Blended cements containing high volume of natural zeolites: Properties, hydration and paste microstructure

https://doi.org/10.1016/j.cemconcomp.2011.08.009Get rights and content

Abstract

In this study, properties and hydration characteristics as well as paste microstructure of blended cements containing 55% by weight zeolitic tuff composed mainly of clinoptilolite mineral were investigated. Free Ca(OH)2 content, crystalline hydration products and decomposition of zeolite crystal structure, pore size distribution and microstructural architecture of hydrated cement pastes were examined. Superplasticizer requirement and compressive strength development of blended cement mortars were also determined. The blended cements containing high volume of natural zeolites were characterized with the following properties; (i) no free Ca(OH)2 in hardened pastes at the end of 28 days of hydration, (ii) less proportion of the pores larger than 50 nm when compared to portland cement paste, (iii) complete decomposition of crystal structure of zeolite at the end of 28 days of hydration, (iv) presence of tetra calcium aluminate hydrate as a crystalline product of pozzolanic reaction, (v) more compatibility with the melamine-based superplasticizer when compared to the naphthalene based product, and (vi) similar 28 days compressive strength of mortars to that of reference portland cement.

Highlights

Blended cements containing high volume of natural zeolite were studied. ► Hydration characteristics and paste microstructure were investigated. ► No free Ca(OH)2 in pastes at the end of 28 days of hydration was observed. ► Crystal structure of zeolite completely decomposed at the end of 28 days of age. ► Similar 28-day compressive strength to that of reference Portland cement was found.

Introduction

Portland cement industry is responsible for approximately 7% of global CO2 emission [1]. Partial replacement of portland cements by one or more additives to obtain blended cements not only provides reduction in CO2 emission and energy saving in cement production but also supplies more durable cementitious systems to construction industry.

The extent of the benefits provided by use of blended cements increases with increasing content of additives in blended portland cements. However, the content of additives in blended portland cements, especially for natural pozzolans, is limited by some factors such as increase in water requirement and decrease in rate of strength development of the cementitious systems. It has been found that the blended cements containing high volume (55% by weight) of natural pozzolans (volcanic tuff) posses lower 28-day compressive strength when compared to the reference portland cement, although they show similar strength values at 91 days of age [2], [3]. Therefore production of high-volume natural pozzolan blended cements which are able to compete against ordinary portland cement requires natural pozzolans exhibiting significantly high strength activity.

Natural zeolites are crystalline aluminosilicates composed of a three dimensional arrangement of silicon–oxygen (SiO4) and aluminum–oxygen (AlO4) tetrahedra [4]. Zeolite group of minerals currently include more than forty naturally occurring species, and is the largest group of silicate minerals [4]. Clinoptilolite, heulandite, analcime, chabazite, and mordenite are the most common types of natural zeolite minerals on the earth. It is known that they show considerable pozzolanic activity despite their distinct crystalline structure. Pozzolanic activity of natural zeolites has been principally attributed to dissolution of zeolitic crystals of three-dimensional framework structure under the attack of OH ions available in hydrating cementitious system [5], [6], [7], [8], [9], [10]. In a recent study, Uzal et al. [11] reported that the clinoptilolite zeolite possesses a lime reactivity which is comparable to silica fume, and higher than fly ash and a non-zeolitic natural pozzolan. They also concluded that the high reactivity of the clinoptilolite is attributable to its specific surface area for certain grinding method and duration as well as its reactive SiO2 content.

It is known that zeolitic tuffs are used in some countries as a blending component during cement production and as a mineral admixture in concrete mixtures. However the knowledge on the properties and hydration characteristics of blended cements containing natural zeolites is limited. Published the literature contains no report on properties and hydration of blended portland cements containing high volume of natural zeolites, more than 50% by weight. One of the objectives of this study is to fulfill this need.

This paper presents the results of an investigation on blended cements containing 55% by weight clinoptilolite-rich zeolitic tuffs. Zeolitic tuffs obtained from two major deposits in Turkey, Gordes and Bigadic, were finely ground so as to blend with portland cement separately. Blended cements containing 55% zeolitic tuff were obtained, and tested for setting time; Ca(OH)2 content and pore size distributions of the pastes; superplasticizer requirement and compressive strength development of mortars in comparison with the reference portland cement. In addition, the hardened blended cement pastes were examined for the crystalline products of hydration via X-ray diffraction (XRD) analysis and for the microstructural architecture using back-scattered scanning electron microscopy (BS-SEM).

Section snippets

Materials

Chemical composition and physical properties of the ordinary portland cement (PC) used in the study are shown in Table 1.

Normal consistency and setting time

Water-to-cement ratios required for normal consistency and setting time of cements are shown in Table 3. Water demand of GZ55 and BZ55 blended cements for normal consistency was approximately 60% and 40% higher than that of reference PC, respectively. Significantly increased water demand of blended cements when compared to PC is probably due to high capacity of zeolite particles to absorb water. Lower water demand of BZ55 could be attributed to lower BET surface area of BZ when compared to GZ.

Conclusions

Based on the experimental results, the following conclusions can be drawn;

  • 1.

    Blended cements containing large amount of clinoptilolite tuff demonstrated faster initial and final setting time than the ordinary portland cement. This fact may be related to consistency loss resulted from high water absorption of zeolite particles indicated by high water demand of blended cements for normal consistency, rather than setting of the blended system.

  • 2.

    In blended cement pastes, Ca(OH)2 formed from hydration of

Acknowledgements

The authors would like to thank Prof. Hayrettin Yücel (Dept. of Chemical Engineering), Prof. M. Cemal Göncüoğlu (Dept. of Geological Engineering) and Prof. Ali Çulfaz (Dept. of Chemical Engineering) at Middle East Technical University for their helpful comments on the characterization of zeolitic tuffs used in the study. This study was funded by The Scientific & Technological Research Council of Turkey under Project No: 104M393. The instrumental analyses in the study were carried out in Middle

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At the time of the study, the author was a Ph.D. Candidate at the Department of Civil Engineering, Middle East Technical University, Ankara, Turkey.

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