Characteristics of some Kenyan sands and their effect on mortar strength

Abstract

In Kenya, portland cement (Ordinary and Rapid-hardening) strength quality control is done to meet the requirements of the British standard BS 12: 1989 or the Kenyan standard KS 02-21: 1976. This M.Sc. Thesis research project endeavours to characterise locally available natural sands, and to study their effect on mortar cube compressive

strength. The primary objecti ve was to compare the strength of mortar made using Kenyan sands to the strength of mortar made using imported standard Leighton Buzzard sand.

A total of fifteen (15) sand samples - ten samples from Machakos, Kajiado and areas around Nairobi and five samples from the coast - were procured and subjected to laboratory testing and analysis. Basic material characteristics including gradation, percentage of silt, dust and clay, presence of organic matter, specific gravity and percentage of acid-soluble matter were determined.

Standard Leighton Buzzard sand and Ordinary Portland Cement were obtained to enable comparative mortar strength testing. Test specimens were extracted from each sand sample and used in casting mortar cubes for compressive strength determination. Casting, curing, and testing was done according to BS 4550 : 1978, the 1 day, 3 days, 7 days and 28 days age strengths being determined. The results were compared with the corresponding strengths developed by the standard Leighton Buzzard sand mortar when cast, cured and tested under nominally the same laboratory environment and conditions.

The results obtained indicate that nine out of ten of the up-country sand samples tested were not admissibly pure due to presence of red ferric oxides and other acid-soluble matter, while all the five coast samples were found to be admissibly pure. The yield in the particle size range of 850 to 600 urn prescribed by BS 4550 was uneeonomically low for all samples. The yield values were found to range from 5 to 21 per cent. River-bed sands from up-country were observed to develop lower mortar strengths than the standard Leighton sand. Dug-up sands from Kwalc ill the coast were found to develop mortar strength values comparable to those of the standard Leighton sand.