Cross-Linking Reactions in Macro-Defect Free Cement Composites
- PDF / 302,893 Bytes
- 6 Pages / 420.48 x 639 pts Page_size
- 8 Downloads / 173 Views
CROSS-LINKING REACTIONS INMACRO-DEFECT FREE CEMENT COMPOSITES P. G. DESAI, J. F. YOUNG and R. P. WOOL Center for Cement Composite Materials, University of Illinois at Urbana-Champaign. ABSTRACT: The key to the processing and properties of Macro-Defect Free (MDF) cement composites is the interaction between the cement and the polymer. In calcium aluminate (CA) / polyvinyl alcohol (PVA) MDF composites, it has been hypothesized that aluminate (AI(OH) 4") ions released from the cement cross-link PVA in a manner similar to the well known interaction of the borate (B(OH) 4") ions with PVA. In this investigation, stable aluminate/PVA solutions at different concentrations have been studied. Results indicate that the nature of interaction of aluminate ions is not similar to that of borate ions indilute and semidilute polymer solutions. 1. INTRODUCTION: Macro-Defect Free (MDF) cement composites are made by combining water with a hydraulic cement and a water soluble polymer followed by high shear mixing and curing/drying of the product [1,2). Although many hydraulic cement/watersoluble polymer combinations could be used to make MDF cement composites, a calcium aluminate (CA) cement / partially hydrolyzed (80%) polyvinyl alcohol (PVA) MDF cement composite has attracted much attention because of superior mechanical properties [2,3]. Although an important function of the polymer is to assist processing with low water/cement ratio, the polymer is also believed to interact chemically with the hydrating cement to improve mechanical properties. There have been two hypotheses on the nature of chemical interaction: (1) Aluminate (AI(OH) 4 ) ions released by CA cement cross-link the hydroxyl groups of PVA. (2) AI(OH) 3 precipitating from supersaturated solution forms strong coordinate bonds with the hydroxyl groups of PVA. Hypothesis (1) was first suggested by Groves and coworkers [4], and later supported by others [5,6]. Supporting evidence was based on solution chemistry, calorimetry, microscopy and microanalysis of the CA-PVA-water system. Hypothesis (2) was suggested by Birchall and coworkers [3]. The evidence cited [1,4,5,6] could be used to support both the hypotheses. Interactions of the Borate Ion with PVA: Since the aluminate ion is isostructural with the tetrahedral borate (B(OH)4") ion, it has been proposed [4] that it may interact in a similar fashion. It is well known that borate ions form ionic cross-links with hydroxyl groups of PVA [7, 8]. Therefore, if the analogy is appropriate, effect of aluminate and borate ions in presence of PVA should be similar. Previous studies [8-10] on the effect of complexing ions on PVA solutions have shown concentration dependence. In dilute solutions, the polymer chains do not overlap, and, intramolecular complexing predominates (see fig. 1)[9,10]. In more concentrated solutions where the polymer chains do overlap, intermolecular complexing predominates [8] as shown in fig. 2.
Mat. Res. Soc. Symp. Proc. Vol. 245. ©1992 Materials Research Society
180
1AM
Fig. 1 Schematic profile of in
Data Loading...
