Expansive clinker

In the system Ca0-Si02-CaS04 the compound 2C2S CaSO is formed [72], stable at the range 1000-1200 °C. This is important in the production of expansive clinkers [73]. As it was mentioned earlier, at low quantities the sulphur presence has advantageous effect on clinkering process, principally by modification of the melt properties. 

In former USSR expansive cements were divided into 3 classes 20,40 and 60 their potential of self-prestressing was 2, 4 and 6 MPa respectively, Expansive clinker containing C A S, Portland cement phases, CaO, and anhydrite, The first producer nowadays this cement is manufactured in many countries among them in significant amount in Russia, AlEv—calcium aluminate expansive cements, PEv—Portland expansive cements, Very similar to that patented by Lafarge... 

There are the three technologies of the K type laments production. Expansive clinker can be produced composed of alite, C4A3S, C AF and CS however, it is not convenient because the calcium sulphoaluminate is decomposed in rotary kiln already at temperature of 1,350 °C. The better solution is to produce expansive... 

Fig. 9.28 Expansive clinker K C4A3S phase, B belite. (photo of B. Trybalska)...

Expansive cements Expansive cements, in addition, Portland clinker, expansive clinker and are designed to offset the effect of the constituent materials. 

Expansive cements It contains, in addition to Portland clinker, expansive clinkers (usually sulphoaluminate clinkers), and are designed to offset the effects... 

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. 

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. 

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. 

Portland cement clinkers contain small amounts of alkalis and sulphates derived from the raw materials and fuel. Both alkalis and SO3 can be present in the major clinker phases, but tend to combine preferentially with each other to form alkali or potassium calcium sulphates, and it is necessary to consider these components together. In addition, silicate and aluminate phases containing sulphate can form either as intermediates or in undesirable deposits in eement making, and a calcium aluminate sulphate is a major constituent of some expansive and other speeial cements. 

Regourd (R34) reviewed structural and other aspects of slags. XRD patterns show an asymmetric, diffuse band from the glass peaking at about 31 20 (CuK d = 0.29 nm) and extending from about 20 to about 37 and a weaker band at about 48 20 (0.19 nm). Crystalline phases, if present in sufficient quantity, give superimposed, sharper peaks melilite and merwinite are the most usual. Neither periclase nor lime is found and, since it is present either in the glass or in these inert, crystalline phases, MgO is not a potential cause of expansion, as it may be in a clinker (S89). 

Probably a majority of practical expansive cements have depended on the modification of a Portland cement in such a way as to increase the formation of ettringite. Single expansive cement clinkers can be made, but it has been more usual to produce admixtures that are blended or interground with a normal Portland cement or clinker. Blending has the advantage that the... 

Type S cements are Portland cements high in C3A and with suitable contents of calcium sulphate they have found little favour as they are too diHicLilt to control. The A1,0, has also been supplied in forms other than those mentioned above. Impure alunite [KAl3(S04),(0H)(,]. which occurs as a natural rock, has been used, either after calcination (V6) or uncalcined (W30). In the latter case it was mixed with Portland cement clinker, anhydrite and pfa or slag and was found to dissolve relatively slowly, thus suitably delaying the expansive reaction. 

The possibilities of using free lime or periclase as expansive agents have also been investigated. Kawano cl al. (K50) and Long (L49) described clinkers in which substantial proportions of free lime are present as inclusions in alite. In both ca.ses, the raw mix included anhydrite. Long investigated and described the processes by which this microstructure was formed. 

Pastes inpregnated with PMMA or sulphur are still sufficiently permeable to water that expansion occurs on long exposure (F46). In polymer-impregnated (S108) and MDF (R64) cement pastes, there is evidence of interaction between Ca ions and carboxylate and possibly other groups of the polymer. In MDF pastes made with calcium aluminate cement, the polymer (PVA) was found to inhibit the normal hydration reactions of the cement, but to react with Ca and AH to give an ionically cross-linked polymer and calcium acetate. TEM showed the material to be essentially a dispersion of grains of clinker or hydration products in a continuous polymer matrix. 

The hmit of MgO content is linked to the lack of respective standardised test of potential expansion, caused by the periclase from clinker. In ASTM standards the method of volume stability testing in autoclave was introduced, and after the United States also nine coimtries have adopted this method, among others Canada, Finland, Belgium and Argentine [27],... 

It was established that in clinkers with low MgO and alkalis content less glass is formed [189]. Simultaneously it is known that rapid clinker cooling diminishes expansion in water solution of MgSO. ... 

Fig. 6.40 Effect of SO content in cements from industrial clinkers on the expansion of mortars subjected to the heat treatment at temperature of 90 °C, and next matured in humid atmosphere at temperature of 20 °C for a long period of time. (According to [149])...

C this phase decomposes [40]. At higher content of spurrite the expansive properties of clinker are reduced. The amormt of C4A3S in clinker can be determined by XRD or by chemical method. This method consists in the extraction of the other clinker components with the solution of maleic acid in methanol and subsequent with the ammonium chloride water solution [107]. 

One problem with the crushing of clinker prior to examination is that microcracks seen in polished-section or thin section study are ambiguously interpreted. Microcracks that are not artifacts of sample preparation may, in some investigations, be related to strain caused by thermal stress (Hornain and Regourd, 1980), crystal reorganization, hydration, and expansion. 

In clinkers and cements, expansive hydration of free lime produces popcorn or cauliflowerlike crystals of calcium hydroxide, called epezite, surrounding the original particle of free lime (see Brown and Swayze, 1938). Formation of epezite via air slaking normally causes clinker disintegration. 

Total MgO content in clinker is normally maintained at less than 2.0% to avoid expansion in the autoclave test. Belite containing periclase (at an early stage of crystallization) indicates coarse grains of MgO-bearing silicates, such as diopside, pyroxene, and hornblende in the raw feed. Dendritic periclase occurs in slowly cooled clinkers from large kilns (200 t/hr) and is hardly observed in clinkers from small kilns (20 t/ hr), according to Ono (1995). 

Hoop-stress cracks around free lime nests Expansion due to lime hydration, typical for aged clinker (Brugan, 1979)... 

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