1 EFFECT OF MIX COMPOSITION ON RHEOLOGY AND SURFACE FINISH OF CEMENT-BASED MORTARS Jacek Kwasny 1 , Mohammed Sonebi 1* , Sofiane Amziane 2 and Julien Plasse 2 1 QueeŶ’s UŶiǀeƌsity Belfast, Belfast, UK 2 Polytech Clermont-Ferrand, Clermont-Ferrand, FRANCE * corresponding author: m.sonebi@qub.ac.uk ABSTRACT Aesthetic of concrete structures is directly related to the quality of their surface finish. The objective of this investigation was to examine the effect of rheological properties on the quality of surface finish of cement-based materials. The study was divided into two phases. Firstly, the influence of the mix composition of mortars, viz. the w/c ratio, the sand content and the superplasticiser (SP) dosage, on their rheology was evaluated. Secondly, the surface finish quality was characterised and related to the rheology of the studied systems. Rheology of these materials, i.e. the yield value, was measured using a vane viscometer. The quality of the surface finish was assessed by quantification of the surface air voids by analysing digital photographs of the mould finished sample surfaces. It was found that an increase in the w/c ratio and the SP content decreased the yield value, whilst the increase in the sand content had an opposite effect. When the surface quality is concerned, an increase in the yield value was found to increase the total content of the surface air voids and especially those with size smaller than 1 mm in diameter. Moreover, the analysis of the location of the surface air voids along the height of the sample revealed that with the increase in the yield value their concentration was higher in the bottom section of the analysed samples. Keywords: mortar; yield stress; bugholes; surface air voids; INTRODUCTION The concrete surface finish is a very important aesthetic feature of concrete buildings and structures, widely recognised by architects, investors, developers and asset owners. If the quality of the surface finish is lower than desired, it can diminish the appearance and/or value of these structures, and in consequence lead to expensive surface repairs. Surface appearance can be affected by major defects, such as cold joints, honeycombs and delamination made during construction process, as well as structural- or durability-related cracks, developed during service live of the structure [1, 2]. However, more frequent are small surface defects, i.e. surface air voids, channels caused by bleed water migration, hairline shrinkage cracks and