Pozzolanicity of selected kaolinites with special focus on pyroprocessing parameters
Gathua, Joseph Kimiri
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In this study, the pozzolanic reactivity of selected clay minerals (kaolin) with lime was investigated mainly to establish their suitability in low cost housing. Clay minerals from eleven different regions in Kenya were characterized using x-ray diffraction, x-ray fluorescence and the differential thermal analysis coupled with thermogravimetry. The X-ray diffraction tracings for all the samples showed features of disorder that made the exact quantification of the clay minerals difficult, but estimates showed that kaolinite ranged between 30-74%. The results also showed that the clay minerals were mixed- layered or interstratified. The results of chemical analysis showed predominance of Si02 and A1203 with Fe203 being the next major constituent. All the samples met the Standards IS-1344-1981 on chemical composition of pozzolanas. The thermal behaviour was monitored using differential thermal analysis coupled with thermogravimetry. Adsorbed water was released in the range 50-90°C. The dehydroxylation reaction occurred in the range 525-600°C and was accompanied by phase transformation from the crystalline kaolinite to the amorphous or quasi- amorphous metakaolin phase. Another change of phase in some samples was observed in the range 900-980°C, which signified recrystallization (fusion) of the collapsed structure into spinel. Three samples with the highest content of kaolinite namely, Mwingi Kaolin (sample A-2), Kisii kaolin (sample A-3) and Nyeri kaolin (sample G) were selected for pozzolanic reactivity tests with lime alongside kaolin Caminau, Kaolin Guttau and diatomaceous earth. Kaolin Caminau and Guttau were sourced as packaged industrial products from Freiberg, Federal Republic of Germany, while diatomaceous earth was sourced as an industrial product from the African Diatomite Company, Gilgil- KenyaThe optimum activation temperature and thermal activation time were established by thermally activating the samples at varied temperatures and time using soak method in a muffle furnace. The two parameters were then determined on the basis of compressive strength of the pozzolanallime mortars cured for 7 days and dissolution method. The latter method entailed dissolution of thermally activated pozzolana in dilute hydrofluoric acid and subsequent measurement of electrical conductivity. The optimum activation temperature and thermal activation time ranged between 650-800°C and 3-4 hours respectively. The results showed that dissolution can be an effective and quick method of establishing the pyroprocessing parameters. The optimum pozzooanal Lime ratio for the six pozzolanas was in the range 2.3-3.0 and the results were in agreement with pozzolanic investigations done elsewhere.. Kaolin Caminau and diatomaceous earth registered high compressive strengths during the first 7 days of curing and limited strength development thereafter indicating a very fast rate of hydration. Kisii, Mwingi and Nyeri kaolin recorded very low strength during the first 7 days but the strength developed progressively upon curing up to 28 days signifying slow rate of hydration. The pozzolanicity of soak calcined Kisii kaolin using the Thattle model of vertical shaft kiln (VSK) was compared with that of electric muffle furnace (EMF). The results indicated that kaolin calcined in EMF was more reactive with lime than the V SK calcined one. Investigations were extended to comparison of the soak and flash calcinations methods. The flash method produced more reactive metakaolin than the soak method. Thermal decomposition under isothermal conditions was also implicitly monitored on the basis of electrical conductivity measurements. The results indicated that this method is a good indicator of the structural changes taking place within a pozzolana upon thermal activation. All the samples attained compressive strength of 4MP after 28 days of curing as per the IS-1344-1981 standards, an indication that they can be used as pozzolanas for ordinary masonry purpose.