**2. Waste to resource potentials of POC**

#### **2.1 POC in geopolymer made structural elements**

Geopolymer is an inorganic material that can be formed through the use of a binder. According to [30], any material that contain silica and alumina can be utilized as a binder. Alkali activators are also important for the production of geopolymers. Numerous high silica and alumina containing waste materials could be utilized for geopolymer production due to their pleasing size, shape and chemical composition. POC, considered a pozzolanic aggregate, has the capacity to create good bond in geopolymer matrix as it possesses the aforementioned characteristics. In contrast to POC with OPC concrete, the use of a geopolymer binder increases the workability and strength of POC concrete thus lowering its water absorbability. A green and long-lasting structural lightweight concrete can be produced by combining POC with Fly ash-based geopolymer binder [31]. Utilizing POC particles in the geo-polymeric specimen results in structural elements with good resistance to water absorption.

Sustainability in high strength concrete production can be achieved by combining POC with fly ash as a geopolymer based binder. Designing and mixing concrete with 100% POC aggregate can give rise to a concrete with compressive strength >30 MPa and a density of 1821 kg/m3 . However, 32% strength reduction was experienced as natural aggregate was substituted by OPC. 75% POC aggregate *Palm Oil Clinker as a Waste by-Product: Utilization and Circular Economy Potential DOI: http://dx.doi.org/10.5772/intechopen.97312*

replacement in geopolymer concrete mixtures has been proven by [31] to be the most effective one. As POC concentration increases in geopolymer concrete mixtures, water absorption increases as density decreases.

POC sand was used for full sand replacement in a geopolymer mortar and achieved comparable mechanical properties showing high resistance to MgSO4 and HCl solutions. 53 MPa was recorded as the 28-day compressive strength with 17% density reduction [32].

A geopolymer concrete that contain 100% POC as coarse aggregate was designed and evaluated. According to the results, 41.5 MPa was the highest compressive strength achieved at 28 days curing with a density range of 1910–2172 kg/m3. Splitting tensile strength increased and UPV values were also good. POC also improved the compressive toughness of the geopolymer mortar. The study concluded that, structural grade lightweight geopolymer concrete could be produced by using POC [22].

### **2.2 POC in conventional structural elements**

The physical characteristics of POC used by several researchers are shown in **Table 2**.

#### *2.2.1 POC as a coarse aggregate*

The application of commercial aggregates was minimized due to high production costs emanating as a result of excessive raw materials and energy consumption. They also increase the dead weight of structures. Therefore, introducing POC, being a porous and lightweight material that contain high volume of solid waste materials are used to produce structural lightweight aggregate with potentials for high strength and good workability concrete. POC density is said to be less than that of normal aggregate [33]. Even though substituting normal weight coarse aggregate with POC wrecks the splitting tensile strength and modulus of elasticity, it however improves the concretes compressive strength [43].

The physical properties of POC aggregate have a notable influence on produced concrete properties. An equivalent of 1 m3 of soil is saved when 1 m3 of POC aggregates is utilized for concrete production instead of being discarded in a landfill. This would substantially lead to a safer and more productive climate [29]. CO2 emissions were said to have decreased by 20% when natural aggregate was totally replaced with POC coarse aggregate [44]. POC aggregates are lightweight and porous by nature, and they contribute to the reduction in concrete structural density [1].

POC increases concrete mixtures porosity and permeability. Compressive strength reduction of about 65% was recorded at full POC replacement. Nonetheless, concretes with lower strength could be used for pedestrian trials and walkways construction [38].

POC aggregate crushing value is three times less than that of gravel aggregate, thereby indicating higher energy consumption [14]. Having a density of 1990.33 kg/ m3, makes POC aggregates ideal for use in lightweight concrete mix proportions [40].

Experimental investigation was carried out on concrete substituted by POC as a filler and an aggregates material for high strength concrete (HSC) creation. The permeable nature and uneven form of POC coarse had a negative impact on the fresh concrete mix's workability. Nonetheless, adding POC powder as a filler improved the workability. Adding POC powder in POC concrete mixes improved compressive, splitting tensile and flexural strengths by 0–13%, 2–10% and 1–9%, respectively compared to POC mix without POC powder. According to Rapid Chloride Permeability Test (RCPT) carried out, both POC concrete mixes, with and without POC powder, have a strong resistance to chloride penetration with very low permeability category <100°C [7].


