The microstructure of porous building materials: Study of a cement and lime mortar

Building materials such as cement mortars and concrets present a very broad distribution of pore sizes, from some tenths of angstroms to several micra. This distribution is very important in establishing their macroscopic properties, e.g., vapor adsorption and desorption and moisture transfer. It is, thus, important to develop procedures to analyze the microstructure of these materials in the full range of pore sizes. In the present work, two complementary methods are used for obtaining the pore sizes distribution of a cement and lime mortar, often used as a building coating material. Electron scanning microscopy is used for pore sizes greater than 1250 å, from a sequence of pictures taken with magnifications from 25x to 12500x, for highly polished surfaces. The heterogeneous spatial distribution of pores is discussed, related to the problem of the geometrical reconstitution of porous structure. For pore sizes smaller than 1250 å, adsorption isotherms obtained at 30 C are used. Molecular physical adsorption is supposed to be the dominant adsorption mechanism in a wide range of relative humidities and modeled using the De Boer and Zwikker theory. This is confirmed by a very high correlation coefficient equal to 0.994 for the present case, for values of RH smaller than 80%. Capillary condensation is supposed to become significant at the point where the adsorption curve deviates from the linear behavior as predicted by the De Boer and Zwikker theory, and the Broekhoff and De Boer theory is used for predicting the pore size distribution from the adsorption isotherm,starting from the deviation point andincreasing RH. The results show the pore size distribution between 200 å and 13m.