Investigation of sodium alginate as a candidate retarder of magnesium phosphate cement: Hydration properties and its retarding mechanism

This work investigated the effect of sodium alginate on the hydration properties of magnesium potassium phosphate cement (MKPC) suspension with a high w/c ratio, and explored its retarding mechanism. At w/c = 10, hydration of MKPC presented a mult-step process, specifically, KMg2H(PO4)(2)center dot 15H(2)O and possibly MgHPO4 center dot 7H(2)O first precipitate, then decomposed to struvite-k. As sodium alginate was added, Na2HPO4 center dot 12H(2)O and KMg2H(PO4)(2)center dot 15H(2)O precipitated at the initial stage, and then destabilized afterwards. Sodium alginate could delay dissolution of KMg2H(PO4)(2)center dot 15H(2)O as well as precipitation of struvite-k. Moreover, sodium alginate tends to transform to alginic acid at low pH, and left a large number of Na+ ions, leading to precipitation of Na2HPO4 center dot 12H(2)O. The alginic acid is difficult to dissolve at low pH, which limited its effect on extending the setting time of MKPC further.

Automated summary

This study investigated the impact of sodium alginate on the hydration properties of magnesium potassium phosphate cement suspension with a high water-to-cement ratio, and explored the mechanism behind its retarding effect. Sodium alginate was found to delay the dissolution and precipitation of key crystalline phases, leading to an extended setting time for the cement.