Influence of milling process of palm oil fuel ash on the properties of palm oil fuel ash-based alkali activated mortar

This work aims to study the influence of milling process on particle size reduction and reactivity improvement of palm oil fuel ash (POFA) as raw materials for the production alkali activated products. Specifically, the quantity of material milled (designated as Q) and the period of milling (designated as M) were the two factors investigated. The amount of milled POFA was examined at six different levels from 1 kg to 5.5 kg (designated as Q1 to Q6). While three milling periods were fixed at 8, 16 and 24 h (designated as M1, M2 and M3, respectively) to obtain the optimum milling process. A total of eighteen mixes of alkali activated mortars were prepared using mixtures of dry source material of milled POFA (GPOFA) and alkaline activator solution from NaOH and Na2SiO3. These eighteen mixes were divided into three different groups based on the periods of milling as M1, M2, and M3. The performance of the samples was evaluated by compressive strength, flexural strength, ultrasonic pulse velocity (UPV), porosity, and water absorption. Chemical and microstructural analyses via X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), thermogravimetry (TG), and scanning electron microscopy (SEM/EDX) were undertaken to further quantify the performance of the alkali activated mortar. The results revealed that the specific surface area of POFA was increased from 193 m2 /kg to 1710 m2 /kg at the optimum milled quantity of 3 kg and milling period of 24 hrs. The alkali activated mortar based on the ultrafine particle of GPOFA produced a maximum compressive strength (57.5 MPa), flexural strength (10.9 MPa), porosity (13.1%), water absorption (6.2%), ultrasonic pulse velocity (3846.1 m/s) and density (2110.4 kg/m3) at 28 days of curing. The superior results were linked to the formation of greater volume of main binding phases consisting of a calcium silicate hydrate (C-S-H) gel, which was formed within the GPOFA-based alkali activated mortar. Therefore, this optimum GPOFA can be used as alternative to other source materials in the manufacture of alkali activated products and become a sustainable option for solving the waste disposal in the palm oil industry.