Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack

Six replicates per series were used to determine the compressive strengths. Testing was conducted using an Ibertest Autest 200/10-SW (Madrid, Spain) test frame, the average and standard deviation of the results were then calculated and reported. Prior to microstructural characterization (Mercury Intrusion Porosimetry (MIP) and Scanning Electron Microscopy with Energy Dispersive X-ray Analysis (BSEM/EDX), one of the specimens, left unbroken, is submitted to immersion in isopropanol for 2 days to stop further hydration reactions. Subsequently, they were dried in a desiccator for a minimum of 48 h to eliminate any residual isopropanol. To prepare the powder samples (for the mineralogical analysis), specimen fragments were ground to pass through a 63 μm sieve as per previous recommendations to stop the reaction processes. Subsequently, they were mixed with isopropanol for 3 min, filtered and placed in a vacuum desiccator until a constant weight was attained. Changes in the pore structure were evaluated by mercury intrusion porosimetry (MIP) on a Micromeritics Poresize 9320 IV.09 mercury intrusion porosimeter (Micromeritics Instrument Corporation, Norcross, GA, USA), assuming a sample–mercury contact angle of 140◦. The microstructure of the samples was studied by Backscattering Electron Microscopy (BSEM) on a JEOL JSM6400 scanning electron microscope (Tokyo, Japan). Additionally, a semi-quantitative analysis of the chemical composition of the reaction products was conducted via Energy Dispersive X-ray spectroscopy (EDX) on a Links ISIS EDX analyzer, collecting at least 40 points from the cementitious matrix per sample; the data were processed using the Bruker ESPRIT 1.9 software, whereas the mineralogical and chemical composition of samples were studied via X-ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR) and Thermogravimetric analysis (TG/DTG). XRD measurements were carried out on a Bruker D8 Advance diffractometer (Karlsruhe, Germany) in a 2θ range of 5–60◦ with a step size of 0.02◦ every 0.5 s using CuKα radiation at 40 kV and 30 mA. The existing phases were identified and quantified using the DiffracPlus EVA 4 2.1 and TOPAS 5.0 software, in conjunction with a chemical reconciliation. A Nicolet 6700 spectrometer (Thermo Fisher Scientific, Waltham, MA, USA, 02451) was employed for FTIR analysis, covering a range of 400 cm−1 to 4000 cm−1 with a resolution of 4 cm−1, using powdered samples embedded in KBr pellets (0.001 g sample/ 0.099 g KBr). Thermogravimetric analysis (TGA) was conducted using a Perkin-Elmer TG analyzer (PerkinElmer, Shelton, USA). Powdered samples underwent heating from room temperature to 800 ◦C at a rate of 10 ◦C min−1 under a N2 flow of 200 cm3/min.