Studies on the Effect of Selected Local Admixtures on Essential Properties of Cement for Housing Construction
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Date
2012-06-15
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Abstract
Current trend in cement production is to introduce inorganic admixtures to improve the technical properties such as workability and reduced cracks in concrete. In addition, replacement of clinker or cement greatly reduces environmental pollution due to release of greenhouse gas emissions such as carbon dioxide, carbon monoxide, Nitrogen oxides (Nox) and Sulphur dioxide (SO2)during cement production. Cement replacement also reduces the high energy cost associated with clinker production and this leads to reduced price of cement. The objective of this study was to determine the effect and suitability of four local inorganic materials; namely, clay, limestone, clam shells and steel slag (all obtained in Ghana) as mineral admixtures in cement for construction purposes. Blended cements, ranging from binary to quinternary mixes, were produced.
The soundness of the blended cement samples were less than 2.0mm. Water permeability tests showed clearly that addition of the admixtures, except slag, reduced the porosity of the cement by at least 4% and as much as 20%. The water demand of the blended cements ranged from 24.5% to 34.5% and that it increased as the percentage replacements of cement were increased. The water demand of the blended cements ranged from 24.5% to 34.5% and that it increased as the percentage replacements of cement are increased. The setting times of all the blended cements were within the standard limits.
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The infrared spectrometry, X-ray Diffraction(XRD), Scanning Electron Microscopy/Energy Dispersive X-ray (SEM/EDX) and Thermogravimetric (DTA/TG) analysis of the study clearly showed that the presence of the four admixtures reduced Ca(OH)2 content considerably and improved impermeability of concrete. The Infrared analysis indicated that when the admixture content was increased from binary to ternarythe accelerating effect of admixtures on hydration was enhanced. This effect manifested in high early strength of the blended cement, especially the CaCO3-based blended cement samples. The ettringite and monosulphonate of CaCO3-based admixtures (that is, limestone and shell cement) bands were almost flat or disappeared compared to others, indicating increased reactivity between CO32- and SiO44- ions. Significantly, the study showed that a combination of three admixtures (5% each) or all four (2.5% each) in the cement almost eliminated portlandite content as compared to 36.9% in the reference cement as provided from the X-ray diffraction (XRD) analysis. This is a novel revelation and it was sufficiently corroborated by the EDX and TG/DTA analysis.The EDX and X-ray analysis also showed increased silica and oxygen composition of the blended compared to the control indicating increasing amounts of calcium silicate components.
The mortar compressive strength tests showed that blending 5%x, 5%x5%y, 2.5%x2.5%y2.5%z and 2.5%x2.5%y2.5%z2.5p by mass of the admixture(s) produced Class 42.5N cement. These cements can therefore be used for early strength concrete
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works and all types of construction works. 1:2:4 blended cement concretes with water cement ratio of 0.6 reached the targeted strength value of 20 ± 2 MPa at 28 days. The study showed that the effect of seawater (NaCl) and Na2SO4 on strength was reduced as the number of admixtures in the cement were increased. In all, the quinternary (2.5%x2.5%y2.5%z2.5%p) cement provided the best blend that could resist the deleterious effect of both seawater and sulphate solution negative action. These give ample proof that a combination of three or four admixtures in cement greatly influenced the reductionofportlandite content leading to high resistance to acidic attack.
Economically, replacing 15% of total clinker imports by these admixtures would amount to a capital saving of at least $33.43 million per annum to Ghana.
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A thesis submitted to the Faculty of Civil and Geomatic Engineering Department of Civil Engineering in partial fulfilment of the requirements for the award of degree of DOCTOR OF PHILOSOPHY,2012