de Luxan, M.P., Sánchez de Rojas, M.I., Frías, M., Saavedra, J., Martín Patino, M.T. and Madruga, F. 1989. Effect of addition of opaline siliceous rocks at different proportions to portland cement. Materiales de Construcción. 39, 213 (Mar. 1989), 21–32. DOI:https://doi.org/10.3989/mc.1989.v39.i213.814.