342 ACI Materials Journal/July-August 2008 ACI MATERIALS JOURNAL TECHNICAL PAPER ACI Materials Journal, V. 105, No. 4, July-August 2008. MS No. M-2006-417.R1 received May 29, 2007, and reviewed under Institute publication policies. Copyright © 2008, American Concrete Institute. All rights reserved, including the making of copies unless permission is obtained from the copyright proprietors. Pertinent discussion including authors’ closure, if any, will be published in the May-June 2009 ACI Materials Journal if the discussion is received by February 1, 2009. The evolution of rheological properties of cement-based materials during the dormant period of cement hydration depends on the coupled effect of temperature and time. Highly flowable concrete is increasingly being used to facilitate construction. Such concrete should be optimized to account for changes in workability during transport and placement at different temperatures. This paper presents a methodology to combine the influence of time and temperature on the variations of rheological properties of cement-based materials. Such variations are expressed as a function of a relative time scale deduced from calorimetric measurements. Four micromortars were prepared at temperatures of 10 to 30 °C (50 to 86 °F) and tested to determine the evolution of rheological properties over the dormant period of cement hydration. The apparent yield stress and plastic viscosity are shown to vary in linear fashion with the relative time scale that combines time and temperature during the dormant periods for the tested micromortars. Keywords: cement hydration; chemical admixtures; plastic viscosity; rheology; self-consolidating concrete; temperature; yield stress. INTRODUCTION The knowledge of the rheological behavior of cement-based materials has evolved significantly in recent years. Several researchers have investigated the influence of the type of high- range water-reducing admixture (HRWRA) and temperature on rheology. 1-5 The benefits of using synthetic polymers, such as polynaphtalene sulfonate (PNS) and polymelamine sulfonate (PMS) as HRWRA to enhance concrete workability are well established. In some cases, cement-HRWRA incom- patibility can occur due to an interaction between the sulfonate groups within the hydration reaction and the cementitious ionic matrix. 2 The interaction of cement with other chemical admixtures, such as viscosity-enhancing admixtures and HRWRA, could lead to loss of fluidity or a delay in setting. Temperature also affects the level of incompatibility between cementitious materials and various admixtures. The influence of ambient temperature on setting and hydration kinetics of superplasticized cement-based materials is well documented. 3-5 Changes in temperature can influence the rheology of cement paste through various mechanisms. The concentration of the nonadsorbed HRWRA, in the case of PNS and PMS, has a direct effect on the rheology of the cement paste and hydration kinetics of portland cement. For mixtures made with PNS-HRWRA, the concentrations of adsorbed and residual HRWRA in the aqueous phase, vary with temperature of the paste, which has direct influence on rheology. 5 Limited information is available on the coupled effect of HRWRA and temperature on the variations of rheological parameters of highly flowable, cement-based systems. RESEARCH SIGNIFICANCE The main objective of the study reported in this paper is to evaluate the combined influence of time and temperature on apparent yield stress (τ o ) and plastic viscosity μ of highly flowable mortar extracted from self-consolidating concrete (SCC). The study aims at proposing a new methodology to couple the effects of temperature and elapsed time on the evolution of rheological parameters of mortar mixtures based on SCC design. Depending on the characteristics of the dormant period, relationships can be established for the evolution of τ o and μ at various temperatures with time that is related to the hydration kinetics of the cement paste at a given temperature. EXPERIMENTAL INVESTIGATION The testing program involved the evaluation of micromortars prepared with two different mixture compositions. Rheological parameters were monitored up to the end of the dormant period. Heat flux was measured to evaluate hydration kinetics for various combinations of HRWRA and temperature. As summarized in Table 1, the effect of temperature variations on changes in rheological parameters, heat flux, and temperature rise was investigated for four micromortars (M1 to M4). The compositions of the micromortars were based on mixture proportions derived from SCC. The nominal size of sand in the micromortars was limited to 315 μm (0.0124 in.). Micromortar M1 is based on an SCC that was used for the extension of a hospital in the City of Bethune in Northern France. During construction, such concrete was cast at ambient temperatures varying between 5 and 30 °C (41 and 86 °F) with no special provisions to adjust the mixture composition for variation in fresh concrete temperature. This practice can be tolerated in some mixtures that are not very sensitive to temperature changes; however, in some cases, the rheological properties exhibit considerable changes with temperature over time and must be taken into account in the mixture design. The SCC was proportioned with a relatively high water-binder ratio (w/b) of 0.52. The term binder is employed herein because limestone filler and blast-furnace slag were used. The specified 28-day compressive strength of the concrete was 32 MPa (4640 psi). The concrete was not air entrained and it was proportioned in compliance with the French NF P 18 305 Standard and the European EN 206-1 Standard for ready mixed concrete in nonaggressive environment. The SCC used to design the micromortar tested in Series 2 to 4 had a w/b of 0.42 and corresponded to SCC used in structural applications. Title no. 105-M39 Methodology to Couple Time-Temperature Effects on Rheology of Mortar by Jean-Yves Petit, Kamal H. Khayat, and Eric Wirquin