Type I Portland cement is a general-purpose Portland cement that may be used for any application if the particular qualities of other kinds are not required. It is employed in situations when cement or concrete is not subjected to certain exposures, such as sulfate assault from soil or water, or an unpleasant temperature rise caused by heat created by hydration. Its low cost makes it suitable for applications where quality or performance is not critical. Type I cements contain approximately 12% calcium oxide by weight and 88% silicon dioxide. They are produced by heating raw materials (quartz sand, limestone, clay, etc.) with sodium carbonate or potassium carbonate at temperatures around 500 degrees Celsius for several hours. The alkalis react with the silica and alumina in the raw materials to produce sodium or potassium silicate and aluminosilicate, which eventually transform into beta quartz and alpha alumina upon cooling. The carbon dioxide gas given off during this process combines with the calcium oxide in the remaining material to create calcium carbonate, which becomes part of the hardened mass. The main difference between Type I cements and Type II cements is the amount of aluminum oxide present in the former. Type II cements have less than 0.5% aluminum oxide while Type I cements can have up to 2.0%. This means that they can resist acid attacks better. Also, Type I cements remain workable for longer periods of time at higher temperatures.