Sulfoaluminate hydrates

Calcium sulfoaluminates hydrates CJAS3H32, ettringite, has the form of small crystals (1-10 pm) with long needles or plates and stripes, its volume corresponds to 10-15%. 

Sulfates will also cause concrete to deteriorate. In addition to being able to produce sulfuric acid, which is highly corrosive to concrete, sulfates are also reactive with some aggregates used in concrete mixes. Sulfate ions react with tricalcium aluminate to form sulfoaluminate hydrate with a large crystallized water content. 

After all gypsum is converted to ettringite, the excess C3A will react with ettringite to form the low sulfoaluminate hydrate. If calcium hydroxide is present the compound C4AH13 will also form. 

The rate of hydration of cements can be determined through the heat development characteristics using a conduction calorimeter. In Fig. 13, the heat effect in the first few minutes is attributable to the heat of wetting and ettringite formation. Within a few hours, another strong exotherm appears due to the hydration of C3S. In some cases, depending on the composition of the cement, an additional peak is observed after the C3S peak. This is related to the reaction of C3A to form the low sulfoaluminate hydrate. 

ASTM C845 Type E-I (K) expansive cement manufactured ia the United States usually depends on aluminate and sulfate phases that result ia more ettriagite formation duriag hydration than ia normal Portland cements. Type K contains an anhydrous calcium sulfoaluminate, C A SI. This cement can be made either by iategraHy burning to produce the desired phase composition, or by intergrinding a special component with ordinary Portland cement clinkers and calcium sulfate. 

The most widely used single component, calcium sulfoaluminate admixture, is composed of 30% CAS, 50% CaSO and 20% CaO with small amounts of glassy phase. Particle s3ize is coarser than that of Portland cement. Larger particle size ensures that the potential expansion due to hydration is extended over a period of time. Chemical and physical properties of the most widely used proprietary product, Denka CSA, are given in Table 6.10 [74], Other CSAs include mixtures of C ASH and 2 CS (monosulfate and gypsum) and mixtures of Type I cement, liigh-aiumina cement, CaSO, 2H O, Ca(OH) and CaO [75], 4 2... 

The hydrated components (C-S-H, portlandite, sulfoaluminates) in the cement matrix of concrete are in equilibrium with the pore liquid that is characterized by a high pH, due to the presence of OH (balanced by Na and K ). When concrete comes into contact with acid solutions, these compounds may dissolve at a rate that depends on the permeability of the concrete, the concentration and the type of acid. In soil with acidic ground water, the rate of refreshing is important Acids that can attack concrete are sulfuric acid, hydrochloric acid, nitric acid, organic acids such as acetic acid and humic adds and solutions of CO2. The rate of attack on the cement matrix depends on the solubility of the salts that are formed... 

Mehta, P.K. Effect of Lime on Hydration of Pastes Containing Gypsum and Calcium Alu-minates or Calcium Sulfoaluminate. J. Am. Ceram. Soc. 56, 315 (1973). (published by John Wiley Sons Ltd, reproduced with the permission of Wiley Sons)... 

Hanic, F., Kapralik, ., and Gabrisova, A. (1989) Mechanism of hydration reactions in the system C4A3S-CS-Ca0-H20 referred to hydration of sulfoaluminate cements. Cement and Concrete Research 19,671-682... 

Kasselouri, V. et al. (1995) A study on the hydration products of a non-expansive sulfoaluminate cement. Cement and Concrete Research 25,1726-1736. 

Lau, W., and Glasser, F.P. (1996) Hydration of calcium sulfoaluminate cements. Advances in Cement Research 8, 127-134. 

Palon, M.T., and Majling, J. (1996) Effect of sulfate, calcium and aluminum ions upon the hydration of sulfoaluminate belite cement. Journal of Thermal Analysis 46,549-556. 

Sahu, S., Majling, J., and Havlica. J. (1992) Influence of anhydrite particle size on the hydration of sulfoaluminate belite cement, in Proceedings 9th ICCC, New Delhi, Vol. 4, pp. 443-448. 

To produce expansion in expansive cements a variety of aluminous phases may be considered, but calcium aluminate or sulfoaluminate phases are most commonly employed. Owing to differences in their reactivities (Mehta, 1973b Odler and Colan-Subauste, 1999), not all of them perform equally well, to produce expansion without cracking. Tetracalcium trialuminate sulfate (C4A3S) appears to be the most suitable, as most of this phase hydrates in the desirable time range of several hours to several days that is, in a period in which the cement paste has already set but is not yet excessively rigid. 

CSH2 (gypsum) is formed in the hydration of anlydrite present in the calcium sulfoaluminate clinker, and may also be directly inteiblended or interground into the cement. Calcium hydroxide (CH) is formed in the hydration of free lime present in the calcium sulfoaluminate clinker, and also in the lydration of alite and belite present in Portland clinker, both being constituents of the expansive cement. 

The heightened compounds of hydrated silica (up to 0.3 g/dm ) and aluminum (up to 0.038 g/dm ) are often recognized in specimens of deteriorated reinforced concrete due to leaching of portland cement matrix. In many cases, it was detected a significant concentrations of sulfate (up to 34.5 g/dm ) that may be caused by deterioration and leaching of hydrated calcium sulfoaluminate of lining backfill. 

In Portland cement, although the average C3A content is about 4-11%, it influences significantly the early reactions. The phenomenon offlash set, the formation of various calcium aluminate hydrates, and calcium carbo-and sulfoaluminates involve the reactions of C3A. Higher amounts of C3A in portland cement may pose durability problems. For example, a cement which is exposed to sulfate solutions should not contain more than 5% C3A. 

Thermogravimetric technique has also been applied to follow the formation of sulfoaluminate products hydrated in a mixture containing tricalcium aluminate and g q)sum.[ l... 

In Fig. 29, the thermal curves of C4AF-CaS04 2H20 mixes hydrated at aw/s ratio of 0.5 or 1.0 are given. Most samples hydrated for 2 days at 25°C contain mainly the low sulfoaluminate.The interconversions... 

In the Portland cement paste containing different amounts of CaCl2, the hydration of the tricalcium aluminate phase was followed as a function time.t The results showed that tricalcium aluminate phase reactedto form compounds such as chloroaluminates, sulfoaluminates, and aluminate hydrates. 

In the hydration of C3A in the presence of gypsum and Na lignosulfonate (NLS), the conduction calorimetric curve indicates that, in the absence of lignosulfonate, ettringite is formed rapidly and converted to the low sulfoaluminate form. In the presence of NLS, the conversion of ettringite to the low sulfoaluminate form is retarded.1 1... 

The effect of 0, 1,2, and 4% SMF on the hydration of C3A + 25% gypsum may be followed by conduction calorimetry (Fig. 4).Iri An exothermic peak appears at 20 hours with 1 % SMF, at about 23 hours with 2% SMF, and at about 15 hours with 4% SMF. A continuous hump is registered for the sample containing no admixture. The retardation effect of C3A-ettringite reaction to produce low sulfoaluminate may be caused by the adsorption of SMF on the hydrating C3A surface. 

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