#### Elaeis guineensis

#### *Palm Oil Clinker as a Waste by-Product: Utilization and Circular Economy Potential DOI: http://dx.doi.org/10.5772/intechopen.97312*


**Table 2.**

*Physical properties of POC.*

POC concrete beams have been known to provide sufficient notice of impending failure by exhibiting traditional structural ductile behavior. At service loads, the crack width (0.24–0.3 mm) of POC concrete beam was found to be within the BS8110 overall permissible value for durability requirements [45].

In an oil palm shell (OPS) lightweight concrete, OPS aggregates were partly replaced with POC coarse aggregates from 0 to 50%. The slum value, density (2–4%), compressive strength and modulus of elasticity (18–24%) of the OPS concrete increases as POC coarse aggregate increases in the mix. More so, at 20–50% POC coarse aggregate addition, grade 35 OPS concrete was upgraded to grade 40. As a result, it's classified as a high-strength lightweight concrete [36]. In a related study, authors reported a positive impact on both workability, UPV and compressive strength. Highest compressive strength of ~63 MPa which is about 43% higher than the control mix was obtained for the OPS:POC mixture. This may be due to the efficient POC and mortar interlocking. With maximum obtainable stress between 0.00173–0.00401 > the normal weight concrete (NWC), the OPS:POC mixture could have better shrinkage crack resisting capacity. Furthermore, a 2.5 fold rise in elasticity modulus could remarkably control deflection [45].

POC aggregate could be used to develop high-strength lightweight concrete with a 28-day compressive strength of 50–60 MPa and an oven-dry density of 1875–1995 kg/m3. At full water curing and air-drying curing conditions, equivalent compressive strengths were recorded. This proves that, POC lightweight concrete is not too delicate to curing method. The study suggests the use of regular sand with a nominal grain size not more than 2 mm. This is to improve elastic modulus of the concrete [36]. Ultrasonic pulse value (UPV) tests value for POC concrete was good with a compressive strength and hardened density of 33–49 MPa and 2074–2358 kg/m3 at 28 days respectively. At 10% POC replacement for coarse aggregate, grade 40 concrete was obtained. However, increasing POC replacement ratio with coarse aggregate reduces the concrete workability. The advantage of applying POC as a lightweight aggregate is to decrease concrete structures dead load by up to 35% without much loss in structural strength. The decrease in dead load can save construction cost without compromising structural integrity. Therefore, applying lighter waste materials such as POC can greatly reduce concrete costs, due to its low cost of RM 0.020 per kg. This will go a long way in reducing the need for non-sustainable natural resources. For structural application, shear failure mode of POC concrete beams were found to be close to that of regular weight concrete beams, and as well in line with ASTM: C330 [28].

Despite the concrete's higher porosity, self-compacting lightweight concrete (SCLWC) had strong UPV values. Tensile splitting strength, compressive strength and flexural to compressive strength ratio also met the strength requirement for SCLWC [25]. Therefore, SCLWC is classified as a form of lightweight concrete with a high strength because 28-day compressive strength >40 N/mm2. As an actively mobilized material, POC was also able to amplify the filling and passing ability of self-compacting concrete. The concrete showed less segregation resistance due to low POC coarse density. Although obtained density values were in an acceptable range, coarse aggregates replacement with POC in SCLWC reduced density in ovendry and saturated surface-dry conditions by 16% and 18%, respectively.

Lightweight aggregate concretes made of POC with 12% less dead load compared to the conventional concrete mix showed an acceptable splitting tensile strengths and workability without any segregation or floating at an average water to cement ratio. Interestingly, even after 28 days of curing, POC concretes did not achieve their maximum strength [34]. Testing the efficiency of POC in concrete slabs, the mechanical interlock (m) and friction (k) between the steel and concrete were found to be 117.67 N/mm2 and 0.0973 N/mm2, respectively. It was also

