Portland cement paste pore structure

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. 

Other components of the cement paste structure are magnesium hydroxide MgH, which may occupy up to 5% of the total volume and capillary pores with volume depending mainly on the w c ratio. Examples of images of the hydrated Portland cement paste are shown in Figure 6.4. 

Odler, I., RoEler, M. (1985) Investigations on the relationship between porosity, structure and strength of hydrated Portland cement pastes. II. Effect of pore structure and of degree of hydration . Cement and Concrete Research, 15 401-10. 

The sensitivity of concrete structures to sulphate attack is strongly related to the exposure conditions. Structures in an environment of high sulphate content in the air or in w ater, for example sewage tunnels, are particularly vulnerable. After sulphate ions penetrate the pore system of cement paste, complex reactions start with C3 A leading principally to two kinds of processes gypsum corrosion and sulphoaluminate corrosion (Mindess etal. 2003). The products of sulphate reactions with cement expand and can cause cracking and destruction. The permeability of the material s structure and the quality of cement decide upon the rate of these processes. Special Portland cements as well as high alumina cements may be used for elements exposed to sulphates (cf. Section 4.1.1). 

Mikhail, R. S., and Selun, S. A., Adsorption of Organic Vapors in Relation to the Pore Stmctrrre of Hardened Portlarrd Cement Pastes, Symposium on Structure of Portland Cement Paste and Concrete, Special Report 90, HRB 123-134 (1966)... 

Ma H. and Z. Li (2013). Realistic pore structure of Portland cement paste Experimental study and numerical simulation . Computers and. Concrete 11 317-336. 

Source Mikhail, R. S., and S.A. Selim. Adsorption of organic vapors in relation to the pore structure of hardened Portland cement pastes. Highway Research Board Special Report 90, 1966 Thomas, J. et alConcrete Science and Engineering, 1,45-64,1999. 

The structure of concrete-like materials may be considered at different levels (cf. Section 2.4). The lowest level, where the required resolution of observation is of the order of a few Angstroms, is used mostly for examination of the microstructure of cement paste. The main interest is concentrated between so-called micro- and macro-levels, which means between the size of Portland cement grains or of small pores of a few micrometers in diameter, up to tens of millimetres for maximum grains of coarse aggregate and the diameters of steel bars. 

---