Pengaruh Fasies dan Diagenesis Batugamping Terhadap Kualitasnya sebagai Bahan Baku Semen Portland pada Tambang Terbuka PT. Semen Padang, Sumatera Barat = The Influence of Limestone Facies and Diagenesis on Its Quality as Raw Material for Portland Cement in the Open Pit Mining of PT. Semen Padang, West Sumatra