cement and concrete

Cement and concrete

Cement remains as the key material for building and infrastructure construction. It constitutes the basic ingredient for concrete, which, after water, is the most consumed material worldwide. Consequently, the cement industry, being the third largest CO2 emitting industrial sector, is constantly facing challenges in preserving natural resources and reducing its CO2 emissions. Nevertheless, in practice, the cement composition is largely determined by the raw materials locally available (most commonly limestone, clay, and other supplementary materials such as sand or fly ash).

Cement manufacturing becomes a complex process. It starts with the extraction from the quarry of raw materials such as limestone (calcium carbonate) and clay (aluminosilicate). Afterward, these are crushed and ground fine enough (usually finer than 90 micrometers) to produce a powder for blending. This powder is then heated to a sintered temperature as high as 1450 ºC inside the main burner of a rotary kiln. Therein, calcination takes place, the calcium carbonate being transformed into carbon dioxide and calcium oxide. The latter combined with the rest of materials in the blend yield calcium silicates and other cementitious compounds. The clinker is the hard substance obtained as a result. In addition, clinker can be milled to a fine powder in a cement mill and admixed with gypsum (calcium sulfate) to produce cement, typically, Portland cement.

The calcium carbonate (CaCO3) content in limestone can be as high as 80%. Thus, limestone represents the major raw material for the cement manufacture. Likewise, clay, sand, iron ore, shale, slag, or fly ash are some additional raw materials. Slags include blast furnace slag, which consists primarily of silicates, aluminosilicates, and calcium-alumina-silicates. Or Granulated Blast Furnace Slag (GBFS) which results of cooling and solidifying the molten slag by water quenching into a glassy state. This process yields sand-sized fragments. The physical structure of the granulated slag varies with its chemical composition, the temperature at the time of water quenching, and the method of producing the same.

The raw meal sinterization is considered one of the most energy-consuming steps, accounting for about 60% of the total energy required. Thus, the reduction in residence time within the sintering furnace and in temperature for the sinterization process would be a benefit in terms of production costs and carbon emissions. This goal could allow more environmental-friendly cement.

Moreover, concrete is a composite material that results from the mixture of cement and pozzolanic or cementitious materials such as slag or fly ash, with water and aggregates (normally a coarse aggregate from crushed rocks, e.g., limestone, granite, and a fine aggregate, e.g., sand). The allite present is responsible for the early strength development of concretes. Supplementary Cementing Materials (SCMs) confer various properties such as hardness or increased strength or reduced permeability to concretes.

Monitoring the composition of raw materials ensure the quality of final products. Therefore, cement laboratories use Analytical Techniques such as X-Ray Fluorescence (XRF) and X-Ray Diffraction (XRD) to control each step in the manufacturing process.

Some key parameters in the quality control of cement are the following physical properties:

– fineness (the hydration rate of cement is directly related to the cement particle size),
– strength (compressive, tensile and flexural),
– consistency (Vicat Test),
– setting time (the time that cement takes to set and harden when water is added),
– bulk density,
– relative density,
– heat of hydration,
– soundness (ability to not shrink upon hardening).