**3.2 Effect of thermal and chemical treatment on POC structural elements**

Fire resistance of any structural element greatly depends on the stability of concrete ingredients at elevated temperatures. Therefore, researchers usually conduct thermal activation to evaluate its effect on physical and mechanical properties, crystalline structure, minerals, organic carbon content, morphology and chemical composition. Investigating these properties in necessary as they directly or indirectly affect the structural elements compressive strength. This was why Karim et al. [11] studied the effect of temperature on microstructure change and compressive strength of cement paste incorporated with POC. It was reported that, thermal activation at 600°C, and 800°C for a duration of 3 h yield higher residual compressive strength for POC specimen than that obtained for OPC specimen. This could mainly be due to the pozzolanic reaction of POC specimen when heated at elevated temperature. Also, C-S-H gels were more stable in POC containing cement paste after an elevated temperature exposure. This signifies that POC incorporated specimen has higher fire resistance. Crack formation was also higher in OPC paste surface, which is an indication of higher superiority of POCPC in making fire resistant concrete.

In a related study by same authors [61], it was gathered that 580°C for 3 h is proven to be the appropriate condition for thermal activation effect on POCP as the compressive strength of mortar was significantly increased, organic carbon content in POC reduced as inorganic oxides content increases, with an increment rate of 3.4%, 3.5% and 3.4% for SiO2, Al2O3 and Fe2O3 respectively at °C. Porosity reduced as fibers were eliminated and POC color transformed from black to gray. It was also discovered that thermal activation has no significant influence on POC crystalline structure. Therefore, it has been proven that thermal treatment can enhance POC pozzolanic reactivity by elevating the maturing process of hardened specimens and unburned carbon removal.

In concrete specimen prepared with 25%, 50%, 75% and 100% replacement of Oil Palm Shell (OPS) with POC as coarse aggregate at an elevated temperature up to 500°C for 30–60 min, POC aggregate experienced negligible weight loss of <1% with excellent resistance. As the POC content is being increased, number of cracks and crack width decreases. At 100% OPS replacement with POC, the loss of residual compressive strength of only 9% indicates the vast improvement of OPS concrete using POC [2].

In other to investigate the pozzolanic reactivity of POC powder by chemical pretreatment, the powder was replaced at 2.5–15.0% by weight of cement for pretreated and untreated POC powder in mortar mixtures. POC impregnation with low HCl acid concentration was able to enhance its pozzolanic reactivity through the hike of active silica proportion and reduced impurities and traces of metallic elements. The combination of 0.1 M of HCl acid and 1 h of impregnation time was selected as the optimum pre-treatment parameters. The strength activity index of up to 7.5% of cement content replacement with pre-treated POC increased in the hardened mortar. Authors also concluded that, the pre-treatment process would enhance the pozzolanic reactivity of POC powder up to 170% higher, increase the proportion of amorphous silica up to 9.6%, and contribute more to the strength development of mortar compared to the untreated POC powder [17].
