In constructions one of the most common used materials is concrete. In Estonia there is our own cement factory. It was decided that effective is to use different constituents and by-products in cement to reduce amount of Portland cement clinker. One important constituent is oil shale ash as Estonia produces electricity from oil shale. So far it has been used as pulverized combustion which emits significantly pollutants in the environment. Now we use the new technology. It is the fluidized bed boilers combustion and efficacy is higher and there is less environmental pollution. Hence arose the idea of our research work and the need to study how the fluidized bed ash behaves in content of cement. In the book there will be pointed out the fluidized bed combustion ash and its behavior and compared to pulverized combustion ash. Two different combustion temperatures ~ 800 ºC and > 1300 ºC give different properties of various cements. The tests are outlined in the book. The book also provides an overview what does Portland cement generally consist of, which minerals, which impact express, which temperatures give certain minerals.