VARIATION IN PROPERTIES OF DIFFERENT PAKISTANI CEMENTS AND ITS EFFECT ON PROPERTIES OF CONCRETE

Abstract

This research work pertains to study the impact of variations in characteristics of various brands of cement available in Pakistan on the properties of concrete namely, the workability and compressive strength. The main aim was to study the trends of cement using indigenous raw materials and its effect on resulting concrete. Fineness/Blaine values were considered as the key property of cement and its effect on properties of concrete and other cement properties was investigated. Fineness of cement has substantial effect on the properties of fresh and hardened concrete. It is an established fact that the finenesses of Portland cements have increased during the past 50 years and are continuing to increase. Various researchers conducted detailed investigations on the effect of fineness of cement on properties of concrete. Fineness/blaine values were varied by grinding cement clinker with gypsum at laboratory ball mill level and its effect on properties of mortar and concrete was observed. In order to generalize the result, investigation was conducted and averaged over five cement factories. Fresh cement clinker collected from selected batch of each factory was ground with 4% gypsum in a laboratory ball mill. During grinding, fineness was periodically measured using Blaine’s air permeability apparatus. In this way, six cement samples with different fineness values were prepared from each factory. In order to confirm uniformity of their oxide ratios, chemical composition of sample was analyzed by XRF-Cement Spectrometer. Grinding time for each sample was recorded by the built-in timer of ball mills. A comprehensive testing was carried out on cement samples prepared in this way. Comparisons were made between (a) fineness of cement versus consistency, setting time and compressive strength of cement, (b) slump and compressive strength of concrete, (c) Heat of hydration and TG loss measured by DTA/TGA tests and (d) grinding time utilized in preparation of samples in the laboratory ball mill to make relative study of cost of grinding against desirable properties of mortar & concrete at higher fineness levels. The consistency of cement as well as compressive strength of mortar cubes and concrete increase while setting time decreases with increasing fineness of cement. The results obtained are well in accordance with the previous vpublished data made on foreign cements. A fineness of 3200 cm 2 /g has been recommended as optimum value. Up to this value, increase in strength is more and grinding cost is less. However, after this value, grinding cost increases while rate of gain of strength ceases. However, an interesting pattern was noted regarding fineness versus workability for samples from almost all the sources. Workability of concrete increases with fineness up to 2500 cm 2 /g then it remains constant for fineness up to about 3200 cm 2 /g. Afterwards it again starts increasing. In the case of projects where high workability is needed, fineness may be raised above 3400 cm 2 /g but keeping in mind that it is involved with high grinding cost, thus, making cement uneconomical. So, other techniques e.g., use of admixtures to improve workability, may be more economical.