Evolution of phase assemblage of blended magnesium potassium phosphate cement binders at 200° and 1000°C

Abstract

The fire performance of magnesium potassium phosphate cement (MKPC) binders blended with fly ash (FA) and ground granulated blast furnace slag (GBFS) was investigated up to 1000 °C using XRD, TGA, and SEM techniques. The FA/MKPC and GBFS/MKPC binders dehydrate above 200 °C to form amorphous KMgPO 4 , concurrent with volumetric and mass changes. Above 1000 °C, additional crystalline phases were formed and microstructural changes occurred, although no cracking or spalling of the samples was observed. These results indicate that FA/MKPC and GBFS/MKPC binders are expected to have satisfactory fire performance under the fire scenario conditions relevant to the operation of a UK or other geological disposal facility.

Influence of magnesium-to-phosphate ratio and water-to-cement ratio on hydration and properties of magnesium potassium phosphate cements

“…Magnesium phosphate cements (MPCs) are clinker-free cements, which harden through acidbase reaction between calcined magnesia (MgO) and phosphate acid or soluble acid phosphates in the presence of water [1]. Diammonium phosphate ((NH4)2HPO4) [2,3], monoammonium phosphate (NH4H2PO4) [3][4][5][6][7][8], monopotassium phosphate (KH2PO4) [1,3,[9][10][11][12][13][14][15][16][17][18][19][20][21] and monosodium phosphate (NaH2PO4) [22][23][24] are the typical acid phosphates for making MPCs. Compared with other acid phosphates, KH2PO4 has two prominent advantages that make the industrial application of magnesium potassium phosphate (MKP) cements more favorable.…”

“…(1), Mg/PO4 molar ratio equal to 1 is adequate to generate K-struvite, and is expected to lead to good material performance. However, an excess of magnesia over KH2PO4 has been proved to be more beneficial in practice [9,14,15,18,19,21]. High Mg/PO4 molar ratios (≥ 4) have been frequently used for preparing MKP cementitious materials aiming for rehabilitations [9,18,21].…”

“…preferably used for waste immobilizations due to the low pH conditions [14,15]. Excessive unreacted magnesia in MKP cementitious materials, especially those with high Mg/PO4 molar ratios, normally raises the concern over brucite formation and the consequent detrimental volume expansion [12].…”

Thermodynamic data for magnesium (potassium) phosphates

“…Mg/PO 4 molar ratio and w/c ratio are the two important factors controlling pH of the cement system, reaction path (Chau et al, 2012;Lahalle et al, 2016;Le Rouzic et al, 2017a;Xu et al, 2018a), microstructure (Le Rouzic et al, 2017b;Ma et al, 2014a;Ma et al, 2014b;Xu et al, 2015), and thus performance (Le Rouzic et al, 2017b;Lee et al, 2017;Ma et al, 2014b;Qiao et al, 2010;Xu et al, 2017;Xu et al, 2015). A low Mg/PO 4 molar ratio (<4) MKP cement is preferably used as waste solidification agent (Buj et al, 2009;Buj et al, 2010;Gardner et al, 2015a;Gardner et al, 2015b;Lahalle et al, 2016;Le Rouzic et al, 2017b;Walling and Provis, 2016), as a lower Mg/PO 4 molar ratios decrease pH values, which is favourable for immobilizing metallic wastes (Coumes et al, 2014;Covill et al, 2011;Gardner et al, 2015b;Walling and Provis, 2016) such as aluminium, magnesium, and uranium containing nuclear wastes. However, some MKP cement-based materials with low Mg/PO 4 molar ratio showed problems such as efflorescence, strength decrease at later ages and expansion due to the formation of intermediate phases (Chau et al, 2012;Le Rouzic et al, 2017a;Le Rouzic et al, 2017b;Xu et al, 2019).…”

Response to the discussion by Hongyan Ma and Ying Li of the paper “Characterization of magnesium potassium phosphate cement blended with fly ash and ground granulated blast furnace slag”

“…In the case of emplacement in 500 L stainless steel drums, as is common in the UK, the strength required for storage/disposal will be supplied by the stillage (overpack that contains four drums), with no load to be directly placed onto the waste packages [13]. Therefore, the mechanical properties of the blended MKPC binder was not the dominant factor in designing our formulations [1,14].…”