Particle packing and rheology of cement pastes at different replacement levels of cement by α-Al2O3 submicron particles
Introduction
Several different approaches have been developed for the mitigation of the negative environmental effects associated with the production of portland cement and concrete, i.e. high energy consumption and high emissions of greenhouse gasses, and heavy exploitation of natural resources [1], [2]. One of these approaches involves the partial substitution of portland cement (hereafter referred to as “cement”) by naturally occurring or artificially produced powders, with a particle size distribution comparable to cement or extending down to the submicron-size range. In terms of their origin, these powders can be either waste materials from various industrial processes (e.g. spent abrasive dust from abrasive blasting applications, or stone cutting powder [3], [4]), or custom-made materials produced by using a top-down or a bottom-up method [5]. In terms of their hydraulic activity, these materials can be classified either as reactive or as inert powders. In the case of the latter, no large volume applications have yet been introduced, although the use of one such powder has been proposed in the construction sector. The technical feasibility and environmental benefit of blending inert powders with cement-based binders has been investigated by several authors [3], [4].
Over the last decade, research has been increasingly aimed at reducing the cement content of hydraulic binders, by partially substituting these binders by submicron- and/or nano-sized particles. Materials in this size range lie between bulk materials and atomic or molecular structures, and are characterized by a high surface area to volume ratio. They are frequently reactive, and can exhibit different physical properties to those of the comparative bulk material [6].
Most reported work concerning the implementation of chemically inert or near-inert submicron- or nano-sized particles in cement composites has dealt with the mechanical and durability aspects of cementitious materials incorporating particles of TiO2 [7], [8] Al2O3 [9], [10], [11], [12], [13], CaCO3 [14], [15] and Fe2O3 [16]. The results obtained in these studies are rather controversial. In a review by Rashad [17], for instance, it was shown that the replacement of up to 5.0 wt% of cement by α-Al2O3 particles has a beneficial effect on the compressive and flexural strength of the hardened cementitious material. A significant increase in mechanical properties was reported by Arefi et al. [18], as well as by Nazari and Riahi [11], whereas in other studies just a slight increase in these properties has been reported [10]. Other authors found that the effect of blending cement with submicron- to nano-sized particles is negligible [19], [20], or even detrimental [9]. However, in the case of cement pastes incorporating α-Al2O3 particles more encouraging results have been obtained with respect to pore size distribution and pore refinement [10], [16], [21]. Mortars incorporating such particles have shown increased surface hardness and increased resistance to abrasion [13]. The scatter which has been observed in strength properties remains unclarified, indicating that there is a need to use more advanced investigation techniques, including those involving the rheological properties of the fresh state, in order to obtain a better insight into the mechanism of particle packing and self-assembly. When cement is mixed with water a concentrated paste is obtained, which then develops over time into a hardened monolithic mass through a complex chemical process, which involves dissolution of the anhydrous phases and precipitation of the hydration products [22]. The mechano-physical properties of such hardened cement pastes, such as strength and permeability, are known to be affected by the particle size distribution of the solid constituents and by the particle packing.
The packing of powder particles of a given size distribution has been described by several authors [23], [24], using various empirical models. Generally, on widening the overall particle size range, e.g. by adding particles that are significantly smaller in size compared to the original ones, the amount and size of interstitial voids is reduced. As a result, the volume of water needed to fill these voids is decreased and the surplus water can improve the flowability of the suspension [25], [26]. In complex cement-based binders, in which cement has been partially replaced by relatively smaller chemically inert particles, the assessment of particle packing, the determination of the amount of water needed for a given flowability, and the control of particle interaction and of the precipitation of a sufficient amount of hydration products, are needed to design advanced formulations [27]. When adding chemically inert particles, attention should be paid to the avoidance of excessive cement dilution, which could reduce the hardened state mechanical properties even in the presence of satisfactory fresh binder properties. The maximum amount of cement that can be replaced by chemically inert submicron- to nano-sized particles should be determined [28]. The research presented in this paper is focused on experiments which were performed on samples of differently formulated cement pastes incorporating α-Al2O3 submicron particles, as a partial replacement for cement. Its aim was to assess, using a number of different experimental techniques, to what extent the quantity of such particles can affect the packing density of the granular skeleton of the paste, and to determine the consequences of such modifications. The initial assumption was that α-Al2O3 submicron particles act as fillers between the cement grains, replacing water and thus improving the performance of the cement paste, with increased mechanical strength and reduced capillary porosity. Based on the results of this work, the introduction of submicron- to nano-sized particles in cement composites will be given a rationale.
Section snippets
Materials and characterization methods
The study was conducted using a Portland cement CEM I 52.5 R (SIST EN 197-1:2011 [28]) that was supplied by Salonit Anhovo d.d. (Slovenia). Its elemental and phase composition was analysed by X-ray fluorescence (ARL 8480 S, Thermo Fischer Scientific Inc., USA) according to SIST EN 196-2:2013 [29], and by X-ray diffraction (X’Pert PRO MPD, PANalytical B.V., the Netherlands), respectively. Quantitative phase analysis was performed on a cement sample into which 30 wt% of the internal standard
Rheological measurements and the maximum solid volume fraction
One of the assumptions underlying the validity of the K-D method is that the particles do not interact, and that they behave as rigid bodies. In shear flow experiments this implies Newtonian behaviour, as observed by Qiu et al. 2011 [42], apart from measurement aberrations. This was found in the flow curves of the investigated cement suspensions in the shear rate descending branches. In the ascending branches a small amount of shear thinning (microstructural destructuration) was observed
Conclusions
This study was concerned, in general, with the packing of cement/α-Al2O3 submicron-sized particles in modified cement pastes. By using several different investigation techniques, i.e. rheometry, centrifugal consolidation, isothermal calorimetry, compressive strength tests, and porosimetry, the following results were obtained:
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The maximum solid volume fraction ϕm,K-D showed a decrease as the replacement level of cement by α-Al2O3 particles was increased from 0 to 10 wt%, for all the six
Acknowledgements
This work was financially supported by the Slovenian Research Agency (ARRS) within the scope of the Young Researcher programme No. 1000-12-1502. The authors would like to thank Salonit Anhovo d.d., Anhovo 1, 5210 Deskle, Slovenia, for a generous supply of cement, V. Zalar Serjun for her assistance in the quantitative phase analysis of cement, T. Tomše for performing the mercury intrusion porosimetry analysis, and P. Sheppard for his extensive help in the editing and proof-reading of the
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