Polyphosphate Chain Stability in Magnesia-Polyphosphate Cements
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POLYPHOSPHATE CHAIN STABILITY IN MAGNESIA-POLYPHOSPHATE CEMENTS E. D. DIMOTAKIS, W. G. KLEMPERER AND J. F. YOUNG Departments of Chemistry, Civil Engineering and Materials Science and Engineering, University of Illinois at Urbana-Champaign, Urbana, IL 61801. ABSTRACT Polyphosphates of the general formula NanH2PnO3n+l.H20, n = 6, 15, 70, where n is the average degree of polymerization, have been synthesized and characterized by 31p NMR and HPLC. Aqueous solutions of these polyphosphates react at room temperature with magnesia to form cement pastes that harden to amorphous materials. The compressive strengths and porosities obtained are similar to those of MgO-(NH4)H2P3010-H20 cements. Although polyphosphate chain length does not have a significant effect on the strength of the cement, 31p MAS-NMR spectroscopy showed that the degree of polyphosphate chain degradation decreases as the average degree of polymerization, n, increases. INTRODUCTION Recent studies of magnesia-tripolyphosphate cements prepared from MgO(NH4)3H2P3010-H20 show that these materials have greater compressive strength than magnesia-orthophosphate cements prepared from MgO-(NH4)2HP04-H20, namely 13,200 vs. 3,300 psi [1,2]. Their superior mechanical properties are not molecular in origin, however, but arise from lower porosities. In fact, 31p MAS NMR studies showed that triphosphate chains were largely degraded under normal processing conditions. The present investigation was undertaken in order to determine whether long chain polyphosphates show greater chemical stability than the short chain tripolyphosphates previously examined. To this end, the MgO-NanH2Pn-03n+l-H20 system was examined, with average degrees of polymerization n = 6, 15, and 70. Sodium salts were selected to obtain higher solubility and also to avoid the ammonia release observed in MgO-(NH4)nH2PnO3n+lH20 systems. EXPERIMENTAL Sodium polyphosphate mixtures Nan+2PnO3n+l.xH20 with average degrees of polymerization n = 6, 15, and 70 were prepared from stoichiometric quantities of Na2HPO4.7H 2 0 and NaH2PO4-H20 using van Wazer's method [3]. These were converted to protonated NanH2PnO3n+l salts using procedures described in ref [2], but using more concentrated solutions (12 g phosphate in 40 mL water), a macroporous ion exchange resin (Dowex MSC-1), and back titration with NaOH to pH 3. Solid materials were isolated by precipitation with ethanol, and purified Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
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by washing with anhydrous ethanol. Calcd (%) for Na6H 2P60 19 .5.7H20: Na 18.83, H 1.84, P 25.37. Found (%): Na 18.97, H 1.99, P 25.25. Calcd (%) for Na15H2P15O46.13H20: Na 19.36, H 1.58, P 26.08. Found (%): Na 19.50, H 1.49, P 25.98. Calcd (%) for Na7oH2P700211.105H20: Na 17.79, H 2.36, P 23.96. Found (%): Na 17.96, H 2.69, P 23.90. Cements pastes were prepared by adding saturated NanH2PnO3n+l solutions to MgO powder (Martin Marietta 10CR). In each case, about 2.3 g of each polyphosphate were dissolved in 0.9 mL of water. This solution was added to MgO (7.
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