It is now established that the essential raw ingredients of Portland cement are limestone, 75 to 77%, and alumina (A12O3) and silica (SiO2) 20 to 25%. The alumina and silica are commonly found combined as * clay, which is a silicate of aluminum (Al2O32SiO2) + 2H2O. All cements contain other ingredients due to the essential materials not being pure in nature and from contact with fuel in calcination. Some of these accidental ingredients may improve the cement, others are inert and harmless except that they exist in such quantity as to displace essential ingredients, and others are positively harmful, if present in quantity, because of the chemical action they produce. There are accidental ingredients usually amounting to about 5%, but these must not include over 1.75% anhydrous sulphuric acid (So3), nor over 3% of magnesia (MgO).

Excess of clay causes a cement to "set" quickly, while excess of lime causes it to "set" slowly or imperfectly. Iron oxide gives cement its color. Portland cement is improved in dry storage, as any excess of lime particles are air slaked.