Clinker porosity

Portiand cement clinker structures (18,19) vary considerably with composition, particle size of raw materials, and burning conditions, resulting in variations of clinker porosity, crystallite sizes and forms, and aggregations of crystallites. Alite sizes range up to about 80 p.m or even larger, most being 15—40 )J.m. 

High clinker porosity Low degree of burning (Gille and others, 1965)... 

In the cement industry, the term hydration is used to describe a range of reactions between cement and water to produce a hardened product. A cement clinker particle is a multiphase solid having massive calcium silicate grains (50-100 pm) in a matrix of interstitial aluminate and ferrite. This is described as analogous to a distorted clay sequence, which traps regions of porosity-pore size distribution from nanometer to micrometer. 

The most important characteristic of cement is its pore structure and aqueous phase hence, the microstructure of the hardened cement paste via the pore system. It is highly alkaline (pH >13) due to rapid and almost quantitative dissolution of Na and K salts from the cement clinker. The porosity of the paste comprises interconnected and isolated pores, the pore sizes of which are important to the strength and dimensional stability of cement products. Different types of cement are used to meet different performance criteria. Properties can be estimated from compositions and fineness (i.e., particle size and size distribution). In the past, additives... 

Fig. 12.1 Microstructural gradients in the interfacial region of a concrete (a) unreacted clinker phases, (b) porosity, determined using SEM with image analysis. Each point represents a mean from 50 determinations, and the standard errors were approximately 10% relative. Scrivener et al. (SI 14).

Large belite nests were shown to be indicative of poor grindability, even though the same clinker may have relatively high porosity. Clusters of belite crystals remained undivided even at the final grinding stage. 

Ono (1991) listed some of the characteristics of poorly burned clinker free lime greater than 4.5%, tightly packed large free lime nests, belite nests with a surrounding of small alites, high porosity, a loose framework of free lime and alkali aluminate, and a flow pattern of matrix into the free-lime nest. Well-burned clinker was said to contain small free lime and octahedral periclase from dolomite and coarse aluminate and ferrite, resulting from equilibrium crystallization. 

Three zones in clinker (a) Dense core (b) Intermediate porosity (c) Cokelike (a) Primary nodules (kiln ring fragments or nodules formed before burning zone) (b) Forms in burning zone (c) Forms between burning zone and cooler due to heavy dust load (Fundal, 1980)... 

Dusty clinker high porosity, breaded nodules, agglomerated fine particles, alite-rich, large alite crystals, and relatively scarce liquid phase (a) Decomposition of outer clinker shell and concentration of liquid phase in clinker core (Allegre and Terrier, 1960) (b) Recycling of precipitator dust, reducing zones, lack of AljOj in raw slurry (Hofmanner, 1973)... 

Dusty clinker, coarsely crystalline silicates, low-porosity nodules, poor grindability Slow temperature rise, higher clinkering temperature, longer time in burning zone and transition zone (Wolter, 1985)... 

Photograph 7-14 Randomly scattered silicates in well-made clinker. Small, angular, brown alite Relatively large, round, multicolored Type A belite well-differentiated matrix of aluminate (CjA) and ferrite (C AF). Moderate porosity not shown. See proposed belite classification on p. 35. (S A6634)... 

C. Megascopic properties— hardness, color, porosity, shape note variations in these properties within and between clinkers and establish predominant characteristics agglomerated or single nodules. 

Unlike ordinary Portland clinker, alite-fluoroaluminate clinker is brownish in color, rather than dark gray. It typically possesses a very low porosity, and the alite and behte crystals that are present are distinctly smaller than those in ordinary Portland clinker. Only some of the fluorine is in the CjjA2.Cap2 phase the rest is incorporated into alite (Odler and Abdul-Maula, 1980). 

In systems that contain ground Portland clinker without interground calcium sulfate the rheology of the paste may be controlled by adding cal-cium lignosulfonate in combination with an alkali carbonate to the mix (see low-porosity cement, section 2.16.1). The mix exhibits good flow even at low water/cement ratios. 

The produced chnker is polymineralic in nature, and contains—in addition to calcium sulfoaluminate— limited amoimts of dicalcium silicate, anhydrite, and free lime. 8ulfospurrite may also occasionally be present (Herrick et al., 1992 8u et al., 1992). The chemical composition of the sidfoaliuninate phase may vary over a limited range, with a partial substitution of Ca by Mg ", and Al by 8i", Fe, or Ti" (8u et al., 1992). The clinker has a high porosity, with a liter weight of 650-850 g/L, and is easy to grind. 

The bulk density of a particular particle size fraction of clinker (e.g., 5—7 mm), obtained by screening, provides a check on the degree of burning. Depending on the raw meal (chemical composition) and characteristics of the kiln plant (porosity of the clinker, etc.), the values for the bulk density of adequately burned clinker range between 1.2 and 1.6kg/dm . The permissible minimum value in any given case has to be determined empirically. 

Porosity of the OPC paste is determined by the ratio of water to cement a w/c ratio of 0.4 allows complete reaction of the OPC clinker with water and the gel pores with a diameter of several nm remain (Powers, 1954). Total porosity and capillary porosity for OPC cement paste (Fig. 8-3) increases... 

The total mass of solids does not stay constant during cement hydration free water is bound into hydration products (Figure 4.16) and the initial porosity is filled. Rietveld results normalised on a total solids base for dried samples will need to be corrected. Therefore, the final step in the data analysis is to recalculate the QPA to a common basis. This base of comparison may be the initial paste (free water + anhydrous binder) content, the initial binder (SCM + portland cement) or the initial portland cement or clinker content. The recalculation depends on whether free water is removed to stop the hydration reactions. If the sample is assumed to be undried and... 

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