that has a strength below 2 N/mm 2 can be used but used in a passive environment [38].

Conclusion
This study reviews previous studies on the sound absorption properties of porous mortar and its potential use in industry.The study focused on thickness less than 75 mm with porosity greater than 15%.It can be concluded that some PMs has potential to be absorbers because SAA > 0.2, especially those that use mix design cement, aggregate, and water as recommended by ACI 522R but use aggregate sizes between 1.25 mm -5 mm.The replacement of natural fine aggregate using waste aggregate also produces a better NRC, especially porous aggregate such as BA, ACBFS and ceramic, but requires more water due to its porous nature.This PM also has good compression strength as required by one of the layers in the multilayered sound absorber.However, some PM can be a reflective material although the porosity is greater than 15% and potentially used as a hard backing material in a multi-layered sound absorption system.PM made using foam or aerogel has a good NRC but has very low strength and it is suitable for use in a passive environment.

Figure 5 .
Figure 5.Comparison between the SAC curve of selected PM.

Table 1 .
Typical PM with its design mix, density, porosity and noise reduction coefficient.

Table 2 .
Modified PM by the substitution of natural aggregate with waste material with its design mix, density, porosity and noise reduction coefficient.
a Total porosity.

Table 3 .
Modified PM by the substitution of natural aggregate with lightweight aggregate with its design mix, density, porosity and noise reduction coefficient.Geopolymer foam porous mortar PM can be made using the foaming technique to create enough pores to absorb sound.Hung et.al.

Table 4 .
Geopolymer PM with its design mix, density, porosity and noise reduction coefficient.