Pore structures composite cement pastes

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

Studies on other materials show that MIP determines the width distribution of pore entrances and not of the pores themselves (D34). The intrusion of mercury may also coarsen the pore structure this need only imply that, at the higher pressures employed, some of the foils of the gel are displaced so that some pores are widened and entered while neighbouring ones are closed up. The combined result of these processes would be to produce a distribution narrower than that existing before the intrusion began, and a value for the porosity at maximum pressure that corresponded to a minimum pore width before intrusion of less than 3.5 nm. Experiments in which the mercury was removed and subsequently reintruded have indicated that the structure is usually not altered in the case of Portland cement pastes, though it is in that of pastes of composite cements (F35,D32), but cannot show whether an irreversible change occurred during the first intrusion. 

Studies on the pore structures of pastes of composite cements have presented... 

C. L. Page, 0. Vennesland, Pore solution composition and chloride binding capacity of silica fume cement pastes . Materials and Structures, 1983, 16, 91. 

The porosity and pores structure will be of special importance for liquids transport in concrete. These both properties relate directly to w/c ratio and the degree of hydration, because porosity of concrete is affected mainly by cement paste the porosity of aggregate is generally very low. Continuous pores volume is increasing with w/c ratio and decreases with degree of cement hydration. There ate the permeability controlling factor, because the transport of liqitids in concrete composite occurs principally in continuous pores. 

It is difficult to determine the maximum matrix strain before cracks are open because it may vary considerably. In a more developed material structure, composed of inclusions, pores, fibres, etc., the value of is higher than in homogeneous ones like cement paste. As to the heterogeneity of the matrix, not only are the differences between hard inclusions, weaker paste and voids considered, but also the fact that certain macroscopic composite regions have considerably different mechanical properties than others. 

The factors that influence the mechanical behavior of concrete are shape of particles, size and distribution of particles, concentration, their orientation, topology, composition of the disperse and continuous phases, and that between the continuous and disperse phase and the pore structure. The important role played by cement paste is already described. 

---