Abdul Kudus, Sakhiah, et al. “Influence of Palm Oil Fuel Ash on Mechanical Properties of Ultra-High-Performance Concrete”. International Journal of Sustainable Construction Engineering and Technology, vol. 13, no. 4, Nov. 2022, pp. 44-53, https://publisher.uthm.edu.my/ojs/index.php/IJSCET/article/view/12242.