Supersulfated cements

Activation of the latent hydraulic properties of foundry sand can also be achieved with gypsum (sulfate activation). Mixtures of foundry sand with a high aluminum oxide and lime content with ca. 15% calcium sulfate (as raw gypsum or natural anhydrite) are known as supersulfated cement. 

Supersulfated cement is produced by intergrinding or intimately blending granulated blast furnace slag with calcium sulfate and Portland clinker. Usually the cement is more finely ground than Portland cement, to enhance its reactivity. 

The strength development of supersulfated cement is rather sluggish and its final strength is relatively low. The rate of strength development increases with temperature up... 

If used above ground, proper curing of concrete made from supersulfated cement is essential. The surface must be kept damp, as otherwise a friable surface layer, which tends to dusting, may develop. The tendency to dusting may be reduced by increasing the Portland clinker content in the cement. 

Supersulfated cement is highly resistant to a variety of aggressive agents, especially sulfate solutions. It also exhibits a fair resistance to diluted solutions of inorganic and organic acids down to a pH of about 3.5. 

Supersulfated cement is particularly suitable for underground apphcations, especially where a contact of the finished construction with water containing significant amounts of corrosive constituents is to be expected. It has also been successfully used in harbor and breakwater constractions, where its resistance to seawater is advantageous. 

Mohan, K., and Ghosh, S.P. (1992) Evaluation of physico-chemical and hydration characteristics of supersulfated cement, in Proceedings 9th ICCC, New Delhi, Vol. 3, pp. 331-337. 

The ettringite that is formed exhibits distinet eementing properties, but may cause expansion at a different rate and to a different degree, depending on the Al source involved in the reaction. Cements that eontain significant amounts of calcium sulfate include expansive cements (see seetion 21), regulated set cement (see section 5.2), supersulfated cement (see section 8.4), sulfobelite cement (see section 4.2), and sulfoalite cement (see section 4.3). Limited amounts of ealeium sulfate are also present in Portland cement (see section 2) and blended eements (see section 7). 

Portland cement and related binders should not be used in applications where the concrete surface is permanently exposed to pH values lower than 7. Calcium aluminate cement (see section 10) resists acids somewhat better than Portland cement, and may be applied down to pH values as low as 4. Supersulfated cement (see section 8.4) also performs relatively well in diluted acid solutions down to pH=4. Binders highly resistant to acid solutions include alkali silicate cement (see section 15.3), geopolymer cements (see section 15.7.1), and alkali-activated fly ash-slag cement (see section 9.1.5, and Lu and Li, 1997). 

Hydrated supersulfated cement is highly resistant to snlfate attack, and may be used in applications in which the concrete is exposed to highly concentrated sulfate solutions. TTie high sulfate resistance is dne to the fact that most or all of the AI2O3 (except that in the residual, non-reacted slag) is present in the form of ettringite, or is bound within the formed C-S-H phase, and is not available for a reaction with sulfate ions. Such sulfate resistance is limited to alkah and calcium sulfate solutions, or to moderately acid solutions (down to abont pH=3.5), in which the sulfate ions are present in the form of ammoninm sulfate or even as free sulfuric add of low concentrations. 

Galvanized steel is susceptible to chloride-induced corrosion in highly alkaline environments, as exist in mixes made with ordinaiy Portland, Portland-pozzolana, or Portland-slag cements. The corrosion may be reduced significantly by the use of supersulfate cement (Dass et al, 1992). 

Blast-furnace slag can also be used with 10% calcium sulfate and 5% Portland cement to produce supersulfated cement. However, it is much more commonly used to extend and improve Portland cement. 

Stark, J Frohburg, U Mielke, I. Supersulfated cement with and without cement clinker. In Proceedings of the International Symposium on Non-Traditional Cement and Concrete, held Brno (Czech Republic), (ed. V. Bflek and Z. Kerner), 2001 25-138. 

Supersulfated cement has a lower heat of hydration and shows better resistance to sulfate attack than normal portland cement. It has lower CH contents and most of aluminum is bound as ettringite. This cement may contain 80-85% slag, 10-15% anhydrite, and 5% activator. The main hydration products are C-S-H and ettringite. A comparison of the conduction calorimetric curves of the supersulfated cement with that of normal... 

The DTA and X-ray methods have proven useful in the examination of set supersulfated cements.l Thermograms of supersulfated cement hydrated to different times are plotted in Fig. 18. The cement exhibits an endothermal effect at 150-170°C that is mainly attributed to ettringite. A large amount of ettringite is present even at 28 days and is lower at later periods. The increased strength in cement at 28 days is due mainly to the C-S-H phase and alumina gel. A quantitative estimation of products of hydration of supersulfated cement has also been carried out by thermal methods,... 

C. Baux, A. Phelipot-Mardele, C. Lanos, A. Pierre, J. Guilbert, M. Guillaume, Performances of supersulfated cements, CONSEC13,7" Int. Conf. on Concrete under Severe Conditions, 23-25 sept., Nanjing, China, pp. 1857-1870 (2013). 

Figure 8.38 The effect of matrix composition on the stress-strain curves of AR-GRC composites containing 5% by weight fibres, after ageing in natural environment (after Majumdar [17]). (a) Portland cement (b) alumina cement (c) supersulfated cement (d) Portland + 40%flyash (e) polymer modified cement.

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