#### *Palm Oil Clinker as a Waste by-Product: Utilization and Circular Economy Potential DOI: http://dx.doi.org/10.5772/intechopen.97312*

discovered that horizontal shear-bond strength and structural behavior are satisfactory, nearly comparable to the conventional concrete slabs and could be used for composite slabs construction. Compared to conventional concrete slabs, POC concrete slab possess a reduction in weight of 18.3% [35]. Under absolute air, water, and 3 days water curing, the abrasion resistance and strength properties of concrete comprising POC coarse aggregate were investigated. The compressive strength of POC concrete cured in air and in water for 3 days displayed comparable conduct, with a maximum loss in strength of about 5% and an acceptable abrasion resistance. Interestingly, abrasion resistance was improved when cured in full water [44]. Air curing application in a tropical environment permit POC concrete to achieve the desired strength due to the surroundings high humidity. However, water curing is the most appropriate curing method for POC light aggregate concrete, because it contains enough water to ensure proper hydration and pozzolanic reactivity [41].

#### *2.2.2 POC as a fine aggregate*

Palm oil clinker (POC) has in recent times being used for partial replacement of fine aggregates in structural elements. This was possible due to the grading features and particle size distribution similitude between sand and POC fine aggregate [5]. The particle size distribution of POC ranging from 100 to 400 mm, indicates that they are suitable for use as fine aggregates. A study by [4] found the compressive and flexural strength of concrete to be increasing with the sand replacement with POC. The study further confirms that fine aggregate replacement with POC had no remarkable impact on compressive strength. However, it decreasing concrete workability [28].

Sand was totally replaced by POC in a mortar designed using the volume-based approach. At 28-day curing, 41 MPa was recorded as the compressive strength of the mortar. POCS aids the gain of early-stage strength development for up to 77%. With 4.09 km/s as the POC mortar velocity, well-compacted specimens were obtained. The poriferous structure and rough nature of POC aids in the formation of a stronger bond with cement paste. The price of the mortar could be reduced by 16% when POC is utilized [20]. When POC was partially replaced with sand from 0 to 40% by weight of sand to investigate its effect on fly ash cement sand brick engineering properties, it was found that up to 30% POC usage enhanced the brick strength due to the pozzolanic effect of the fine clinker. Calcium hydroxide and silicon dioxide were responsible for the pore refinement and higher brick strength development [27]. Replacing OPC with fine aggregate increased the mortar sorptivity, initial and final water absorption because of its high porosity. OPC replacement changes the cement mortar thermophysical properties. At 100% sand replacement, compressive strength development ((7th day)*/ (*28th day)) was higher than samples containing lesser amounts of OPC. Under the same conditions, the specific heat capacity of mortar boosted by ~41%. Thermal conductivity and diffusivity lessened by 72% and 76% respectively. This show*s* that, OPC replaced mortar has the potential to lower heat transfer and energy consumption in buildings [46].

In a related study at 100% sand replacement, it was reported that POC fine has the potential to produce 86% and 78% compressive strength at 28 and 56 days curing respectively and providing almost 97% durability when compared to the conventional mix. POC fine durability showed a satisfactory outcome with good resistance against corrosion risk. POC fine is capable of lowering the carbon emissions of mortar by 50%. More so, POC fine can improve the engineering economic index and engineering environmental index by 11% and 95%, respectively. Life cycle impact assessment (LCIA) shows POC's potential to encourage a healthier and safer community with substantial reduction in ecotoxicity [18].

Incorporating POC sand in OPS concrete is beneficial to reduce its sensitivity to lack of curing. OPC was used as a replacement for sand at 0–50% in an oil palm shell (OPS) lightweight aggregate concrete. It was discovered that the replacement does not affect the drying shrinkage strain. High percentage of POC replacement increased the water absorption of the concrete. The concrete was proven to possess high splitting tensile strength [42]. In comparison with normal mining sand, POC fine aggregates have lower density and higher water absorption. Surprisingly, the slump value of concrete containing 25% POC fine showed good workability. The POC fine replaced concrete was classified as high strength because 69–76 MPa was obtained as compressive strength for 28-day curing. 12.5% POC fine replacement in concrete is said to be practical and cost effective [47].

