Compatibility between polycarboxylate-based admixtures and blended-cement pastes M.M. Alonso a,⇑ , M. Palacios a,b , F. Puertas a a Eduardo Torroja Institute for Construction Science, (IETcc - CSIC), Spain b Institute of Building Materials, ETH Zurich, Switzerland article info Article history: Received 27 September 2010 Received in revised form 16 March 2012 Accepted 18 August 2012 Available online 7 September 2012 Keywords: Adsorption Superplasticizers Blended cement Rheology Calorimetry abstract Compatibility between three structurally different PCEs and four commercial cements: one non-blended cement and three blended cements, was studied by adsorption, zeta potential, rheological and calorimetric methods. According to the adsorption curve results, the higher the percentage of carboxylates groups in the admixture, the more intensely it is adsorbed on cement pastes. Moreover, admixtures were shown to be adsorbed by the additions as well, being most effectively adsorbed in limestone. From the rheological point of view, the optimum carboxylate group/ester group ratio for the admixtures used in the present study was found to range from 0.7 to 1.2. The fluidizing effect of the admixtures on cement pastes is conditioned by the presence of mineral additions. Despite the low adsorption rates of the admixtures in slag-blended cements, the inclusion of PCEs generated the steepest declines in the rheological parameters. The delay of admixtures on cement hydration intensifies with rising PCE dosage. This admixture- mediated retarding effect was also observed to vary depending on the nature of the addition, and was most intense in slag-blended cement. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The development in recent years of higher performance and self-compacting concretes has been possible thanks to the use of superplasticizers, mainly polycarboxylate-ether superplasticizers (PCEs). These admixtures decrease water content in concrete (up to 40%) much greater than afforded by conventional lignosulfo- nate-, melamine- or naphthalene-based superplasticizers. This reduction in water content leads to a decline in porosity, thereby raising concrete mechanical strength and durability. These admix- tures also improve concrete workability and rheology, facilitating casting in hard-to-reach places, covering reinforcement more effectively [1]. PCE admixtures structure consists in a linear hydrocarbon back- bone with carboxylate and ether group side chains. Their adsorp- tion on cement particles, mediated by their carboxylate groups, disperses cement grains as a result of the steric repulsion gener- ated by the long ether group chains [2–4]. Nonetheless, the use of superplasticizers may pose problems in terms of variations in flowability, uncontrolled setting, anomalous rheological behaviour and so on, due in most cases to cement–admixture incompatibility. Several authors have reported that cement–admixture compat- ibility depends primarily on factors attributable to both the admix- tures and the cements. The factors associated with admixtures that determine their performance and fluidizing effect are their dosage, the manner and timing of inclusion in the mix and their chemical and structural composition [4–8]. The wide variety of formulations in commercial PCE admixtures, however, is largely responsible for the present incomplete understanding of their effect on rheology, hydration and microstructure of cement systems. The factors attributable to cements that affect compatibility, in turn, include their fineness [9], chemical and mineralogical composition, particularly their C 3 A content [10–12], and the amount and type of components such as calcium sulphate and alkaline sulphates [13–15]. The partial replacement of cement clinker with mineral addi- tions is an increasingly common practice. The aim is to produce more eco-efficient, less energy intensive cements whose manufac- ture involves the re-use of industrial by-products such as fly ash and granulated blast furnace slag. The inclusion of such mineral additions may also enhance paste flowability and durability. Of the 27 cements listed in the existing European standard on the subject (EN 197-1:2000), 26 contain some manner of mineral addition. Consequently, one factor that may affect cement–admixture compatibility is the presence of mineral additions, since 0958-9465/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconcomp.2012.08.020 ⇑ Corresponding author. E-mail address: mmalonso@ietcc.csic.es (M.M. Alonso). Cement & Concrete Composites 35 (2013) 151–162 Contents lists available at SciVerse ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp