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 sev eral micra. This distribution is very important in establishing their macroscopic properties, e.g., vapor adsorption and desorption and mois ture 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 obtainin g the pore sizes distribution of a cement and lime mortar, often used as a building coating material. Electron scanning microscopy is used f or pore sizes greater than 1250 angstrom, from a sequence of pictures taken with magnifications from 25x to 12500x, for highly polished surf aces. The heterogeneous spatial distribution of pores is discussed, re lated to the problem of the geometrical reconstitution of porous struc ture. For pore sizes smaller than 1250 angstrom, adsorption isotherms obtained at 30-degrees-C are used. Molecular physical adsorption is su pposed to be the dominant adsorption mechanism in a wide range of rela tive humidities and modeled using the De Boer and Zwikker theory. This is confirmed by a very high correlation coefficient equal to 0.994 fo r the present case, for values of RH smaller than 80%. Capillary conde nsation is supposed to become significant at the point where the adsor ption curve deviates from the linear behavior as predicted by the De B oer and Zwikker theory, and the Broekhoff and De Boer theory is used f or predicting the pore size distribution from the adsorption isotherm, starting from the deviation point and increasing RH. The results show the pore size distribution between 200 angstrom and 13 mum.