Retarders calcium aluminate cements

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). 

Western-world bauxite production in 1988 totaled about 90 x 10 t, approximately 90% of which was refined to aluminum hydroxide by the Bayer process. Most of the hydroxide was then calcined to alumina and consumed in making aluminum metal. The balance, which constituted about 2.3 x 10 t in 1988 (Table 2), was consumed in production of abrasives (qv) adhesives (qv) calcium aluminate cement used in binding ceramics (qv) and refractories (qv) catalysts used in petrochemical processes and automobile catalytic converter systems (see Petroleum Exhaust control, automotive) ceramics that insulate electronic components such as semiconductors and spark plugs chemicals such as alum, aluminum halides, and zeoHte countertop materials for kitchens and baths cultured marble fire-retardant filler for acryhc and plastic materials used in automobile seats, carpet backing, and insulation wrap for wire and cable (see Flame retardants) paper (qv) cosmetics (qv) toothpaste manufacture refractory linings for furnaces and kilns and separation systems that remove impurities from Hquids and gases. 

In seawater, calcium aluminate cement is more durable than Portland cement Conversion also takes place here, but it is usually veiy slow, except in the tidal zone or in warm waters (Baker and Banfill, 1992). If seawater is employed as mixing water the initial hydration may be retarded, but the final microstmcture of the hardened material is very similar to that made with fresh water. Chloroaluminates are formed in the hydration reaction (Raise and Pratt, 1986). 

According to Ding and Beaudoin (1996), the addition of small amounts of sodium silicate promotes the formation of strathngite (C2SAHg) in calcium aluminate cement pastes. At the same time the hydration of the cement is strongly retarded. The additive delays, but does not prevent completely, the formation of the hydrogamet phase in the cement paste. 

Additives for calcium aluminate cement formulations have been described. Additives with primarily retarding and accelerating effects are tartaric acid, sodium carbonate, and lithium carbonate. Additives that primarily affect the consistency of the composition... 

Other examples of set retarders are ethylenediamine tetraac-etic acid and nitrilotriacetic acid, or a combination of these compounds (11). StiU another example of a set retarder for Portland cement is a copolymer formed from a monomer of 2-acrylamido-2-methyl-l-propane sulfonic acid. Examples of set retarders for calcium aluminate cements are polymeric phosphate salts such as sodium tripol q3hosphate (12). 

Sugama, T., Carciello, N. R., and Gray, G. Alkali carbonation of calcium aluminate cements influence of set-retarding admixtures under hydrothermal conditions. J Materials Sci. 27,4909-4916, 1992. 

Dry mix mortars often exhibit a quite complex mix composition, especially if they are rapid setting and/or rapid hardening. In the latter case, they generally contain binary or ternary binders based on calcium aluminate or calcium sulfoaluminate cements in blends with calcium sulfate without and with portland cement. Isothermal calorimetry is an efficient method to use for optimising mix designs of such mortars with respect to the hydration kinetics. As only small cement mortar or paste samples are used, the influence of the binder composition as well as of different combinations of accelerators, retarders, water reducers, plasticisers, etc. can quickly be tested. Two examples of how the amount of calcium sulfate addition is able to influence hydration kinetics are shown for blends of calcium aluminate cement with hemihydrate (Figure 2.22) and ternary binders based on port-land cement, calcium sulfoaluminate cement and anhydrite (Figure 2.23). 

In cements, incorporation of calcium carbonate is permitted in some countries. In Canada, the maximum limit is set at 5%. Calcium carbonate is not an inert filler. It is known to react with calcium aluminate. In a study of the hydration of tricalcium silicate in the presence of finely divided calcium carbonate, Ramachandran observed that the carbonate acted as an accelerator. Ushiyama, et al.,t examined the effect of carbonates of Na, K, Li, Cs, and bicarbonates of Na, K, and Li on the hydration of alite. Although small amounts retarded the hydration, larger amounts acted as accelerators. 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO and the tricalcium aluminate keeps the SO level near zero concentration for the first 10-20 min causing rapid stiffening and quick set (approximately 15 min). It is also reported that chemical... 

Similarly influence the soluble fluorides for example the F ions released from NaF to the solution form the insoluble CaFj which covers the cement grains. The complex fluorosilicates Na2SiFg and NajAlFg can additionally result in the precipitation of siUcate or aluminate gel on the surface of cement grain poor in calcium. Phosphogypsum, soluble calcium phosphates and sodium fluoroaluminates containing by-products exhibits the retarding effect too [96]. 

Hydration of fly ash cement differs from pure cement in terms of the hydration rates of the clinker phases, amount of calcium hydroxide formed, composition of the clinker hydration products, and additional hydration products from the reaction of the fly ash.I l Lower amounts of lime are formed in the presence of fly ash because ofthe pozzolanic reaction between the fly ash and lime formed in cement hydration. Fly ash generally retards the reaction of alite in the early stages and accelerates the middle stage reaction. The accelerated reaction is attributed to the existence of nucleation sites on fly ash particles. The aluminate and ferrite phases hydrate more rapidly in the presence of fly ash, and also there is a significant difference in the hydration rate of the belite phase up to 28 days. Table 1 gives the relative hydration rates of cement compounds in the presence of fly ash as derived from conduction calorimetry. [" 1 It can be seen that the earlier rates of hydration are generally retarded, and the later stage hydration rates are accelerated. 

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