Portland cement pore solution composition

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. 

Since the pH of the concrete pore solution may vary in the interval where remarkable changes in the behaviour of zinc occur (Section 2.1.1), the behaviour of galvanized steel may be influenced by the composition of the concrete and, especially by the cement type and its alkali content. In practice, however, the pH of the pore solution in concrete usually is below 13.3 during the first hours after mixing, due to the presence of sulfate ions from the gypsum added to the Portland cement as a set regulator. A protective layer thus can be formed on galvanized bars. 

Dining manufacturing of cement-based pipeline products, the silicates and alumi-nates present in the cement react with water to form products of hydration and, in time, these set to a hard mass. The various solid phases formed come into thermodynamic equilibrium with the interstitial water (pore water), which is rich in calcium, sodium, and potassium hydroxide phases. The presence of these hydroxides raises the pH of the pore water solution to about 13 or 13.5. Table 4.15 gives typical compositions of pore solution for two types of cement Portland and blast furnace slag cement. 

Calciiun aluminate cement has been suggested as a binder for glass fiber reinforced composites. (Majumdar et al, 1981). This cement appears to be more siritable for this purpose than Portland cement, as the pH of its pore solution is distinctly lower. Also, it does not liberate calcium hydroxide in its hydration, which is resportsible for the embrittlement seen in such materials after prolonged times of ciuing. 

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... 

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