Pore solutions Portland cement pastes

Table 2.1 shows the ionic concentrations measured by different researchers in the pore solution of cement pastes, mortars and concretes, obtained both with Portland cements (OPC) and blended cements [4-14]. Measurements were carried out by chemical analysis of the liquid extracted under pressure, using specific pore-extraction devices. 

Figure 3.4 Relation between the equivalent percent of alkali in Portland cement and OH concentration in the pore solution of cement paste [18]...

Fig. 7.8 Concentrations in the pore solution (scaled as indicated in the cases of OH and SiOj) of a Portland cement paste of w/c ratio 0.5. After Lawrence (L. 2).

The hydrated material has been analysed by X-ray microanalysis and analytical electron microscopy. In a 3-day old paste, that formed in situ from alite or belite did not differ significantly in composition from the corresponding product in pure Portland cement pastes (H4). but at later ages Ca/ Si is lower and Al/Ca higher (R25,R26,T44,U 17,U 18,R42). Ca/Si is typically about 1.55, but the value decreases with age and ratio of pfa to clinker. Uchikawa (U20,U17) reported a value of 1.01 for a 4-year-old paste with 40% replacement of cement by pfa. Several of the studies (R25,T44,U20,U 17) showed that the C-S-H was higher in alkalis if pfa was present, but one cannot tell to what extent potassium or sodium apparently present in the C-S-H has been deposited from the pore solution on drying. For material close to the pfa particles in a 10-year-old mortar. Sato and Furuhashi (S92) found a Ca/Si ratio of 1.1-1.2. 

Figure 5. Evolution ofthe relative under-saturation index a according to the duration of gamma irradiation of a Portland cement paste for two dose rates (simulation). The solubility product Ks is reached for the highest dose rate that causes an earlier regulation ofthe radiolysis in the pore solution.

The alkalinity of the pore solution in hardened alinite cement pastes lies at around pH=ll (Kostogloudis et al, 1998), and thus is distinctly lower than that common in ordinary Portland cement pastes (pH 12.5-13.5). 

Lothenbach, B., F. Winnefeld, C. Aldei E. Wieland and P. Lunk (2007). Effect of temperature on the pore solution, microstructure and hydration products of Portland cement pastes . Cement and Concrete Research 37(4) 483—491. 

Page el al. (P52) determined the CP/OH" ratios in the pore solutions, CP diffusivities and corrosion rates of embedded steel in pastes of cements of several types. With Portland cements, the CP/OH" ratios decreased... 

Table 2.1 Ionic concentration (in mmol/L) measured in the pore solution extracted from cement pastes, mortars and concrete made with ordinary Portland cement (OPC) and with additions of blast furnace slag (GGBS), fly ash (PFA) and silica fume (SF). n.a. = concentration not available [15]... 

There are generally more alkalis in fly ash than in Portland cement. Most of these alkalis occur in the vitreous phase and hence only a part is water soluble, usually about 0.1% Na20g [60]. On the other side one should remember that the glass is the most reactive component of fly ash in cement paste. It is not clear what part participate in the pozzolanic reaction and what is released to the pore solution. However, there is a dominant opinion that the release of alkalis to the solution from fly ash occurs slowly and hence they cannot participate in the reaction with aggregates [60]. Therefore Hobbs [110] proposes to take only 0.2% Na20 as an income of alkalis from fly ash, when the total alkalis content in concrete is calculated. However, better effect are showing fly ash with low alkalis and CaO content [60, 121]. 

As a result of this process the calcium aluminate hydrates will convert to calcium carbonate and hydrous alumina. The process is associated with a lowering of the pH value of the pore solution, which in turn may result in corrosion of the steel reinforcement. Rocks that may include freeable alkalis include granite or mica. As well as alkalis derived from the aggregate, alkalis may also migrate into the paste from an outside soitrce, for example if placed in contact with Portland cement concrete. 

Steel fibers guarantee good properties of the fiber-reinforced composite material, but must be used only in combination with binders that protect the fibers from corrosion. This requirement is generally fulfilled in cements in which the pore solution of the hardened paste has a pH higher than about 11. Portland cement is one of the binders that meets this requirement, and is most widely used in combination with steel fibers (Bentur et ai, 1985b, 1985c Bentur, 1989 Marchese and Marchese, 1992). Both the compressive and the flexural strength of steel fiber reinforced concrete made with Portland cement... 

---