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, KMg 2 H(PO 4 ) 2 •15H 2 O and possibly MgHPO 4· 7H 2 O first precipitate, then decomposed to struvite-k. As sodium alginate was added, Na 2 HPO 4 •12H 2 O and KMg 2 H(PO 4 ) 2 •15H 2 O precipitated at the initial stage, and then destabilized afterwards. Sodium alginate could delay dissolution of KMg 2 H(PO 4 ) 2 •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 Na 2 HPO 4 •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.