S. M. Siregar, S. Humaidi, N. Bukit, and E. Frida, “Palm Oil Fuel Ash and Fly Ash for a Partial Replacement of Cement in High-Quality, Environmentally Friendly Mortar as a Solution to Industrial Waste”, sci. technol. indones., vol. 9, no. 1, pp. 59–68, Jan. 2024.