Setting of Cement

Hardened cement is a porous sohd with a density of about 2.5 g/mL and a surface area of 100-300 m /g. Such pores can be fully sealed by polymer impregnation that has numerous applications. 

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R. B. Carpenter and W. N. Wilson. Method of accelerating set of cement by washover fluid containing alkanolamine. Patent US 4976316, 1990. 

It is this unpredictable and puzzling chemical reactivity which makes freshly formed silica dust a chemical poison that causes silicosis when it is inhaled. In many processes which deal with mineral products—e.g., the setting of cements, the milling of enamels and of pigments, the slaking of lime, etc.—solids with freshly formed surfaces are brought into contact with water. For understanding these phenomena the kinetics of hydration of incompletely screened surfaces has to be considered. 

Cement chemistry is sensitive to the components of the combustion products. Some contain chlorides. Chlorides and other anions hinder the setting of cement. 

The carbon content of ash is also a major factor. High carbon ash hinders setting of cement. In particular, ash produced by low-NO burners contains a high proportion of carbon. To meet the Clean Air Act requirements, future industries may opt for low-NOj burners that will produce ash unsuitable for cement. 

A small percentage of gypsum is added during grinding to control or retard the time of setting of cement in the field. 

Fig. 5.3 -2. Processes in the setting of cement, [according to F, W. Lochcr in ZcmcnUaschenbuch (Cement Handbook) 1984, p. 67, Bauverlag. Wiesbaden (1984)]...

Design and synthesis of macrocyclic ligands for specific interaction with crystalline ettringite and demonstration of a viable mechanism for the setting of cement, J. L. W. Griffin, P. V. Coveney, A. Whiting and R. Davey, J. Chem. Soc., Perkin Trans. 2, 1999, 1973. 

Retarding admixtures are water soluble chemicals which delay the setting of cement. 

In many cases, particularly for very small particles, surface area is a more appropriate characteristic to assess than some size based on an equivalent diameter. Particle surface area is important, for example, in paints and pigments or when chemical reactivity is an important property, as in the setting of cement. Precipitated materials are often characterized in this manner. Amongst the several techniques available, those based on permeability and gas adsorption are probably the most popular. 

Air-setting of cement during storage, poor workability Alkali-water combination, alkalies developed upon quick cooling of reduced clinker (Long, 1982a)... 

Relatively rapid set of cement Clinker burned and cooled under reducing conditions highly reactive, well-crystallized, more abundant C3A, slowly cooled (Sylla, 1981)... 

It has long been recognized that, even in the absence of calcium sulfate, flash setting of cement may be prevented and a cement suspension of flowing consistency obtained by combining finely groimd Portland clinker with an alkali metal carbonate or hydrogen... 

FIGURE 2.98 Spin lattice relaxation time, T, as a function of the specific surface area of a set of cement pastes. (Drawn from the data shown by Gran and Hansen, Cement Concrete Res., 27,1319,1997.)... 

H NMR spectroscopy (Figure 2.98) was applied by Gran and Hansen (1997) to study a set of cement pastes of different structural and hydration characteristics, the freeze/thaw cycling effects on the pore structure. 

The kinetics of setting of cement depends on the rate of hydration of the individual... 

In order to accelerate or retard the setting of cement, water-soluble admixtures may be used (see Section 5.3.1). Whilst many inorganic salts, such as calcium chloride, can act as accelerators, lead and zinc salts, borates and phosphates, as well as hydrocarbonic acids (e.g., citric acid or tartaric acid) and, most importantly, sugar derivatives such as raffinose or sucrose, will tend to retard the setting process. As the rate of setting can be determined by recording the heat evolved during hydration, a plot of (in Wkg ) versus 1/t will yield a close to linear relationship. 

Structure which facilitates the formation of hydrates, a property crucial to the setting of cement. 

Active metals are highly reactive metals. Oxides of active metals react with water to produce metal hydroxides. Calcium oxide, CaO, also known as lime or quicklime, is manufactured in large quantities. The addition of water to lime to produce Ca(OH)2, which is also known as slaked lime, is a crucial step in the setting of cement. 

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