Low sulfoaluminate

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 low sulfoaluminate phase ean form by the reaetion of excess C4AF with the high sulfoaluminate phase. 

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... 

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. 

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. 

Mudbhatkal, G.A. et al. (1986) Non-alitic cement from calcium sulfoaluminate clinker— optimization for high strength and low temperature applications, in Proceedings 8th ICCC, Rio de Janeiro, Vol. 4, pp. 364-370. 

Sharp, J.H., Lawrence, C.D., and Yang, R. (1999) Calcium sulfoaluminate cements — low-energy cements, special cements or what. Advances in Cement Research 11,3-13. 

Wang, Y. et al. (1997) Sulfoaluminate cement with low alkalinity, in Proceedings 10th ICCC, Goteborg, paper 4iv030. 

In cases where the initial reactivity may be high, and the later reactivity is low, it is often useful to prepare two separate samples of each mixture one smaller sample for capturing the initial reactivity and one larger sample for capturing later age reactivity. Examples of such materials include calcium aluminate or sulfoaluminate-based binders and high-calcium fly ash. 

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