The utilization of palm kernel shells (PKS) as an alternative to conventional materials for construction is desirable to promote Coarse Aggregate sustainable development. The purpose of this study is to investigate the properties of lightweight concrete produced with different sizes of PKS of 6, 8, 10, 12 mm and mix (consisting of 25% each of the four sizes). RPK sizes were used to replace coarse aggregate in the concrete and cured for 7, 14, 21 and 28 days. The tests performed on the concrete are dry density, compressive strength, flexural strength, EDS and SEM. It was revealed that the densities of the concrete specimens were all less than 2000 kg/m³, which implies that the PKS concrete satisfied the requirement of lightweight concrete for structural application. The compressive strength of the 12 mm PKS concrete specimens at 28-day of curing was 10.2 MPa which was 4% to 15.9% better than the other PKS sizes concrete. The flexural strength of the 12 mm PKS concrete specimens at 28-day of curing was 2.85 MPa which was also 3.2% to 57.07% better than the other PKS sizes concrete. It was also revealed by the SEM analysis that there was a good bond between the palm kernel shells and the mortar. A high calcium-silicate content was found in the concrete which resulted in a Ca/Si ratio of 1.26 and Al/Si ratio of 0.11. The study therefore concludes that size variations of PKS as replacement of coarse aggregate have an influence on the properties of the lightweight concrete and recommends 12 mm PKS for use by construction practitioners for lightweight concrete structural application.

Production of concrete is a major concern in recent times due to the incessant depletion of the raw materials involved. Concrete is one of the most important materials used on earth, and the aggregate in concrete accounts for about 75% of the entire volume. Concrete mainly consists of cement, water, fine aggregate, coarse aggregate and sometimes admixtures. Due to the increase production of housing infrastructure globally, there is a high demand for natural aggregate utilization. The cost of concrete production is mostly dependent on the constituent of the concrete [1]. According to Ogundipe et al. [2], aggregates are the main constituents in concrete and contribute greatly to strength development. The increasing cost of aggregate has contributed to the shortage of housing infrastructure in most developing countries. Danso [3] estimated that, about 60% of the population of Africa resides in favelas and informal settlements, which is primarily caused by the rapid growth of urbanization and increase in population, particularly in Sub-Sahara Africa (SSA) without corresponding increase in housing infrastructure. Hence, there is an urgent need to look for alternative construction materials which can produce low-cost housing and sustainable buildings for everyone [4]. This links with Sustainable Development Goal 11 (SDG 11) which promotes safe sustainable cities and communities. According to Jackson et al. [5], the use of alternative building materials as substitute for natural aggregate in concrete contributes to sustainable construction. There is therefore, the need to source alternates materials that are cheaper and environmentally friendly to substitute for natural aggregate in concrete production [6].

Several studies have been carried out recently on the usage of palm kernel shell (PKS) waste in lightweight concrete as a sustainable construction material. Oyejobi et al. [1] investigated the effect of mix proportions to predict the compressive strength of lightweight concrete with PKS as a substitute for coarse aggregate. Jackson et al. [5] examined the density, workability, and strength properties of concrete with PKS and coconut shells, and obtained compressive strength of between 6.85 to 13.29 MPa for the palm kernel shell concrete. Odeyemi et al. [7] studied the flexural strength and bond properties of self-compacting concrete containing PKS as a substitute for coarse aggregate; and recorded the highest flexural strength of 6.88 MPa at 28-day curing. Ogundipe et al. [2] studied the strength properties of concrete prepared with PKS and periwinkle as replacement of coarse aggregate; and found a clear trend of decreasing strength development. Azunna [8] investigated the water absorption and compressive strength properties of concrete with PKS as replacement of coarse aggregate; and found between 10% and 25% water absorption and between 4.44 and 4.78 MPa compressive strength. Fanijo et al. [9] investigated the strength properties of laterized concrete produced with PKS as a substitute for coarse aggregate; and found that the concrete with PKS partial replacement (20%) obtained better results. Ntenga et al. [10] examined the elemental composition and microstructure of PKS and coconut kernel shells.

Furthermore, Olanipekun et al. [11] compared the properties of concrete prepared with PKS and coconut shells. The flexural characteristics of PKS concrete with and without mineral admixture were reported by Alengaram et al. [12]. Traore et al. [13] also reported the durability properties of lightweight concrete using PKS as aggregates. Oyejobi et al. [14] examined the influence of the PKS ranges of sizes and mix ratio on lightweight concrete. In the past research works, all sizes or range of sizes of PKS have been used as a complete or partial replacement for coarse aggregate. Conversely, there is a lack of studies on the specific sizes (size variation) of PKS in concrete as a replacement of coarse aggregate. This study therefore intends to determine the influence of specific sizes of PKS on the production of lightweight concrete as coarse aggregate replacement.

PKS is the by-product of the edible seed of the oil palm fruit or palm nut which is obtained from the palm tree (Elaeis guineensis). The PKS is obtained from the nuts of oil palm fruit after the extraction of the palm oil leaving the shells as waste [15]. These waste shells are usually burnt in the open space as a means of disposal which contributes to the pollution of the air resulting in the release of carbon dioxide (CO₂) in the environment. Incorporating the PKS waste in concrete as a replacement of coarse aggregate will therefore provide environmental and economic benefits, thereby contributing to knowledge in sustainable concrete production. The study aims at investigating the engineering properties of lightweight concrete produced with size variations of PKS as replacement of coarse aggregate. In this study, specific sizes (6, 8, 10 and 12 mm, and mix sizes) of PKS were used as the replacement of coarse aggregate in concrete and their engineering properties such as density, compressive strength and flexural strength were determined. Furthermore, scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) analyses were also conducted on the concrete specimens.