Effect of Sodium Hexametaphosphate and Trisodium Phosphate on Dispersion of Polycarboxylate Superplasticizer

Abstract

Enhancement in dispersion of polycarboxylate superplasticizer (PCE) could be obtained by incorporating retarders in normal concrete. The generally believed reason was that the consumption of free water and polymer at the beginning was reduced by retarding cement hydration. This theory could not convincingly explain why sodium hexametaphosphate (SHMP) was able to promote the dispersion capacity of PCE, while trisodium phosphate (TSP) could not, despite that both TSP and SHMP could obviously retard the cement hydration. The adsorption behavior of PCE and phosphate was investigated and the mechanism was analyzed in order to gain deeper understanding. The results showed that TSP and SHMP delayed the cement hydration, impeded adsorption process of PCE, and increased thickness of adsorption layer. It was interesting that TSP reduced the dispersion, but SHMP enhanced. The reason for this contradiction was due to the difference in composition of adsorption layer. In the PCE-TSP system, this layer was composed of the precipitates (formed by TSP and Ca 2+ ) and the invalided PCE (caused by these precipitates in the immediate vicinity of the cement grains); the invalided PCE was due to the decrease of PCE dispersion. In the PCE-SHMP system, "Inner-phosphate (multi-layers) + Outer-PCE (single layer)" structure was formed to make the PCE work more effective, hence enhancing the dispersion. These results were expected to be useful for the design of highly efficient dispersants. dispersion capacity [26][27][28][29][30]. Based on this, the retarder might hinder the adsorption process of PCE and it was referred to as competitive adsorption [31][32][33][34][35], thereby reducing the dispersion. In addition, the thickness of the adsorption layer should also be accepted as a factor influencing the dispersion of PCE [36,37]. In spite of the decline in adsorption amount of PCE in the presence of retarder, as reported in previous study, the adsorption layer was obviously thickened to enhance the dispersion [37]. On the basis of discussion above, the conclusion could be made that the dispersion capacity of the PCE-retarder system should be associated with the retarding effect of retarder, adsorption behavior of PCE, and the thickness of the adsorption layer.Poly-phosphates, such as sodium tripolyphosphate and sodium hexametaphosphate (SHMP), have been widely used to promote the dispersion ability and dispersion retention ability of the superplasticizer system in real engineering practice. However, mono-phosphates, such as trisodium phosphate (TSP) and sodium hydrogen phosphate, cannot play the same roles as poly-phosphate in commercially available PCE products. If the retardation was the main reason for the enhancement, poly-phosphate and mono-phosphate would both have a similar effect on dispersion of PCE system, because both TPS and SHMP could retard the cement hydration [38]. Therefore, the reason why the poly-phosphate could effectively enhance the dispersion, whilst the mono-phosphate failed cannot be simply explained by the retard...

Transport properties of moisture and ionic chelators in concrete

“…SHMP is a commonly used admixture in cementitious materials. SHMP acts as a retarder (Zhang et al, 2019), dispersant (Jia et al, 2016) and other agents in concrete (Bhattacherjee et al, 2022), and it is a chelating agent that forms chelates with cations in concrete. The chelating ions in EDTA-2Na were chosen as the influencing factor for the internal transport of SHMP in concrete.…”