Special purpose admixtures

Other types of admixtures used with calcium aluminate cements include water reducers and superplasticizers (Section 11.4), which also act as retarders, and thickening agents, such as carboxymethylcellulose. Complex formulations may be used for special purposes for example, a ready-mix mortar for high-performance road repair might contain 55% Ciment Fondu and 45% sand, with glass fibre, aluminium powder, Li2C03, sodium gluconate and methyl ethyl cellulose (M93). 

Styrene-butadiene latex is used as an admixture for materials applied to the repair of concrete structures, for protective overlays and for industrial floors. A typical application would be for the repair of bridge decks damaged by de-icing agents and freeze-thaw cycles. The PCC is also applied for special purposes, such as protective spray-on coatings for various kinds of structures or decorative elements. The latex is added as 5% and 20% of cement mass during mixing. If the latex content is below 5% of cement mass the term PMC is used instead of PCC. 

Successful use of various high performance materials based on the cement matrix has a considerable positive influence on the production of ordinary concretes. New components and technologies developed for special purposes are now, at least partly, applied in everyday production in ready-mix concrete plants. A large variety and better quality of admixtures, improved precision of execution and adequate curing are the bases for ordinary concretes that are becoming inexpensive, strong and ensure improved durability of buildings and civil infrastructure. 

Modem high-performance concretes (such as non-dispersible underwater concrete, self-compacting concrete, and concretes used at below freezing temperatures) have necessitated the use of special admixtures, often requiring on-site addition. Figure 7.57 shows a special dispensing unit with storage tank used for this purpose. 

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