Portland cement pastes fresh

Fig. 4.3 Backscattered electron images of polished sections of (A) a Portland cement clinker and (B) grains of a Portland cement in a fresh paste. In both sections, alite is the predominant clinker phase. In (A), the relatively large, darker areas are of belite, and the interstitial material consists of dendritic ferrite (light) in a matrix of aluminate (dark) cracks and pores (black) are also visible. In (B), the belite forms well-defined regions, which are rounded, striated and darker than the alite the interstitial material, present, for example, in a vertical band left of centre within the larger grain, consists mainly of ferrite (light) and aluminate (dark). Scrivener and Pratt (S28).

Structure and properties of fresh and hardened Portland cement pastes... 

The sludges were formulated to contain 0.04 moles/L each of cadmium, chromium, and lead. Metals were added as cliromium chloride, cadmium nitrate, and lead nitrate and converted to their hydroxide precipitate forms in the sludge by neutralization of the solution to pH 8.5 with sodium hydroxide. Some samples al.so contained sodium arsenite, to evaluate the leaching of anionic metals. The sludges were mixed with type II portland cement to form cement pastes with water/cement (w/c) ratios of 0.5 and 1.0 u-sing ASTM procedure C30582. The freshly mixed pastes were placed in 3.8 cm diameter by 7.6 cm tall PCV cylinders and allowed to set for 24—48 hours at 100% humidity before they were removed from their molds and returned to the 100% humidity environment. 

Portland cement Mixing 0,1,3 and 5 wt% of HO2 with cement. NS-Ti02 was PC-105 from Millennium (anatase, BET 5+10 m /g, a crystallite size of 15-25 mn). The cement pastes were mixed with a water/cement ratio of 0.33. After mixing, the fresh materials were cast into 20 x 20 xl cm moulds made of expanded polystyrene Decolorization of Rhodamine B [529]... 

Silica fume. Silica fume (SF) is a waste product of manufacturing ferro-sihcon alloys. It consists of an extremely fine powder of amorphous silica. Average particle diameter is about 100 times smaller than that of Portland cement and the specific surface area is enormous 13000-30000 m /kg compared to 300-400 m /kg for common Portland cements. Silica fume shows an elevated pozzolanic activity and is also a very effective filler. For these reasons, addition of silica fume to Portland cement may lead to a very low porosity of the cement paste, increasing the strength and lowering the permeability. It is usually added in the proportion of 5 to 10 % and it is combined with the use of a superplasticizer in order to maintain adequate workability of the fresh concrete. 

If ground together with Portland clinker, limestone will predominate in the fine fractions, as it is significantly softer than the clinker. In a fresh paste made from such a cement, the fine limestone particles will fill the spaces between the coarser particles of clinker, and consequently the amount of water needed to attain a given consistency will decline, up to an optimum limestone addition. This amount of limestone may vary in different cements, up to as much as 25 wt%. 

The Portland clinker used should contain a high amount of tricaldum silicate, preferably more than 45%. This is necessary as the hydration of this phase produces the calcium hydroxide needed for a pozzolanic reaction of the ash. The hydration of the clinker minerals is mainly responsible for the setting and initial strength development of the cement, as the reaction rate of the fly ash is rather slow. The lydration of the ash contributes to strength only at longer hydration times, but also affects other properties of the hardened material. The calcium sulfate added in the form of gypsum or anhydrite serves to control the setting of the fresh paste in a similar way as in plain Portland cement. 

Both fresh and hardened cement pastes exhibit electrical conductivity, which is brought about by the movement of ions in the liquid phase. The limiting factors are the size and tortuosity of the existing pore system, rather than the mobility of the ions involved and their concentration in the liquid phase (Tumidajski et al, 1996). The electrical conductivity increases with increasing water/cement ratio and declines with progressing hydration, as the volume of the pores declines. The following conductivity values [in ( fl.m) ] were reported for Portland cement pastes made with different water/cement ratios and hydrated between one and 28 days (Tumidajski, 1996) ... 

---