Accelerators calcium aluminate cements

In calcium aluminate clinkers the lower or higher Cj2A, amounts are present, which causes set acceleration of cement paste. Sometimes in the sintered clinkers even the CjA phase can be present, due to the local inhomogeneity of the raw mixture, in which the larger limestone grains can occur. Calleja [16] proposed the simplified formulae for the calculation of calcium aluminate cement phase composition. 

There is an opinion that the concrete should be matured for at least 24 h at the temperature of 20 °C or close to 20 °C. The calcium aluminate cement concrete shows the lowest strength at the temperature of 800-900 °C, because the calcium aluminate hydrates are decomposed and the ceramic bond has not sufficient strength. The CJ2A7 phase is detected as a first dehydration product, and at temperature of about 600 °C CA2, formed as a result of active AI2O3 (in statu nascendi) reaction this AI2O3 is the product of AH3 decomposition [12]. The sintering is greatly accelerated at temperatures above 800 °C. 

The total heat of hydration of calcium aluminate cement is in the range 450-500 J/g, and is similar to that of Portland cement. However, 70-90% of it is liberated within the first 24 hours (at 20°C), making dissipation of the heat into the environment more difficult than in situations where Portland cement is employed, where the liberation of hydration heat takes place much more slowly. This may be critical, especially in the erection of massive stmctures, where a significant rise of temperature may take place shortly after mixing. Maximum temperatures of up to 80°C may be reached in the bulk of the concrete stracture. This in turn may accelerate the conversion of the calcium aluminate hydrates formed. Wet curing has to be employed to prevent superficial dehydration and dusting of the hardened concrete. 

In the CAC-rich region setting is due to hydration of the calcium aluminate cement, which is accelerated in the high-pH environment produced by the presence of Portland cement in the mix. 

The hydration process in OPC+CAC blends may also be accelerated by adding small amoimts of a lithium salt, such as Li2C03, to the mix. The hthium salt, which accelerates the hydration of the monocalcium aluminate phase very effeetively, is also highly effective in mixes in which the calcium aluminate cement is the minor component. 

To accelerate the process of hardening it has been suggested that carbon dioxide should be injected into the spreadable mix of wood, cement, and water, or that ammonimn, sodium or potassium carbonate should be added (Simatupang et al, 1995). Under these conditions the cement sets in a very short time, owing to the formation of calcimn carbonate. Effective acceleration of the hardening process may also be achieved by the use of a fast-setting cement produced by combining Portland cement with hmited amounts of calcium aluminate cement (see section 10.10.1). 

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... 

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). 

Alite reacts with water to form calcium silicate hydrate and calcium hydroxide, which is also known as portlan-dite. The hardened paste has high strength when the reaction is completed, and because alite is the most abundant compound in cement, it also makes the dominant contribution to the mechanical properties of the final product. The hydration reaction proceeds at an appreciable rate a few hours after the addition of water and lasts up to about 20 days. The reaction of alite with water is accelerated by aluminate and gypsum. 

In mixes in which calcinm aluminate cement is the main constituent, the reaction mainly responsible for fast setting and strength development is a very fast hydration of the calcium alirminate phases. The accelerated hydration responsible for fast setting and strength development is dne to the increase of the pH value of the mix, brought about by... 

Many substances are known to act as accelerators for concrete. These include soluble inorganic chlorides, bromides, fluorides, carbonates, thiocyanates, nitrites, nitrates, thiosulfates, silicates, aliuninates, alkali hydroxides, and soluble organic compounds such as triethanolamine, calcium formate, calcium acetate, calcium propionate, and calcium butyrate. Some of them are used in combination with water reducers. Quick setting admixture s used in shotcrete applications and which promote setting in a few minutes may contain sodium silicate, sodium aluminate, aluminum chloride, sodium fluoride, strong alkalis, and calcium chloride. Others are solid admixtures such as calcium aluminate, seeds of finely divided Portland cement, silicate minerals, finely divided magnesium carbonate, and calcium carbonate. Of these, calcium chloride has been the most widely used because of its ready availability, low cost, predictable performance characteristics, and successful application over several decades.In some countries the use of calcium chloride is prohibited, in some others, such as Canada and the USA, the use of calcium chloride is permitted provided certain precautions are taken. Attempts have continued to find an effective alternative to calcium chloride because of some of the problems associated with its use. 

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 banning of calcium chloride during the last decade provided the impetus for the development of alternative materials which accelerated the hydration of cement without the potential for corrosion. A number of inorganic and organic compounds including aluminates, sulfates, formates, thiosulfates, nitrates, silicates, alkali hydroxides, carbonates, nitrites and calcium salts have been evaluated. Commercialization and field experience, however, is limited to only a few of these materials. 

The alkali content in clinker is usually on the level of 1 % Na O with dominating soluble alkali sulphates. In the solution rich in CH, they increase the solubility of gypsum, while the solubility of calcium hydroxide decreases. In this condition the SO4 ions can be bonded with C-S-H. That is why the gypsum addition should be higher. When the specific surface area of cement is high and the reaction of tricalcium aluminate is therefore accelerated, the gypsiun addition should be higher too. 

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