Increasing the reactivity of metakaolin-cement blends using zinc oxide Sarah C. Taylor-Lange a,⇑ , Kyle A. Riding b , Maria C.G. Juenger a a Department of Civil, Architectural, and Environmental Engineering, 1 University Station C1748, The University of Texas at Austin, Austin, TX 78712, USA b Department of Civil Engineering, 2107 Fiedler Hall, Kansas State University, Manhattan, KS 66506, USA article info Article history: Received 12 July 2011 Received in revised form 3 March 2012 Accepted 7 March 2012 Available online 17 March 2012 Keywords: Metakaolin Kaolinite Supplementary cementitious materials Zinc oxide Dehydroxylation Pozzolan abstract This study aimed to improve the reactivity of metakaolin-cement mixtures using ZnO additions. Kaolinite samples with 0.1–1 wt% ZnO were calcined at temperature intervals of 50 °C from 500 to 650 °C for 1 h. The resulting metakaolins were examined for structural changes after calcination and for their pozzolanic reactivity, influence on the hydration behavior of cement pastes, and impact on the compressive strength of mortar cubes. ZnO behaved as a delayed accelerator for cement paste. However, when ZnO was com- bined with highly amorphous metakaolin, chemical retardation was eliminated while acceleration was maintained. Such systems also had increased 28-day compressive strengths. ZnO additions did not affect the degree of dehydroxylation or the pozzolanic reactivity of the metakaolin. These results could facilitate the use of less pure calcined clays as SCM by providing a mechanism for improving reactivity and may also impact the ability to use zinc-contaminated materials in concrete. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The most commonly used calcined clay as a supplementary cementitious material (SCM) is calcined kaolinite, called metakao- lin. Using metakaolin as a partial substitute for Portland cement in concrete has been shown to increase long-term strength and dura- bility compared with Portland cement alone and reduce the quan- tity of cement required, thereby lowering concrete carbon dioxide footprint associated with cement manufacturing [1–3]. Kaolinite is classified as a 1:1 clay mineral, consisting of one tetrahedral sheet and one octahedral sheet held together by weak van de Waals forces [4]. Ground kaolinite [Al 2 Si 2 O 5 (OH) 4 ], which is theoretically composed of 46.54% SiO 2 , 39.50% Al 2 O 3 and 13.96% H 2 O, forms metakaolin [Al 2 Si 2 O 7 ] through the loss of the lattice oxygen and hydroxyl groups following heat treatment at 500–800 °C [5,6]. During the most active period of cement hydration, from 3 h to 24 h, cement reacts to form C–S–H, 1 calcium hydroxide, and ettringite, yet metakaolin is relatively inert [7]. The amorphous metakaolin pozzolanically reacts with the Ca(OH) 2 formed from cement hydration forming (i) C–S–H gel, (ii) crystalline calcium alu- minate hydrates [C 4 AH 13 ,C 3 AH 6 ], and (iii) crystalline calcium alumi- nosilicate [C 2 ASH 8 ] [7–10]. Generally, metakaolin has a positive effect on concrete strength following 48 h of hydration, increasing strength relative to neat cement systems due to the pozzolanic reac- tivity [8]. It was of interest in this study to improve the reactivity of metakaolin-cement mixtures. Influencing the reactivity of an SCM-cement blend can be achieved by either (1) increasing early ce- ment hydration to compensate for the dilution effect of the SCM and/or (2) enhancing the pozzolanic reactivity of the SCM. The idea in this case was to enhance the reactivity of metakaolin-cement blends to enable the use of calcined impure, kaolin-containing clays as SCMs. Calcined impure clays have a lower reactivity compared to pure metakaolin [6,9], however they are desirable as SCMs due to higher availability and lower cost compared to pure kaolinite. Enabling the use of impure clays, by compensating for the lower reactivity may facilitate their use as SCMs. Some limited research has been conducted on altering kaolinite mineralogical properties, in turn potentially changing the reactiv- ity of the SCM [11–14]. San Cristóbal et al. [11] pretreated kaolinite with 6 M HCl at 90 °C for 3 h, followed by centrifugation at 10,000 rpm, and then calcination; they found that the acid substantially altered the physico-chemical, mineralogical and mor- phological properties of the metakaolin. However, San Cristóbal et al. [11] did not test the effects of this treated kaolinite on pozzo- lanic reactivity. Similarly, Lenarda et al. [12] treated kaolinite with 1MH 2 SO 4 at 90 °C, followed by calcination at 850 °C, resulting in metakaolins with a high surface area and good catalytic properties. Kuechler [13] used cone fusion to demonstrate that the addition of titanium dioxide and iron oxide prior to calcination lowered the refractoriness of calcined kaolinite (i.e. thermal resistance). Chatterji et al. [14] demonstrated that the addition of 0.001 wt%, 0958-9465/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.cemconcomp.2012.03.004 ⇑ Corresponding author. Tel.: +1 310 499 3924; fax: +1 512 471 4555. E-mail address: staylorla@gmail.com (S.C. Taylor-Lange). 1 Cement chemistry notation is used throughout, whereby single letters represent oxides: A = Al 2 O 3 , C = CaO, F = Fe 2 O 3 , H=H 2 O, S = SiO 2 , and S ˆ = SO 3 . The dashes in ‘‘C–S–H’’ indicate variable stoichiometry. Cement & Concrete Composites 34 (2012) 835–847 Contents lists available at SciVerse ScienceDirect Cement & Concrete Composites journal homepage: www.elsevier.com/locate/cemconcomp