The Setting of Cement

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. 

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. 

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. 

Xu L 1984 [Arabinogalactan from two larch timbers - contents, molecular properties, and effects on the setting of cement.] Linye Kexue 20 57-65... 

Zinc oxide, in small amounts, retards the setting of cement very effectively. Calorimteric investigation has shown that, at an addition of 0.25% ZnO, hydration is almost nil up to 12 hours, and, at 1% addition, hydration is not initiated even up to 2 days. l... 

The physico-chemical phenomena associated with the setting of cement is complex. The time of setting is defined in ASTM in terms of the penetration of a needle to a specific depth. Ramachandranl l examined by DSC several cement samples containing water reducers, retarders, and superplasticizers that had just set. In all the mixes, an endothermal peak typical of Ca(OH)2 was detected, suggesting that at the time of set at least some CjS must hydrate. It was also observed that the superplasticized cement paste had hydrated to a lesser extent than the reference cement paste at the time of setting. This indicates that physical forces are also involved in the setting phenomenon. 

The trials that afforded the two exceptions were with mortars containing a smaller proportion of cement than the six others. These facts seem to show that the addition of bitter-water, within certain limits, improves the cement, but that bejond these limits it is injurious and that where the proportions of cement are great, an increased addition of bitter-water may be advantageous. These particular experiments were made in consecjuence of finding that the addition of a little bitter-water hastened the setting of cement A when immersed. 

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