Characteristics And Suitability Of Using Kenya's Natural Pozzolana Rocks And Burnt- Agro -Residues As A Substitute To Ordinary Portland Cement Clinker.

Abstract

The pozzolana materials considered in this research work were of two categories namely; natural and artificial. The natural pozzolana rocks were classified at the Department of Geology, t (UON) as Trachytic tuff (TT), lithic tuff (LT), light brown grey tuff (LBGT) and light grey tuff (LGT). The chemical compositions were characterized mainly using the wavelength dispersive xray spectrometer (WDXRF) equipment of the type rigaku geigerflex, which is a sequential automatic x-ray spectrometer modeled by Siemens srs-l. Other instruments used were the flame photometer for analysis of alkali metals and the gravimetric methods for sulphate. The results showed silica oxide as the dominant compound, i.e., TT, 64.68%; LT, 66.15%; LBGT, 63.20%; and LGT, 63.21%. Alumina was the second major component in the chemical composition with 13.56%,12.78%,18.99%, and 14.89% in TT, LT, LBGT and LGT, respectively. The agro-based pozzolana was obtained from Rice husk ash (RHA) and Sugar-Cane Bagasse ash (SCBA). They were incinerated at temperatures between 500 and 600°C to obtain a reactive silica, hence the name artificial pozzolans. Their chemical compositions were similarly determined using the Wavelength Dispersive X-Ray Spectrometer (WDXRF) instrument; Silica oxide was also noted to be the dominant compound in RHA (76.23%) and SCBA (65.46%). Compressibility and Microchemical analysis of the pozzolana materials were determined using a 2000 N compressive machine and EDX instrument, respectively. Compressibility data were a result of cement mortar formulation of pozzolana material with a known proportion of OPC-clinker. Formulations considered for each pozzolan materials were 15%, 20%, 25% and 30%. The formulation of 30% result showed RHA's 2-days compressive strength of 11.45 N/mm2. The control set had a compressive strength of20.26 Nzmm", which was 76.94% higher than the mortar strength ofRHA. The 2 days minimum standard strength requirement is 10 N/mm2. Twenty eight (28) days curing showed compressive strength value of 46.98 Nzmm", which was 3.75% higher than the control set. Thirty percent (30%) formulation of SCBA showed 28 day's compressive strength of 30.05 N/mm2. The results were 33.56% lower than the 45.22 N/mm2 compressive strength of the control set. X-ray powder diffractometer (XRPD) results indicated RHA to be mainly in amorphous state confirming the non-crystalline material. The results obtained compared well with the high strength development obtained in the cement mix. A 50:50 pozzolana -filler ratio formulation of25% (i.e. 12.5% LGT+ 12.5% Filler) and 30 % (i.e. 15% LGT + 15% filler) indicated 97.96% and 9l.27% compressive strengths, respectively, compared to the 28 days standard strength requirement of 32.5N/mm2 . The reactivity of a pozzolana material may not be established on the basis of total silica content, but on the reactive the silica. Microchemical analysis showed RHA to contain the amorphous silica, SCBA was mainly crystalline with minor traces of sanidine. Both TT and LGT contained sanidine, LBGT showed Analcime and Merlioite as the major phases whereas LT showed amorphous silica with traces of Calcite, quartz and Bayerite. All the pozzolans considered showed high combined silica content (Ab03+Si02) ahove 70%, therefore meeting Kenya's requirement ofa pozzolana material. The RHA had 77.24 % silica, whereas LT had 78.93% silica. Paste physical appearance showed OPC, TT and LGT to be light grey, whereas RHA and SCBA were dark grey and LBGT was light brown grey. The use of an agricultural residue as a substitute to clinker in ordinary portland cement (OPC) requires large plantations of the crop plant. Therefore, RHA cement having strength enhancing properties requires joint efforts from the rice farmers and the cereal boards, for the production of sufficient ash material for use in cement. In addition, clinker reactivity needs a further study on the various types ofpozzolana materials.