Portland cement consists mainly of__________________?
A. CaO & SiO2
B. SiO2 & Al2O3
C. CaO & Al2O3
D. CaO & Fe2O3
A. CaO & SiO2
B. SiO2 & Al2O3
C. CaO & Al2O3
D. CaO & Fe2O3