Home page > Information sources > Civil engineering > In the heart of cement concrete     In the heart of cement concrete Cement concrete is a material providing a good durability when it is fairly well formulated and implemented. However, even after a very long hardening time, it is, in no case, a material that remains inert, and it changes and evolves with time according to the aggressivness rate of its surrounding environment. Physical and chemical deteriorations may be due to external agents (acid, pure water, sea water, salt solutions, etc.). Par ailleurs, ces altérations sont de deux types : soit une érosion qui entraîne une partie du liant, soit un gonflement souvent accompagné par une fissuration due à la formation de composés expansifs. Furthermore, the study of concrete by scanning electron microscopy and X analysis make it possible to provide information on a cement texture that has changed or on the precise nature of some deleterious species, either crystalline or amorphous (gel). (These pictures are freeware and NOT for commercial use) Silicious aggregate (in the center of the picture) coated by cement paste (magnification: X143) Fly ash in the form of perfectly spherical balls hiden inside the cement paste. Those fly ashes are issued from thermal plants and are used as fines in order to improve the nature of the mineral skeleton (magnification: X482).   Calcite crystals (CaCO3) in the form of a bunch. They result from the reaction between carbon dioxyde and alkaline components of the cement paste (magnification: X 1310).     Fine hexagonal calcium chloro-aluminate crystals due to the penetration of chlorides into concrete (magnification: X625)   calcium chloro-aluminate crystal due to the chloride action on the cement paste (magnification: X4400)     Solid gypsum crystals (CaSO4.2H2O) resulting from an environment full of sulfates (magnification: X2030)   Imprint of an aggregate covered by dense products that are more or less crystallized. This is ettringite (calcium trisulfoaluminates) that may be expanding (magnification: X2030)     Chestnut-bur looking ettringite needles after concrete was attacked by either sea water or an environment rich in sulfates (magnification: X326).   Imprint of an aggregate full of products resulting from alkali-aggregate reaction. They are expansive products resulting from the interaction between the soluble silica contained in some aggregates and the alkalines of the pore solution of concrete (magnification: X24).     Products issued from the alkali-aggregate reaction ; you can see a smoothed and cracked gel plate in the center of the picture and products that are more or less well organized at the extreme sides of the picture (magnification: X57).

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