Role of sodium hexametaphosphate in MgO/SiO₂ cement pastes

The extent of reaction between magnesium oxide (MgO) and silica fume (SiO₂) is normally limited and mixes require high water contents to give suitable rheology. The use of considerably lower water contents and the formation of magnesium silicate hydrate (M-S-H) gel as a binding phase is made possible by adding sodium hexametaphosphate (Na-HMP) to the mix water prior to the addition of MgO and SiO₂. This results in the formation of extensive reaction products and cured samples with high compressive strength and low porosity. In this work, the effect of Na-HMP on the hydration of MgO/SiO₂ mixes is investigated using high water to solids ratio samples to allow monitoring of pH and the solution chemistry during hydration. It is shown that a relatively small amount of Na-HMP inhibits the formation of Mg(OH)₂ when MgO is hydrolyzed. It is proposed that this is due to adsorption of phosphate species on the MgO which inhibits the nucleation of the Mg(OH)₂. This gives rise to high Mg²+ species in solution and elevated pH (> 12) conditions relative to when Mg(OH)₂ forms. In contrast, the phosphate does not suppress formation of M-S-H gel. In combination with the enhanced dissolution rate of SiO₂ at high pH, M-S-H gel can form quickly without competition for Mg²+ ions by Mg(OH)₂ precipitation. Incorporating the optimum concentration of Na-HMP into the mix water therefore transforms the properties of cement paste and mortar samples formed by reacting MgO and SiO₂.

• Record URL:
  • https://doi.org/10.1016/j.cemconres.2016.08.003
• Record URL:
  • http://www.sciencedirect.com/science/article/pii/S0008884616302484
• Availability:
  • Find a library where document is available. Order URL: http://worldcat.org/issn/00088846
• Supplemental Notes:
  • Abstract reprinted with permission from Elsevier.
• Authors:
  • Jia, Yuan
  • Wang, Baomin
  • Wu, Zhenlin
  • Han, Junnan
  • Zhang, Tingting
  • Vandeperre, Luc J
  • Cheeseman, Chris R
• Publication Date: 2016-11

Language

• English

Media Info

• Media Type: Web
• Features: Figures; References; Tables;
• Pagination: pp 63-71
• Serial:
  • Cement and Concrete Research
  • Volume: 89
  • Publisher: Elsevier
  • ISSN: 0008-8846
  • Serial URL: http://www.sciencedirect.com/science/journal/00088846

Subject/Index Terms

• TRT Terms: Cement paste; Hydration; Moisture content; pH value; Silica fume
• Uncontrolled Terms: Magnesium oxide; Magnesium silicate hydrates; Reaction products; Sodium hexametaphosphate
• Subject Areas: Highways; Materials;

Filing Info

• Accession Number: 01617742
• Record Type: Publication
• Files: TRIS
• Created Date: Nov 26 2016 3:50PM