Compressive strength and hydrate assemblages of wollastonite-blended magnesium potassium phosphate cements exposed to different pH conditions

Magnesium potassium phosphate (MKP) cements maybe used in various service conditions. The long-term behavior, compressive strength and hydrates formed under different environments was investigated using diluted nitric acid, water and simplified Portland cement pore solution. The main hydrate K-struvite in MKP cements is destabilized to bobierrite if exposed to solutions with pH ≤ 7, and to brucite and hazenite at pH > 7. The formation of additional K-struvite with time leads to cracks and strength loss. Addition of wollastonite to MKP cements slows down compressive strength development, but leads to comparable strengths at later ages, and prevents strength loss with time. Under acidic conditions the formations of calcium hydroxyapatite (or calcium pyrophosphate) and likely M-S-H are observed, while under basic conditions wollastonite accelerates K-struvite destabilization to brucite. The presence of wollastonite in MKP cements prevents cracking and improves the resistances to aggressive acid and alkaline solutions.