Self-Generating Zeolite-Cement Composites
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SELF-GENERATING ZEOLITE-CEMENT COMPOSITES
JUDITH L. LAROSA, STEPHEN KWAN, MICHAEL W. GRUTZECK * Materials Research Laboratory, The Pennsylvania State University, University Park, PA 16802
ABSTRACT Zeolites can be synthesized in mixtures containing 80 wt% Class F fly ash and 20 wt% ordinary Portland cement if they are mixed with a concentrated NaOH solution and cured at temperatures of 60-90'C. Zeolite Y and NaP-type zeolite were grown in situ in a coexisting calcium silicate hydrate matrix. Those samples made with NaOH, which contained the zeolites, had
higher compressive and flexural strengths than equivalent samples made with water.
INTRODUCTION Previous work by the authors [1] has shown that zeolites, namely NaP-type, Zeolite Y, and chabazite, readily nucleate and grow in systems containing 80 wt% Class F fly ash and 20wt% Type I ordinary Portland cement (OPC) which have been mixed with concentrated NaOH solution and cured at room temperature (RT) for two days followed by six days at a temperature in the range RT-90°C. The mechanical properties of these so called self-generating zeolite-cement composites are currently under investigation. 4 3 Zeolites are framework silicates in which A1 + are present on Si + sites. Charge is 2 compensated by exchangeable cations such as Na+, K+, Ca2+, and Mg +. The framework is rigid and very open, consisting of channels and cavities which are able to accomodate both the cations and water molecules. Traditionally, zeolites have been utilized as pozzolans, i.e., highly reactive silicates blended with cement to reduce free lime content and to aid in the control of the alkali-silica reaction. Compared to nonzeolitic pozzolans, zeolites can have a higher pozzolanic activity. In addition, zeolitic pozzolans can produce higher compressive strengths in cement as compared to OPC and nonzeolitic-pozzolan cements [2,3]. However, in the current study, the zeolites play a very different role from that of a pozzolan. The purpose here is to promote their growth within a calcium silicate hydrate (C-S-H) matrix in order to develop a particulate-composite with zeolites as the particulates. In general, an alkaline aqueous medium is required to synthesize zeolites. Some of the reaction variables include: temperature, composition, pressure, pretreatment of reactants, low-temperature aging of reaction mixture, pH, seeding, and time[41. The starting materials can be pure oxide/hydroxide end members, silicate minerals such as clays, natural glasses such as volcanic glass, or synthetic glasses [5-81. For example, in an investigation by Yoshida and Inoue[71, zeolites were synthesized from ground Shirasu volcanic glass. An extensive study of zeolite formation from basaltic glass, rhyolitic glass, nepheline, and oligoclase was performed by Barth-Wirsching and Holler[8]. They established a correlation between the type of zeolite formed and the experimental conditions including starting material composition, composition and concentration of solution, pH, and curing temperature. In another study investig
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