### *2.2.3 POC powder*

POC powder is obtainable by grinding dry POC for ~8 h in a controlled ball mill at 150 RPM. It has been confirmed by several authors through microstructure analysis that POCP particles are blackish in color, irregular in shape and contain small pores with fibrous materials present [6]. SiO2, Al2O3, Fe2O3, MgO and CaO are the major components found in POC powder with oxide composition >71.09%. This proves that the powder satisfies the chemical requirement of Class F fly ash [6]. POCP and cement generally have similar fineness. However, the suitability of using them in concrete relies on their pozzolanic activity [7]. Strength activity index result proved that POCP is a pozzolanic material [48]. To ensure that the required workability can be attained when used for partial replacement of cement in mortars, POCP being a pozzolanic material would require more water [49]. The crystallinity index of quartz in POC powder utilized by [6] was 0.97 indicating partial disorderliness of quartz and pozzolanic reactivity of the powder. The major component in POCP present in quartz and cristobalite phases at 2θ angle of 26.87° and 20.45°, respectively is SiO2 [6]. A significant hump in XRD pattern from 10–35°C demonstrates the presence of an amorphous fraction that is reactive due to pozzolanic activity [48, 49].

The addition of POC powder to replace cement and quarry dust has greatly increased the fresh and hardened density and compressive strength of produced blocks. Classified as thermally efficient and light weight blocks, the properties of the produced blocks meet the required thresholds and were higher than those of the common stabilized compressed earth blocks [16]. The use of POCP for cement replacement of about 40% in a cement-lime masonry mortar is recommended based on fresh density, consistency and air content requirements. Split tensile strength at 90 days of curing was greatly improved due to pozzolanic reactivity of POCP at longer duration. Flexural bond strength of the POCP mortar attained about 70% of control mortar. It also reduced 32% carbon footprint, 20% cost saving and save reasonable amount of energy [13].

A study attempts to investigate the durability performance and microstructure behavior of masonry mortars where POCP was used for cement replacement. With a compressive strength of 12.5 MPa, 40% cement replacement appeared to be a reliable mortar in terms of durability front with similar 28-day drying shrinkage to control mortar mix. The mixture possess extremely good electrical resistivity [49].

POC powder significantly enhances concrete compactness. At 15% increment, it improves the modulus of elasticity for up to 60% as compared to normal concrete. This could be attributed to concrete stiffness enhancement. At same increment, highest splitting tensile and flexural strengths in the range of normal weight concrete were recorded. Also, 15% and 30% strength enhancements were obtained for flexural and compressive strengths (65 MPa). The study also found that utilizing

#### *Palm Oil Clinker as a Waste by-Product: Utilization and Circular Economy Potential DOI: http://dx.doi.org/10.5772/intechopen.97312*

POC powder of ~15–20% as a filler or cementitious materials in producing 45 grade lightweight concrete, CO2 was reduced [12]. In a similar study trying to improve concrete strength, authors used varying proportions of nano-palm oil clinker powder (NPOCP) for cement replacement. It was discovered that, as NPOCP content is increased in the concrete mix, density decreases. This is because, cement has higher specific gravity than NPOCP. However, increase of NPOCP content increases concrete workability. The highest and lowest compressive values were obtained at 10% and 40% NPOCP replacement levels [16].

POCP replacement level of up to 30% enhanced the resistance of recycled aggregate-based concrete against water absorption and risk of corrosion decreased to a "moderate" level after 90 days curing period. In terms of compressive strength, POCP optimal replacement level to attain satisfactory result is 20% in comparison with the normal mix [23].

In a study by [5], the surface voids of POC coarse were filled and coated with POCP as a filler material. This mixture could decrease the quantity of aggregates derived from primary sources that are continuously exploited. It also increases the paste content necessary to make the mixes more cohesive. A notable increment in flexural strength was attained between 5 and 25% higher as compared to the POC concrete with 20–30% attained for compressive strength. However, supplementing POCP led to a decrease in water absorption value by decreasing the pore size, thereby producing highly densified paste. Specimens that contain POCP were reported to exhibit greater chloride-ion resistance.

POC powder can reduce the cost of mortar by 41%, save 3.3% of cement production, 52% carbon emission reduction. 50% POC powder replaced mortar could achieve 70% strength and 60% structural efficiency as compared to normal mortar [14]. The pozzolanic reactivity, microstructure properties investigation and strength activity index result confirmed that POC powder has pozzolanic property and good for utilization in cement-based applications.
