Admixtures and special uses of cements

Many materials have been used as admixtures and the potential of many more has been examined. The literature, including the patent literature and other descriptions of proprietary materials, is voluminous a useful review published as long ago as 1980 includes over 600 references (M103). We shall only attempt to describe the effects of the most important classes of admixtures in general terms and to discuss the chemistry underlying those effects at a fundamental level. Grinding aids are similarly considered. 

The rest of the chapter deals with the hydration chemistry of Portland and composite cements at temperatures outside the range 15-25 C, including that of autoclave processes, and with specialized uses of cements in casing oil wells and in making very high strength materials. 

---

All the cements considered in this book fall into the category of hydraulic cements they set and harden as a result of chemical reactions with water, and if mixed with water in appropriate proportions continue to harden even if stored under water after they have set. Much the most important is Portland cement. Chapters I to 4 of the present work deal mainly with the chemistry of manufacture of Portland cement and with the nature of the resulting product. Chapters 5 to 8 deal mainly with the processes that occur when this product is mixed with water and with the nature of the hardened material. Chapters 9 to 11 deal with the chemistry of other types of cement, of admixtures for concrete and of special uses of cements. Chapter 12 deals with chemical and microstructural aspects of concrete, including ones relevant to processes that affect its durability or limit its service life. 

Such an evaluation is particularly important where (1) the admixture has not been used previously with the particular combination of materials (2) special types of cement are specified (3) more than one admixture is to be used (4) mixing and placing is done at temperatures well outside generally recommended concreting temperature ranges (5) there is a limit on the amount of chloride ion that is permitted in concrete as manufactured. 

For special high strength appHcations, ie, up to 69 MPa (10,000 psi), special formulations of Pordand cement concretes have been developed. These ate based on the use of chemical and mineral admixtures. The typical mineral admixtures ate fumed siUca and other po22olanics. The chemical admixtures ate generally chemicals termed supetplastici2ets that allow very low water to cement ratios, ie, between 0.4 and 0.25, and reduce the amount of water needed to provide plasticity or dow to the concrete. PubHc works appHcations take just under 32% of the total Pordand cement market streets and highways represent 68% of this usage, and water and waste account for 23%. 

Specifically, the improved solidification (cementation) technology involves the use of (a) a special dry powder admixture for the generation of a nonsoluble crystalline formation deep within the pores and capillary tracts of the concrete—a crystalline structure that permanently seals the concrete against the penetration or movement of water and other hazardous liquids from any direction (b) special nonmetal reinforced bars for enhancing the concrete block s tensile and compressive strengths and (c) a unique chemical crystallization treatment for the waterproofing and protection of the concrete block s surface. 

Air-entraining water-reducing admixtures require special consideration the presence of entrained air leads to a reduction in compressive strength, whilst the water reduction results in a compensatory increase in strength. The effect can be quantified, however, by considering the amount of entrained air in terms of an equivalent volume of water to calculate the (air and water)-cement ratio. This new factor can be used to estimate the expected strength from Fig. 1.37. 

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. 

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). 

These admixtures are used in hot climates to slow down the cement hydration and to reduce the rate of heat liberation. A similar effect is needed in the execution of external surfaces, which are finished by special procedures, like exposing coarse aggregates. In other cases these admixtures are used when more time is needed for casting fresh concrete before cement hydration takes place. 

In the mix composition of advanced cement-based materials a few admixtures for improving their special properties in fresh and in hardened states are normally included (cf. Section 4.3). In particular, different water reducers and plasticizers are used in a large majority of concrete production. This trend will intensify in the future, based on the results of investigations. 

---