The influence of potassium oxide contamination of sugar cane bagasse ash (SCBA) on the hydration of cement-based pastes and the compressive strength of mortars was investigated. Three SCBAs with different potassium oxide contents (2.7–12.6%) were used. The SCBA samples were initially characterised by their particle size distribution, BET specific surface area (SSA), pore volume, mineralogy and pozzolanic activity based on the Chapelle method. The hydration of cement-based pastes containing 20 wt% of SCBA was investigated using isothermal calorimetry and chemical shrinkage. The compressive strength of the mortars was evaluated at 3, 7, 28 and 90 days. The pozzolanic activity of SCBA was found to increase with decreasing potassium oxide content due to the higher silicon dioxide content and SSA of the SCBA dosed with a low potassium oxide content in comparison with other samples. The potassium oxide dosage had a limited influence on the paste hydration, except for a slight increase in the induction period observed by calorimetry. Cement replacement with SCBA with a high potassium oxide dosage led to maintenance of the compressive strength in relation to the reference up to 90 days. The mortars with low and medium potassium oxide contents showed good performance in terms of compressive strength, with significant increases compared with the reference after 90 days of curing.
Brazil is a major producer of alcohol from sugar cane, a fuel with low environmental impact. The production of alcohol generates a large amount of bagasse, the biggest waste of Brazilian agriculture. This bagasse is usually burned for energy production providing nearly 3% of residual ashes. The potential use of these ashes like mineral admixture of cementitious composites depends on calcination conditions. The present work identifies the physical and chemical characteristics of ashes from the furnace exhauster, obtained in an industry located at southeast region of Brazil. The ashes were obtained from bagasse of sugar cane harvest in two different seasons. Chemical composition analysis, X-ray diffraction, grain size distribution, loss on ignition, thermogravimetry analysis (TGA), differential scanning calorimetry (DSC), specific surface measurements (BET) and Fourier transform infrared spectroscopy (FTIR) tests were used to characterize the ashes. The pozzolanic activity was estimated by pozzolanic activity index tests with cement and lime, by the modified Chapelle test, electrical conductivity in lime solution tests, TGA and FTIR. The ashes presented different chemical compositions and degree of amorphicity. The ashes with a higher content of silica, the lower organic material content and high degree of crystallinity (1st harvest), for the same size, have higher pozzolanic on electrical conductivity in lime solution tests than the ashes with lower silica content and higher amorphicity (2nd harvest). However, the results of differential scanning calorimetry (DSC) and modified Chapelle method would indicate the pozzolanicity of the ashes of the 2nd harvest. The results of pozzolanic activity index (with cement or lime) indicated the ashes were not pozzolanic.
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