Hydration of sodium phosphate-modified high alumina cement
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High strength can be achieved in high alumina cement (HAC) through the incorporation of phosphate-based additions at levels of 10 and 20 wt. %. In order to establish the mechanism that results in higher strength, the effects of a variety of condensed sodium phosphates (NaPO 3 ) n , (NaPO 3 ) n • Na 2 O, Na 5 P 3 O 10 , and (NaPO 3 ) 3 were studied. The influence of these additions on the kinetics of hydration was studied using isothermal calorimetry. The phosphatic additions enhanced reactivity, but x-ray diffraction analyses did not reveal evidence of new crystalline phosphate-containing hydration products. Microstructural evolution was examined in real time using environmental SEM, and hydration products exhibiting distinct morphologies were observed. The features exhibited ranged from amorphic to polygonal shapes, plates, and fibers. These frequently formed between crystalline calcium aluminate hydrate grains and by doing so appear to provide a means to enhance the strengths of these cements. In spite of the morphological variations, companion energy dispersive x-ray analysis showed that the compositions of these products did not vary widely. Their ranges of compositions are 52-60 wt. % AI2O3, 20-26 wt. % P 2 O 5 , and 20-24 wt. % CaO.
I. INTRODUCTION It is generally recognized that conventional cements exhibit useful compressive strengths but low tensile and flexural strengths.1 The mechanical properties of cementbased materials are controlled by the compositions of starting materials, the manner in which these are mixed with an aqueous media, the chemical bonding of the reaction products, and the microstructures of the final assemblage. Strategies to improve the mechanical properties of cement-based materials have been addressed by a large number of studies.2 A wide range of compositions have been studied in the development of phosphate-bonded materials.3"6 Phosphates have a long history of use as binding agents in ceramic systems.7 Their reactions have resulted in the formation of materials which have been used in such diverse applications as alumina refractories,8 bioceramics,9 and concrete repair.10 The [PO4] group provides a basic building block that results in a series of compounds that are a remarkable series of analogues to silicate compounds formed on the basis of the [SiO4] building block. Various phosphate-based cements, where HAC was mixed with phosphoric acid and cured at elevated temperature, were studied by Roy.11 Tensile splitting strengths reached 27 MPa after 1 day of moist curing at 90 °C. The chemical reactions at room temperature between HAC and NH 4 H 2 PO 4 were studied by Sugama and Carciello.12 They found that ammonium calcium pyrophosphate is the principal product responsible for the development of strength. Hydrothermal treatment at J. Mater. Res., Vol. 9, No. 5, May 1994
200 °C led to the formation of hydroxyapatite as the major phase and anorthite as the minor one and contributed to the development of compressive strengths in excess of 70 MPa. We previously determined the flexural
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