**1. Introduction**

Oil palm (*Elaeis guineensis* Jacq.) is one of the world's most efficient and versatile crop in the world. It is cultivated in continents of Asia, Africa and South America. In Asia, Malaysia, Indonesia and Thailand produced 91% of total palm oil worldwide [1]. In Malaysia, oil palm is planted on 5.45 million hectares, Indonesia 11.95 million hectares, while Thailand 820,000 hectare in 2020 [2]. 1 hectare oil palm plantation annually produces about 55 ton of dry matter in the form of fibrous biomass while yielding 5.5 ton of oil [3]. These include shells and fibres (**Figure 1**) [4]. Shells and fibres of palm oil are burned together in mills as fuel for firing the furnace of the mill to heat up the boiler, thus producing more waste materials, such as palm oil clinker (POC) and palm oil fuel ash (POFA) (see **Figure 2**) [1, 5]. About 1.1 ton of POC per every ton of oil produced were generated [6]. Palm oil Clinker is a large grey chunk that resembles a porous stone with irregular and flaky shape [1, 5].

**Figure 3** shows a literature review search in Scopus data base and mapping results using VOSviewer software [7] show most research in relation to palm oil clinker is focused on their usage as aggregate replacement and investigation on their

properties and strength for light weight concrete, mortar and sustainable concrete. Some of these research use combined dust POC and fly ash to replace cement and also used for self-compacting mortar [8]. The properties of acoustic concrete containing POC have just been initiated by [9].

POC is widely used as a lightweight aggregate due to its lightweight nature. POC is estimated to be 25% lighter than river sand and 48% lighter than crushed granite stone [10]. Thus, the density of mortar containing 100% POC sand is reduced by 7% compared to that of river sand [11]. The light nature of this POC aggregate is due to the physical properties of POC that contains micro-pores [12]. Due to the porosity of POC, concrete containing POC has lower compressive strength and tensile strength. POC also has an aggregate crushing value (ACV) of between 15 to 30 kN which is considerably lower than the values for the river sand. Therefore, there

were researchers who coated POC to cover the macro pores of POC to slightly increase its compressive strength [12].

In fact, aggregate porosity can be utilised for the development of sound control materials. This has been stated by previous researchers where pores in aggregate is an important feature that influences the sound absorption [13–16]. For example porous-expended shale aggregate size of 12–19 mm increased the sound absorption value by 6% [14] compared with porous concrete using natural aggregate (lime stone) size 13–19 mm. This is because the extended shale aggregate has a porosity of 14.1% compared to the regular limestone aggregate of only 5.6%. Bottom ash also yield in a 13% increase in sound absorption [15] when replaced limestone aggregate with an aggregate-cement ratio of 20%. While, porous basalt stone with porosity 42% was found increased the porosity of concrete from 18 to 22% and caused an increase of sound absorption [16]. Preliminary studies of the sound absorption properties of concrete containing POC showed an increase in SAC at 1000 Hz [9].

Noise control materials are an important element component in reducing the environmental noise in urban areas such as noise barrier systems to reduce reflection from traffic noise. The reflective sound barrier system produces continuous reflections to create a "canyon" environment where users and the housing community near the road will be disturbed. According to the study, street canyon produces reverberance condition with RT30 between 1.2 to 1.4 s [17] which is a measure of annoyance. Road noise is also dominantly at 900 to 1100 Hz [18] which is in the range of human hearing sensitive between 20 Hz to 4000 Hz. Recently, it was found that middle frequency range between 200 and 630 Hz especially the 315 Hz produced high annoyance to resident, in particular on the elderly people [19].

The best noise control material is one that has porous properties because it can absorb sound and produce less reflection and at the same time avoid the 'street canyon'situation. Sound absorption is measured through a sound absorption coefficient (SAC) which indicates that the capability of material absorption between 0 to 1 in which the previous represented perfect reflection while the latter indicates perfect absorption. The nature of good sound absorption is when the value of SAC exceeds 0.35 [20].

The porosity of the aggregate causes an increase in the porosity of the concrete material and according to [16] interconnected porosity has a significant relationship with the sound absorption properties of the concrete. Further, Tie et al. [21] and Gonzalez et al. [22] stated the characteristic sound absorption properties related to

the density of the material. Based on the sound absorption properties by concrete containing POC from a preliminary study by [9], it may be preferable for noise control materials. Therefore, this study aims to further investigate the potential of concrete POC as a noise control materials in alleviating the problem of noise pollution from roads and railways. In this study, further research on two main parameters related to SAC namely porosity and density and their relationship with sound absorption in POC concrete will be discussed further. By using regression analysis of the relationship between SAC, porosity and density can be established. Further, concrete POC mixtures suitable as sound absorbers can be identified.
