Clinker sulfates

Sulfate minerals in clinkers, observed as floating particles in refractive index oil, were said to occur abundantly in underbumed clinker. Opti cal characteristics of clinker sulfates compared with sulfate phases formed in the laboratory led to the conclusion that the low-index mineral in clinker is an alkali sulfate with a variable but small amount of calcium sulfate held in solid solution. 

Increase in alite size and idiomorphism Lowered viscosity of melt resulting from increase in clinker sulfate and degree of sulfatization (sulfate to alkali ratio) (Strunge, Knofel, and Dreizier, 1985)... 

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. 

Class G Intended for use from surface to 8,000 ft (2,440 m) depth as manufactured, or can be used with accelerators and retarders to cover a wide range of well depths and temperatures. No additions other than calcium sulfate or water, or both, shall be interground or blended with the clinker during... 

Rapid cooling of the clinker is preferred for many reasons, notably to prevent the reversion of alite to belite and lime in the 1100 1250 °C regime and also the crystallization of periclase (MgO) at temperatures just below 1450 °C. The magnesium content of the cement should not exceed about 5% MgO equivalent because most of the Mg will be in the form of periclase, which has the NaCl structure, and this hydrates slowly to Mg(OH)2 (brucite), which has the Cdl2 layer structure (Section 4.6). Incorporation of further water between the OH- layers in the Mg(OH)2 causes an expansion that can break up the cement. Accordingly, only limestone of low Mg content can be used in cement making dolomite, for example, cannot be used. Excessive amounts of alkali metal ions, sulfates (whether from components of the cement or from percolating solutions), and indeed of free lime itself should also be avoided for similar reasons. 

Uncalcined Gypsum and Anhydrite. Calcium sulfate, generally in the form of gypsum, is added to Pordand cement (qv) clinker to stop... 

In order to obtain the desired selling qualities in Ihc finished cement, there is added to the clinker about 2% of gypsum (calcium sulfate. CaSOj 2H2O), and ihe mixture is pulverized very finely. For every ton of Portland cement shipped, over two and one-half tons of raw materials and cement clinker must be ground very finely. See Table I. 

The relative reactivity of the different mineral phases of cement with water is usually given as C A>C S>C S>C AF. Aluminate phases and their hydration products therefore play an important role in the early hydration process. Because of the high reactivity of calcium aluminate, the aluminate hydration reaction is carried out in the presence of sulfate ions. The latter provide control of the reaction rate through the formation of mixed aluminum sulfate products (ettringite and monosulfoaluminate) Calcium sulfate which is added to the cement clinker hence controls the properties of the aluminate hydration products. Sulfates thus play a crucial role in cement hydration and the influence of chemical admixtures on any process where sulfates are involved may be expected to be significant [127],... 

Portland cement is the product obtained by pulverizing clinker consisting essentially of calcium silicate, usually containing one or more forms of calcium sulfate, and there are five types of portland cement ... 

Portland cement clinker potential phase composition is presented in Table 4. It could be seen that the C3A content in the clinker was 9.46% which is important for the cement hydration rate and cement sulfate resistance. Common Portland cement is not resistant to the sulfate influence because of the significant C3A content, whose hydrates react with sulfate ions resulting in expansive compounds. Portland cement with the higher resistance to sulfates must have low C3A content. Moderate to high content of mineral alite - C3S (54.72%) is usual for the Serbian cement plants and enables the addition of higher quantities of mineral admixtures without influencing the quality of final cement. 

Calcium sulfate may be decomposed to cement clinker and sulfur dioxide gas in a coke-fired rotary kiln at 900-1400°C (Miiller-Kiihne).28 However, the unfavorable economics of this process relegate it to countries that do not have other sources of sulfur. Phospho-gypsum (gypsum produced by the acidulation of phosphate rock) may be decomposed in this way as a means of recycling the sulfur values in the large waste phosphogypsum piles at fertilizer plants (OSW-Krupp and FIPR/Davy... 

XRD (Cu Ka) data for (A) raw Portland cement clinker, (B) Residue 1 from which most of the C3S has been removed and (C) Residue 2 from which C3S and C2S have been removed. Silicate phase extraction was conducted using a salicylic acid methanol (SAM) mixture using the method described by Taylor." In Residue 2 the presence of the alkali sulfate phases [denoted Arc for arcanite K2SO4 and Ap for aphthitalite K3Na(S04)2] is clearly evident. Notably, the material used in these plots was derived from a different cement plant to the one illustrated in Figure 10. 

Alkali Sulfates. Since the silicate phases (C3S and C2S), present in total at up to 85 wt.%, normally dominate clinker samples, all minor phases are significantly concentrated in Residue 2. This includes the important alkali sulfate phases which can (i) affect setting times and final strength, and (ii) be used to assess kiln operating conditions. Since these are normally present at a total of about 0.5 wt.% in clinker, and are often distributed across several Na and K sulfate phases, they are not easily identified in raw clinker XRD patterns. However, their presence may be more easily detected in the XRD pattern of Residue 2. By optimizing the parameters of the alkali sulfates from Residue 2 data, and then constraining them in the on-line analysis system, these phases can be measured at the <0.5wt.% level (Madsen, Scarlett and Storer 2001, unpublished results) even when rapidly collected on-line data is used. 

A stain technique proposed by Poole and Thomas (1975) for detecting sulfates in aggregates has been modified and found to be quite appropriate for gypsum, plaster, and anhydrite in portland cement and, to some extent, alkali sulfates in clinker. The 6% stain solution is made from an aqueous mixture of BaCl and KMnO in a 2 1 ratio. 

Therefore, any cross section of typical clinker displays (1) the more or less loosely tied framework of alite crystals, (2) belite that occurs as single crystals and as concentrations, and (3) a matrix of aluminate and ferrite formed as the molten liquid cools and crystallizes. Microscopical observations clearly suggest aluminate (CjA) crystallizes after the ferrite, the latter forming a prismatic crystal mesh, the holes of which are partially filled with aluminate. Ferrite can be seen within alumi nate and, extremely rarely, vice versa. The matrix com monly contains secondary belite and shows effects of reaction with alite. Voids remain in areas not filled by the liquid, forming sites for crystallization of alkali sulfates on the cavity walls. Thus, the typical clinker is a somewhat porous mass of interlocking crystals, a truly glassless crystalline mosaic. Recent studies of the sequence of crystal development in the production of Portland cement clinker can be found in papers by Imlach and Hofmanner (1974), Moore (1976), Ono (1981, 1995), Chromy (1974, 1982), and Maki (1982, 1995). 

For details of the extraction procedures for alkali sulfate from clinker, x-ray diffraction analysis, and specific chemical requirements for the formation of these phases, see Gartner and Tang (1987). 

Chen, Conjeaud, and Lehoux (1985) estimated burn ing zone temperatures with three techniques by burnability studies of the raw mix, by electron micro probe analysis of alite in clinker, and by birefringence of alite. The latter method was said to be the least accurate, due possibly to clinker magnesium oxide and sulfate contents, kinetics of alite crystallization from a melt, raw mix particle size, and variable calcination rates. 

---