Architecture:Clay-Based Construction Products.pdf
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1、491 491 1.0INTRODUCTION The performance of clay-based construction products, e.g., bricks and roofing tiles, can be monitored using thermal methods. The types of raw materials, viz., clay and accessory minerals, and their reactions that occur during the firing process and the durability of clay prod
2、ucts can be examined through the application of DTA, TG, TMA, and dilatometric methods. This is particularly important for quality control as physical and chemical behaviors are dependent on the raw material characteristics, e.g., composition, particle size, and morphology. This chapter focuses on t
3、he application of thermal methods specific to raw materials suitable for industrial clay products that are suitable for brick-making. Details of thermal processes for the following materials are discussed: singular, binary, and ternary clay-based systems; structural ceramics; solid waste in clay bri
4、cks; significant archaeological clay prod- ucts; and brick clay products from Central Europe. 12 Clay-Based Construction Products 492Chapter 12 - Clay-Based Construction Products The durability of clay bricks to freezing-thawing cycles is also presented. Emphasis is placed on the use of thermal meth
5、ods, e.g., dilatom- etry to determine the factors that contribute to freezing-thawing resistance. A method for determining the firing temperature characteristics of clay brick is presented. The use of brick particulate additives to portland cement concrete for enhancement of freezing-thawing resista
6、nce and the applica- bility of thermal techniques to assess the bloatability of clays is also described. 2.0THERMAL BEHAVIOR AND IDENTIFICATION OF CLAYS AND ACCESSORY MINERALS 2.1DTA of Clay Minerals Thermal methods (DTA, TG, TMA, and dilatometry) are well- established investigative tools in clay sc
7、ience and related industrial applica- tions.12 Clay brick manufacturers have employed these techniques to optimize their plant production procedures. The successful firing of clays in industrial processes involves considerations related to volume change, phase changes, and crystalliza- tion phenomen
8、a. Drying shrinkage of clay minerals occurs due to pore water loss in the temperature range of 100150C. The oxidation of any organic material takes place in the range of 200600C. The oxidation of sulfides begins between 400 and 500C. The hydroxyl water is removed from the clay minerals starting at t
9、emperatures somewhat below 500C and continuing to temperatures approaching 900C. The nature of the clay minerals in clays and the particle size influences the specific temperature and rate of hydroxyl loss. Some of the general conclusions that can be drawn from the DTA studies of various clays may b
10、e summarized as follows.3 An endothermic reaction below about 200C usually indicates the presence of montmoril- lonite or illite. A clay containing these components will have high plasticity and shrinkage, will probably be non-refractory, and will burn red. In general, the larger this fraction the h
11、igher the plasticity and shrinkage. Endothermic reactions between 300 and 500C usually indicate a hydrox- ide of alumina or iron. If the component is a hydroxide of alumina, the clay 493 will be refractory and will have low shrinkage. A broad exothermic reaction between 200 and 600C is due to the pr
12、esence of an organic material. Clays yielding such thermal reactions will frequently be very plastic and will require careful burning to insure complete oxidation of the carbon. A sharp exothermic reaction between 400 and 500C may indicate the presence of pyrite or marcasite. An intense endothermic
13、reaction at about 600C followed by a sharp exothermic reaction at about 975C indicates the presence of kaolinite. A clay with a peak of low intensity at about 500 or 700C followed by another endothermic reaction at about 900C and then a final exothermic reaction may indicate the presence of illite o
14、r montmo- rillonite. A clay containing either of the two is not refractory or light firing and is apt to have a short vitrification range. If the component is mainly montmorillonite, it will also exhibit high plasticity and shrinkage. A small endothermic break at 575C is typical of quartz which redu
15、ces plasticity and shrinkage of the clay. The presence of calcium carbonate is indicated by an intense endothermic peak at about 850C. If it is present, the clay requires careful preparation and firing technique. It has to be emphasized that very careful study and experience are required to interpre
16、t the thermal curves. In some clays, thermal inflections cannot be easily interpreted unless addi- tional analytical data are obtained with x-ray, chemical, and other methods of analysis. Kaolinite is the most prevalent mineral in ceramic formulations.45 It shows pronounced thermal effects on heatin
17、g and generally has a more ordered structure than other clay minerals. Figures 1 and 2 illustrate typical DTA data for kaolinite, halloysite, and montmorillonite.2 Kaolinite and halloysite lose their hydroxyls between 450 to 600C. Variations within this range are attributed to differences in entrapp
18、ed water vapor that is depen- dent on sample size and shape factors. The loss of hydroxyls from montmo- rillonites in the range of 450 to 650C is typical for dioctahedral forms of these minerals. Dehydroxylation is more gradual for trioctahedral forms and can continue to temperatures up to 850C. The
19、 crystallization of new high temperature phases for fired kaolinite and holloysite occurs at about 9501000C. It has been shown that a significant variation in high temperature effects develop in montmorillo- nites with different exchangeable cations. The nucleation of the high temperature phase is o
20、ften accompanied by a considerable release of energy and is shown on the thermogram by a sharp exothermic peak. Often the temperature must be raised above the nucleation temperature to enable the new structure to grow and develop. For example, mullite in a refractory brick produced from a kaolinite
21、clay begins to form at about 1000C, but Section 2.0 - Clays and Accessory Minerals 494Chapter 12 - Clay-Based Construction Products only develops rapidly at about 1250C. DTA methods are capable of indicating the formation of a new high temperature phase before it is detected by x-ray diffraction mea
22、surements. It shows pronounced thermal effects on heating and generally has a more ordered structure than other clay minerals. Kaolinite changes at 600C with water evolution, becomes isotropic between 900 and 1000C, takes on a granular appearance at 12501300C and transforms into sillimanite above 14
23、00C.6 Mellor and Scott7 describe the thermal events as follows: completion of dehydration above 500C; completion of alumina transformation accompanied by an exothermic reaction at 900C; solid solutions development (sillimanite with 3Al2O32SiO2) below 1200C; formation of 3Al2O32SiO2 above 1200C in Ge
24、orgia kaolin. -Al2O3 has also been identified after heating in the range of 9601000C.8 Fluxing impurities present in brick clays tend to suppress the thermal reactions.9 Some oxide additions accelerate the formation of mullite. The relevance of the exothermic peak at about 980C has been the subject
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