Behavior of sugarcane waste fiber ash as a Cementing material
Mwero, John N
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The aim of this research was to investigate the possibility of using Sugarcane Waste Fiber Ash (SWFA) as a pozzolanic material in concrete. Mechanical strength, consistency, durability, and chemical reaction characteristics of concrete samples containing various quantities of SWFA between 0 and 20% by weight of the total cement in the mixes have been investigated through laboratory tests. The microstructure of 18 hour old hydrating cement pastes containing various quantities of SWFA has also been investigated using Scanning Electron Microscopy. A computer model for the prediction of the heat of hydration and chemical shrinkage of cement pastes containing between 0 - 20% SWFA, and strengths of concrete made from cementJSWFA mixes containing between 0 - 20% SWFA by weight has also been created. Chemical analysis was carried out usmg X-Ray Fluorescence and Atomic Absorption Spectrophotometry, and particle size distribution was carried out using hydrometer analysis. The chemical composition and particle size distribution of SWFA were found to be within the ranges of a good pozzolan. Slump and compaction factor tests carried out on concretes containing 0 - 20% SWFA showed that whereas the slump dropped by up to 70% of the control at 20% SWFA content, compaction factor only reduced by 7%. Concrete was found to stiffen with increase in SWFA. Concrete made from cementJSWFA mixes with up to 20% SWFA content was however found to be still workable though a bigger compaction effort would be required to achieve compaction similar to concrete without SWFA. Standard cube tests carried out showed that the concrete strength increased up to 105% with increasing SWFA content of 6%. Strength was then observed to drop to 72% of the control cube strength with a further increase in SWFA content at 20% replacement. Tensile splitting and modulus of rapture results showed a steady drop of 31% and 65% strength drop at 6% SWFA content respectively, indicating a more drastic effect of the SWFA on tensile strength of concrete. Reinforcement bond strength also was observed to fall in a pattern similar to the tensile strength. Durability tests including exposure to sea water, concentrated hydrochloric acid, dilute carbonic acid and UV light indicated an improved resistance to these environments with increase in SWFA up to about 6% SWFA content after which the resistance gradually becomes comparable to that of concretes without SWFA. Chemical shrinkage tests gave higher shrinkage values for cement pastes containing 4 - 6% SWFA than for the control paste (without SWFA) and other pastes with higher SWFA content. SEM micrographs taken for 18 hour old cement pastes containing SWFA between 0 - 20% by weight showed denser microstructures in the cement pastes with higher SWFA content. SEM micrographs for 5, 10 and 15% SWFA by weight showed most dense microstructures with highly reduced pore sizes. Micrographs for pastes containing 20% SWFA showed unreacted particles in the size range of the cementlSWFA particle sizes indicating a reduced reaction. The computer model coded in Visual Basic and based on the chemical reaction rules of the hydration reactions was able to predict the chemical shrinkage and heat of hydration of hydrating cement pastes containing 0 - 20% SWFA, and also the 28 day compressive strength values of concrete containing 0 - 20% SWFA by weight of the total binder. The study showed that concrete compressive strength and resistance to corrosive environments are both improved by the inclusion of SWFA in the concrete mix. The author therefore recommends replacement of cement with SWFA in concrete mixes at 6% by weight, to achieve higher strength and better resistance to corrosive environments, at a lower overall concrete cost. The 6% SWFA optimum is thought to have been caused by an interaction of the oxides within the ash-cement mix.. SWFA is high in Si02 content, and low in CaO. The Si02 is thought to have reacted with the Ca(OH)2 resulting from the hydration of C3Sand C2Sto form C-S-H causing the increase in strength between I% and 6% SWFA content. Beyond 6%, the reduction of cement resulted in the reduction of CaO and consequently the reduction of C3S and C2S which in turn reduced the amount of Ca(OH)2 available for reaction with Si02.This in turn led to a reduced amount of C-S-H within the mix. This is thought to have caused the reducing strength beyond 6% SWFA content. Tensile strength for all tensile strength tests was at a minimum at approximately 6% SWFA content. In hydrating cement pastes setting occurs towards the end of the acceleratory period occurring at about 3 - 6 hours after mixing with water. During this period hydrating cement starts becoming hard and consequently brittle. Chemical shrinkage test results for cement pastes containing SWFA showed that maximum shrinkage occurs in the cement-SWFA pastes containing around 4% SWFA. The results also suggested that the large shrinkage in the paste containing 4% SWFA occurs after the fourth hour well after setting has started. It is possible that the shrinkage might have cracked some of the already formed bonds resulting in a reduction in tensile strength. This effect was not much pronounced in the compressive strength results since compression is thought to have worked to "close" the cracks as opposed to opening them. The contribution of this work to knowledge is in the information regarding durability characteristics of concretes containing SWFA between 0 - 20% by weight of the cementing material; the compressive strength increase, and corresponding tensile strength decrease, of the concrete at about 6% SWFA content in the cementing material. In addition, a simplified approach is suggested in form of a computer model with capacity of predicting the chemical shrinkage and heat of hydration of cement pastes containing SWFA between 0 - 20% by weight, as well as the strength of concretes made from similar cementlSWFA mixes.