Homogenisation of concrete in a batch plant: the influence of mixing time and method on the introduction of mineral admixtures

Abstract

This paper presents a study conducted on the effects of mixing within a batch plant operation for ‘building industry’ type concretes containing mineral admixtures (fly ash, limestone filler or blast-furnace slag). The primary objective of this study was to evaluate the technical bases of French standard XP P 18-305, which stipulates that the mixing time of these concretes is to be increased whenever a mineral admixture is introduced, as opposed to the admixture being mixed with Portland cement to create a blended cement. Testing was carried out in a semi-industrial plant at a 1:1 scale; various techniques were employed both to evaluate the homogeneity of the batches and to characterise the performance of the materials produced. Results suggest a slight advantage in homogeneity in favour of the blended cements, yet this advantage at the scale of the completed structure is not necessarily significant. Moreover, the feasibility of concretes with a binder consisting of at least two-thirds blast-furnace slag, added into the mixer under typical batch plant operations, has been confirmed. The paper concludes with a few research directions in the area of concrete mixing.