Elsevier

Powder Technology

Volume 383, May 2021, Pages 43-55
Powder Technology

Characterization and stabilization of nano-metakaolin colloidal suspensions

https://doi.org/10.1016/j.powtec.2021.01.024Get rights and content

Highlights

  • The use of suspensions minimizes the adverse effects of handling ultrafine powders.

  • Particle size, surface charges, and pH are critical factors to stabilize suspension.

  • The stability of suspensions was analyzed for zeta potential and rheology over time.

  • Polycarboxylate promoted the greatest stability of nano-metakaolin suspensions.

Abstract

Nanometric powders bear significant problems in the efficiency of dispersion. The ideal situation would be to ensure full nanoparticle separation in suspensions or composites matrix, which is hard to achieve due to the high reactivity and aggregation. This study evaluates the conditions to stabilize nano-metakaolin (NMK) in water suspension by analyzing the influence of solute concentration, pH, type and content of dispersants. Potentiometric titration curves, zeta potential and average particle size measurements were performed, along with rheological measurements, at different times: 0 h, 3 h, 6 h and 24 h. NMK particles showed the same tendency of surface charge distribution, regardless of solute concentration. The dispersed NMK particles presented two isoelectric points, attributed to the multiple arrangements they form when in suspension. Polycarboxylate (PC) was the dispersant that provided greater dispersion stability. Regarding the rheological and zeta potential measurements, the suspensions presented stability up to 24 h.

Introduction

Nanometric powders bear significant problems in the efficiency of dispersion in aqueous solutions or in specific matrices for the production of polymeric and cementitious composites, for example. The ideal situation would be to ensure full nanoparticle dispersion in suspensions or composites matrix, which is hard to achieve due to the high reactivity of the nanoparticles and aggregation. Ensuring morphological control, size distribution and stabilization of nanoparticles is still a challenge in laboratory and industrial scale processing. Different authors have shown that the dispersion of nanoparticles is complex and further studies are needed to evaluate the effectiveness of small-sized particle dispersion [1,2]. It is known that due to the high tendency of agglomeration, caused by the active Van der Waals forces, a poor dispersion of the nanoparticles can cause negative effects on the microstructure of the composites, compromising the final performance of the material [1,2].

There has recently been an increase in the number of studies related to the use of nano-kaolin (NK) and nano-metakaolin (NMK) for several applications, particularly for the construction industry, which is among its largest users [[3], [4], [5], [6], [7], [8], [9], [10]]. Some works recognized the advantages of the MK and NMK inclusion to improve the performance of cementitious materials [[5], [6], [7],[9], [10], [11], [12]], suggesting the necessity of guidelines and commercial techniques to produce NK and NMK. Also, intensive research must be made in order to maximize its potential as an alternative nanomaterial for construction materials [6]. There must be more research particularly on the influence of NMK in composites [7,9,13,14].

There is concern among researchers about dispersing NK or NMK before mixing them with cement. It is noted that there is still no sufficient knowledge to control the final characteristics of these particles, in order to guarantee the benefit of reactivity compatible with their size scale. The size distribution and dispersion efficiency are control parameters of the particles aggregation. They have a direct influence on chemical reactivity, as well as on the rheology of powders and suspensions, and thus on the processing control at all production scales. Besides, the chemical reactivity and microstructure are affected by the type and distribution of surface charges and surface area. The absence of morphological control of the powders modifies and hinders the control of the final product properties.

Different methodologies for the preparation and dispersion of NK and NMK can be seen in Table 1. In general, the resulting powder is mixed directly with the cement or dispersed in water for later incorporation into the cement. Few studies on the characterization, control and efficiency of nanoparticle dispersion under laboratory conditions or processing for mixtures in cementitious matrices have been carried out [2,9,15,16]. Studies show that the most efficient dispersion of nanoparticles involves a combination of methods, using mechanical energy as a primary dispersion, with ultrasonication or a high shear mixer, followed by chemical functionalization or physical coating to stabilize the dispersion obtained in cement-based materials [1]. Korayem et al. [1] still suggest the preparation of suspensions and claim that polycarboxylates (PC) are superior to other types of dispersants. However, studies are still needed to determine the effectiveness of surfactants for different nanoparticles.

The use of suspensions in aqueous medium for the processing of powders has advantages regarding the control of particle size distribution, which improves process control and final material properties. Besides, that can contribute to minimize effects related to the handling of ultrafine and nanometric particles, which could pollute the environment and be harmful to the human respiratory system [15], whether in processing, using or final disposition.

In this work, studies were carried out to optimize the stability of NMK suspensions in water. Thus, pH, particle size distribution, types and content of dispersants, zeta potential and rheological behavior of NMK water suspension were analyzed up to 24 h. NMK was obtained by the calcination of NK, manufactured by the company Caulim da Amazônia. The suspensions are in use for ongoing studies of cementitious composites. To the best of our knowledge, there are not many studies about NMK suspensions in order to achieve and ensure stabilization over time, combining complementary methods of characterization, including rheological parameters relevant for large-scale application.

Section snippets

Preparation of nano-kaolin (NK)

The company Caulim da Amazônia (CADAM) donated the NK manufactured specifically for this study.

Commercial kaolin was treated by physical separation of the sand and kaolin portions, using sieves and centrifuges, followed by chemical treatment to remove iron compounds. After this classification process, the NK product was obtained, with an average particle size of 0.22 μm (D50) - data provided by the manufacturer.

NK has as main oxides, in wt%: SiO2 (50.45%) and Al2O3 (45.24%); the remaining

Characterization of NK and NMK

Thermal analysis of NK (Fig. 3) presented its typical profile of kaolinite, as described in the literature [[20], [21], [22]]. Mass loss was observed within the temperature range of 450–650 °C attributed to kaolinite dehydroxylation and recombination between the silica and alumina groups for metakaolinite formation.

Recently, XRD patterns of metakaolin heated at 550–950 °C have been observed. In the region between 600 and 900 °C, XRD showed amorphous pattern with trace crystalline peak of

Conclusions

NMK particles showed the same tendency of surface charge distribution, with the predominance of negative charges, regardless of the NMK concentration.

From the analysis of the isoelectronic point of the NMK particles in suspension, two i.e.p values were obtained at pH 4.0 and pH 4.6, being attributed to the multiple arrangements that particles form in the suspension.

PC was the dispersant that provided greater stability to NMK suspensions, with smaller variations in both zeta potential and

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgments

This work was supported by FINEP/CNPq [grant number 0111.0142.05/FINEP/FUSP/UFS]; FAPITEC-SE/FUNTEC/CNPq [grant number 019.203.02720/2009-2/FAPITEC-SE/FUNTEC/CNPq]; CNPq [grant number 480981/2013-2/CNPq]; and CAPES. The authors are thankful to the Corrosion and Nanotechnology Laboratory/NUPEG/UFS; UFS Multi-User Nanotechnology Center (CMNano-UFS) for the use of the Electronic Microscopy analysis infrastructure; and the Company Caulim da Amazônia/CADAM that donated the nano-kaolin. We also thank

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