Use of Chemical Admixtures to Modify the Rheological Behavior of Cementitious Systems Containing Manufactured Aggregates

Abstract

The use of local materials is an important part of sustainability for the concrete industry. The declining availability of aggregate resources in many areas has the potential to result in the use of alternative aggregates of lower quality, which can require higher cementitious materials contents, or the use of aggregates shipped from greater distance. In some markets, manufactured sands are replacing natural sands, which can adversely impact the rheology of cementitious mixtures. The use of certain chemical admixtures has been found to often minimize the need to increase cement and water contents in order to overcome the loss of workability that can accompany aggregate sources which feature flat, elongated, angular, and rough particles. In this study, a wide range of natural and manufactured sands were characterized for gradation, mineralogy, shape, texture, and cleanliness, and also evaluated for their effect on mortar rheology with and without a viscosity modifying type chemical (VMA) admixture. Use of the VMA is shown to mitigate the rheological effect of certain sands, and in some cases can allow for optimizing the mixture to lower paste contents. In the case of polycarboxylate (PCP)-based superplasticizers, attention is drawn to the increased dose required to achieve target workability versus superplasticizers based on naphthalene sulfonate condensate (NSFC) when swellable clays are present in the very fine fraction of certain aggregate sources. The use of sands with higher fines contents are also shown to increase the workability provided the fines are of appropriate quality.