Elsevier

Cement and Concrete Research

Volume 75, September 2015, Pages 66-74
Cement and Concrete Research

Parallel-plate rotational rheometry of cement paste: Influence of the squeeze velocity during gap positioning

https://doi.org/10.1016/j.cemconres.2015.04.010Get rights and content

Abstract

This paper reports an experimental investigation regarding the influence of the squeeze velocity, during positioning of the parallel-plate gap, on the rotational shear flow behaviour of a cement paste. The descent of the upper plate was performed using diverse speeds while the normal force generated due to the compression of the paste was recorded. The slower the plate speed, the higher the resulting normal force. This behaviour was caused by liquid–solid separation, which is more likely to occur at slow squeeze velocities. Phase separation was confirmed by assessing, via microwave drying, the water contents of the trimmed portion of the paste sample and of the portion actually subjected to rotational shear cycles. Owing to the variation of water/cement ratio induced by liquid radial migration, paste's Bingham yield stress and plastic viscosity were significantly affected by squeeze speed, and both rheological parameters presented an inversely proportional relationship with this experimental variable.

Introduction

Concentric cylinders, cone-plate, and parallel-plate geometries are normally used either in rotational flow or in oscillatory modes for the rheological evaluation of concentrated suspensions. The first is used for relatively low viscous suspensions as cement slurries, whereas the parallel plate geometries are more appropriate for assessing the rheological behaviour of pastes [1], [2], [3].

Cone-plate setup has the advantage of shearing the material at a constant rate along the sample radius, while in the plate–plate geometry the shear rate varies (being zero at the centre and maximum at the edge) [1], [2], [3], [4]. However, the former geometry also presents a main drawback for the evaluation of granular suspensions, which is the jamming of particles under the apex of the cone [1], [2], [3], [4], [5], [6]. The truncated cone minimizes this effect allowing for its utilisation for suspensions [1], [2], [3], [4], [5], [6], though only for a limited maximum gap, which also restricts the maximum particle size present in the suspension (due to the minimum required gap/particle size ratio of 10). On the other hand, the parallel-plate geometry does not have the jamming problem; the gap can be adjusted from tens of microns to a few millimetres according to the material [1], [2], [3] or to the goal of the experiment [4], [5], [7], [8], and it provides a large shear rate range that is adjustable by changing gap and plate diameter [1], [2], [3], [4].

Because of its suitable features, the parallel-plate geometry has been extensively used for the rheometry of cement pastes in flow [4], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18] and small amplitude oscillatory [4], [16], [17], [18], [19], [20], [21], [22] modes. Most of the studies aimed to evaluate the effects of composition and admixtures [9], [13], [14], [15], [16], [17], [18], [20], [21], [22]; while others focused on the influence of processing parameters like mixing method [4] and temperature [17]; understanding the setting kinetics and microstructural development [11], [16], [17], [19], [20], [21], [22]; or the relationship between rheology of cement paste and that of concrete [7], [15], [18]. The technique was also utilised for the development of a reference material intended for calibration of cement paste rheometers [12]. Roughness [8] and gap distance [4], [7], [8] were the test parameters systematically investigated to date. Rough plates tend to increase the measured Bingham yield stress compared to the values obtained with smooth plates; on the other hand, plastic viscosity decreases with plate roughness [8]. The use of smaller gaps resulted in higher flow resistances [4], [7], [8], especially when the gap approached the size of the particles/agglomerates of the paste, which was considered as a limiting gap related to a sharp increase of flow resistance [4], [7].

Despite the extensive employment of the parallel-plate technique for cement paste rheometry, there is no mention whatsoever in the consulted references on cement pastes about the procedure used to adjust the gap prior to the rotational or oscillatory measurements. As general rules for most materials, the documents from rheometer manufacturers suggest the use of excess material and, then, trimming the squeezed-out portion at a gap 5% higher than the measuring position to ensure a correct filling and total contact between sample and plates [1], [23]. In addition, it is recommended to attempt disturbing the sample as little as possible when loading, in order to maintain its original structure [1], [23], [24], [25]. This mainly consists of descending the upper plate slowly to keep low shear rate and to avoid excessive increase of the normal force and possible modification/destruction of the sample structure during squeezing [1], [25]. This suggestion is worthy particularly for structured fluids, gels, and highly viscoelastic materials. Nevertheless, this may not be the best choice for concentrated suspensions of particles in the micron range like cement pastes.

Actually, the squeeze-flow is a widely used rheometry technique, which has been applied for food, pharmaceuticals, polymer composites, ceramic pastes, and other concentrated suspensions [26], [27], [28], [29], [30]. Its use is becoming more frequent as an alternative/complementary method for cement-based pastes [16], [31], [32] and mortars [33], [34], [35], [36], [37], especially to simulate flow situations associated to geometric restrictions (extrusion, spreading, brick laying, flow through a nozzle during pumping or spraying). The geometry change inherent to the method can induce liquid–solid phase separation, because the liquid may flow radially outwards through the porous structure of packed particles (filtration or drainage), thus having a substantial influence on the squeeze-flow behaviour of concentrated suspensions [27], [28], [29], [30], [32], [33], [34], [35], [36]. The occurrence and intensity of phase separation depend on material characteristics (liquid viscosity and permeability of packed particles) and test parameters, mainly speed and gap [27], [28], [29], [30], [32], [33], [34], [35], [36].

As the oscillatory and flow parallel-plate rheometry of pastes must be preceded by a squeeze-flow, it is important to understand how this pre-test stage may affect these measurements, thus providing useful information to the development of experimental procedures more suitable for concentrated suspensions. For this reason, the objective of this work is to determine the influence of the squeeze velocity, during positioning of the parallel-plate gap, on the rotational shear flow behaviour of a cement paste.

Section snippets

Cement paste

Cement pastes with water/cement ratio of 0.40 were prepared with Portland cement type CEM I 52.5 N (Lafarge Ciments — Usine Du Havre, France) specified according to EN 197-1. Specific gravity (by Helium picnometry), BET specific surface area (by Nitrogen adsorption) and particle size distribution (by laser diffraction in deionized water) of the cement are shown in Table 1.

Mixing procedure

The paste batches were prepared with 50 g of cement and 20 g of water both at room temperature of 23 °C. The materials were

Squeeze-flow

Fig. 3 shows the curves of the cement pastes squeezed using the exponential automatic mode and at a constant rate of 750 μm/s, at 5 and 35 min after mixing. The force-displacement curves of the automatic tests (graphics a and c) present two distinct regions of squeeze behaviour: (i) viscous flow or plastic deformation at low loads, for which the normal force displays a slightly increase followed by a quasi-plateau; (ii) then a transition to strain hardening occurs with a very sharp raise of the

Conclusions

A rheological investigation regarding the influence of the squeeze velocity, during gap positioning of the parallel-plate geometry, on the shear rheological behaviour of a cement paste has been undertaken. The normal forces required to squeeze the paste samples down to the trimming gap of 1050 μm were higher than the instrument limit 50 N at slow speeds, including when the exponential automatic default mode of the equipment was used. On the other hand, fast squeeze velocities in the range of

Acknowledgements

The authors would like to thank the support from the Brazilian research funding agencies FAPESP (Grants n. 2011/00948-9, 2012/18952-5, and 2013/27121-2) and CNPq; and from the company Parex Group (France).

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