Hardening calcium aluminate cements

After reaching a maximum value the strength of hardened calcium aluminate cement starts to decline, owing to the conversion of the primary formed CAHjg and C2AHg to... 

Concrete based on calcium aluminate cement performs rather well if exposed to sulfate solutions, especially if made with a low water/ cement ratio and high cement content. However, cases of expansion and cracking have also occasionally been reported (Scrivener and Capmas, 1998). The reasons for the good sulfate resistance of this type of cement are not obvious. It is mostly attributed to a surface densification of the hardened material, resulting in a very low permeability of the formed surface layer, and/or to the absence of calcium hydroxide in the system (Scrivener and Capmas, 1998). Unlike Portland cement and related binders, magnesium sulfate solutions are less aggressive to calcium aluminate cement than alkali sulfate solutions. This is due mainly to the absence of the C-S-H phase in the hardened calcium aluminate cement pastes, which is particularly sensitive to the action of magnesium sulfate. 

An alternative to silicate-based Portland cement is the calcium aluminate cement, ciment fondu, which originated with the Lafarge company in France in 1908. Ciment fondu is typically made by heating limestone with bauxite, which is mainly AIO(OH) but contains much iron oxide (see Section 17.2). As noted above, calcium aluminate hydrates and hardens much more rapidly than alite, and so ciment fondu, either as such or mixed with Portland cement, can be used whenever a rapidly setting cement is required, for example, for construction at low temperatures. Concretes made from aluminate cements remain serviceable at higher temperatures than Portland cements and so are used to make cast refractories for pyrometal-lurgical applications. 

Ciment Fondu is normally made by complete fusion of limestone and bauxite at 1450-1600 C. In order to produce a cement with the desired rapid-hardening properties, both raw materials must be low in SiO,. The molten clinker is tapped off continuously from the furnace, solidifies and is typically crushed and ground to a fineness of about. 00 m- kg . Some iron is reduced to Fe . The colour of cements produced from bauxite can vary from yellow brown to black, but is commonly greyish black. White calcium aluminate cements are usually made by sintering calcined alumina with quicklime (calcium oxide) or high-purity limestone. 

Calcium aluminate cements harden rapidly as soon as the massive precipitation of hydrates begins. This may be attributed to the fact that, unlike those of Portland cement, the major hydration products are crystalline. Relatively high proportions of water are taken up in the hydration reactions, the theoretical w/c ratios needed for complete hydration of CA being 1.14, 0.63 and 0.46 for the formation of CAHj, CjAHg -F AHj and CjAH -F 2AHj, respectively. For this reason, and also because of the rapid heat... 

Various methods have been used to obtain cements that set and harden rapidly. They include the use of Portland cement with admixtures and of mixtures containing both Portland and calcium aluminate cements, described in Sections 11.5 and lO.I.IO, respectively. Another approach has been the manufacture of clinkers containing either CuAy CaF, or C 4A, S. both of which hydrate rapidly under appropriate conditions with the formation of ettringite. 

Hydraulic cements are another class of technologically important materials. Examples include Portland cement, calcium aluminate cement, and plaster of Paris. They harden at room temperature when their powder is mixed with water. The pastes formed this way set into a hard mass that has sufficient compression strength and can be used as stmctural materials. Their structure is generally noncrystalline. 

In Chapter 2 we mentioned cement and the reactions that occur during the setting and hardening of this material. There is a class of cements known as calcium aluminate cements (CACs) or high-alumina cements (HACs). These ceramics are not used as widely as Portland cement, but their attraction is the rapid hardening reactions. In 1 day CAC achieves the same strength as Portland cement achieves in a month. 

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. 

At low or medium water/cement ratios the porosity and permeability of hydrated non-converted aluminous cement pastes are sufficiently low to confine the corrosive action of any external chemical agents to the surface region of the concrete structure. However, as the porosity increases in the course of conversion, the susceptibility to chemical attack of concrete based on aluminous cement increases. An effective way to prevent this from happening is to use initial water/cement ratios that are too low for complete hydration. Under these conditions the water liberated in the conversion of the hexagonal calcium aluminate hydrate phases, formed initially, reacts with the non-hydrated fraction of the cement, thus preserving a low porosity of the hardened paste. Note that the permeability is the main factor determining the resistance of aluminous cement concrete to chemical agents, and this has to be kept in mind when calcium aluminate cement is used in practice. 

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). 

In conclusion, the resistance of aluminous cement based concrete to most chemical corrosive agents is outstanding, provided that the mix is produced with a sufficient cement content and with a low water/cement ratio, and is properly compacted and cured. Like other hydrated cementitious systems, hardened calcium alununate cement pastes decompose and undergo chemical reactions upon heating ... 

An aggregate particirlarly suitable for use in combination with calcium aluminate cement is limestone. The calcium carboaluminate hydrate (C aCHjj) formed in the reaction of the CaCOj of the limestone with the calcimn aluminates of the cement contributes to an improved bond between the hardened cement paste and the aggregate sitrface. 

Calcium aluminate cement based rapid-setting/hardening binder... 

Menetrier-Sorrentino, D., George, C.M., and Sorrentino, F.P. (1986) The setting and hardening characteristics of calcium aluminate cements studies of the system C3A and Cj2Ay-CA, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 4, pp. 339-343. 

In the most widely used expansive cements the cementitious component consists of ordinary Portland cement, but other binders such as calcium aluminate cement may also be employed. Most commonly, the expansive stresses in the course of hardening are created by the formation of ettringite (C3A.3CS.32H). This phase is formed in the reaction of a constitnent containing alutninnm oxide with calcinm snlfate, calcinm oxide or hydroxide and water. Less common than expansive cements based on ettringite... 

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). 

Many bonds are mixed. For instance, in the case of phosphate-boned ramming materials, a combination of chemical and ceramic bonds are used. Also, in the case of materials that are repaired with a hydraulic bond, the bond is made up of calcium aluminate cements and considerable amounts of clays. This means that the total bond is a mixture of hydraulic and ceramic bonds. When it is a mixed bond, the bond is named after the material that plays the major part during hardening. 

In another system, a colloidal silica solution is used to bond the castable (11). Hardening is achieved by the use of a small amount of a setting agent (< 1%) such as magnesia, lime, or calcium aluminate cement. Again, dewatering is not as critical as with cement systems because the silica sol does not chemically attach the water. Dewatering of this system is reported to be completed at relatively low temperatures (<125°C). 

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). 

Cement and Concrete Concrete is an aggregate of inert reinforcing particles in an amorphous matrix of hardened cement paste. Concrete made of portland cement has limited resistance to acids and bases and will fail mechanically following absorption of crystalforming solutions such as brines and various organics. Concretes made of corrosion-resistant cements (such as calcium aluminate) can be selected for specific chemical exposures. 

---