Title : COMPATIBILTY OF GGBS CEMENT WITH SUPERPLASTICIZER - A CASE STUDY
Authors : Supradip Das
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Price : 250
Abstract : Lately the use of superplasticizer in fresh concrete has been accepted as the fifth ingredients because of its effect on early cement hydration behaviour and also the durability factor of the hardened concrete. This is in addition to the remarkable improvement in the workability of fresh concrete during pouring. However it has been noticed that erratic rheological behaviour coupled with segregation, extended set retardation, loss of entrained air are reported while using blended cement for making high performance concrete. In some cases, incompatibility in terms of sufficient of water reduction has also been reported. Therefore compatibility among concrete materials, an important issue for controlling the workability of concrete for sound placing has been the object of much attention over the past decade. In many occasions, material like pozzolana, metakaoline or granulated blast furnace slag are used as mineral admixture in the concrete or blended cement like Portland Pozzolana and Blast Furnace Slag Cement are often used for bulk concreting. Use of blended cement often shows erratic behaviour with a particular admixture. In some cases it has been found that superplasticizer addition lowers rheological parameters of cement pastes and this decrease of rheological parameters depends on fine fraction contents in cement. In one such hydel project where portland slag cement was recommended for bulk concreting, it was decided to go for compatibility test before carrying out site trials and using the same in the project. The physical and mechanical properties of Portland cement (PC) containing blast furnace slag and the compatibility between such materials and superplasticizers in terms of rheological behaviour were investigated in the present study. After slag is incorporated into PC, the ultimate compressive strength of the blended cement gets enhanced. However, the fluidity of slag based blended cement (PSC) becomes poorer than that of PC at the same dosage of superplasticizer and the same water/binder ratio. Blended cement has been found to be better performing cement as compared to OPC. It has already been established that for bulk concrete or to achieve high performance concrete, the use of blended cement has been substantially increased, but also the fluidity of the blended cement paste was improved. This indicates that ultra-fine slag can improve the physical and mechanical properties of blended cement vis-à-vis the quality of concrete in terms of durability. The incompatibility between cement and chemical admixtures has increased over the last decade. Specifications calling for the use of admixtures in concrete often results in strange occurrences, i.e. rapid set, retardation, accelerated stiffening, etc. It has become a common practice to carry out site trials with cement with varying dosage of admixture. Moreover, high performance concretes, which are in wide use today, almost always incorporate a mineral admixture or filler such as silica fume, fly ash and limestone powder. This further complicates the physico-chemical behaviour of the cement-based system since the mineral admixtures play an important role in the evolution of the hydration reactions and the availability of free water during the early ages of concrete. This paper presents the observations of a study on the effect of two different superplasticizers with respect to the rheological behaviour using Marsh Cone Method and Flow Table was used with Portland slag Cement (PSC) at a variable dosage between 0.2 and 3.0%. This study has also given the indication to use a specific type of admixture with optimum dosage.