Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack
Abstract
:1. Introduction
2. Experimental
2.1. Materials
2.2. Preparation of Magnesium Phosphate Cements MKPC Pastes
- -
- CC: In a climatic chamber at 21 ± 3 °C and 99 ± 5% relative humidity (RH).
- -
- LAB: In the laboratory at 21 ± 3 °C and 52 ± 5% of relative humidity (RH).
2.3. Experimental Methods
3. Results and Discussion
3.1. Mechanical Strengths
3.2. Changes in the Porosity after the Chemical Attack
3.3. Mineralogical Analysis
3.4. Microstructural Analysis
- (I)
- A cluster of compositions close to the theoretical K-struvite (P/Mg = 1 and K/Mg = 1), showing higher P/Mg ratios and possibly higher K/Mg ratios. These compositions are observed in the reference samples and those immersed in solution for 28 days. According to the literature [30], the higher P/Mg ratio (with respect to the theoretical P/Mg = 1) observed in the reference and immersed samples at 28 days could be explained by several reasons: (i) The initial formulation of the system has a higher value than the theoretical P/Mg ratio of 1. (ii) Phosphate anions could be adsorbed on the crystal surface disguising the EDX analysis, or (iii) other phosphates formed with the impurities originally present in the unreacted magnesia. Notably, after one year of immersion, these compositions shits towards a lower K/Mg ratio, particularly in samples immersed in seawater, suggesting a partial dissolution of this phase (which could be associated with a certain amorphization process observed in the BSEM). This suggests that the compositions close to the theoretical K-struvite are more stable in the short term (28 days) but tend to disappear after long-term immersion (1 year), particularly in seawater environments.
- (II)
- A cluster close to the theoretical hazenite (NaKMg2(PO4)2·14H2O). This cluster is observed in samples immersed in seawater and sodium sulfate solutions, but with lower P/Mg molar ratios than the theoretical value. This suggests that these samples tend to form hazenite-like phases under these conditions, though with a slightly altered stoichiometry.
- (III)
- A cluster close to the chemical composition of cattiite (Mg3(PO4)2·22H2O): This cluster is found in samples immersed in deionized water, especially at 1 year. However, these compositions exhibit a higher K/Mg molar ratio, implying that some K+ ions could be adsorbed on the surface, which results in the detection of a small proportion of potassium in the EDX analysis.
- (IV)
- A cluster with a chemical composition close to MgO and brucite Mg(OH)2. With respect to this cluster, we have detected the presence of brucite in small proportion, especially in samples after 1 year of immersion (in the three leaching media) as confirmed by FTIR and TG/DTG analyses. These data points could represent this phase, but with phosphate adsorbed on the surface of magnesium oxide and could affect the surface chemistry and detection in analytical techniques like EDX, leading to a slight deviation in the expected stoichiometry. This phenomenon has also been observed by other researchers [30].
4. Conclusions
- The M/P ratio and the curing conditions affects the microstructure of the cementitious systems and therefore their durability.
- Chemical attacks in water, Na2SO4 solution and seawater negatively affect the mechanical strengths and increase the total porosity of MKPC paste specimens; however, compressive strengths remain above 40 MPa and no significant damage was observed. The system with an M/P molar ratio of 3 demonstrated superior mechanical strengths.
- Regardless of the type of the immersion solution, the main reaction product was K-struvite. Unreacted particles (periclase MgO, magnesite, quartz and dolomite) do not seem to be altered by the chemical solutions after 1 year of immersion.
- After immersion in the three solution media, the paste specimens exhibited slight amorphization of the K-struvite crystals, which is more evident after 1 year of attack (suggesting their partial instability over long-term immersion). The partial dissolution of the K-struvite, together with the presence of ionic species in the medium favor to the formation of secondary reaction products such as hazenite and cattiite.
- The presence of brucite was observed, especially after 1 year of immersion; however, this phase appears in very small proportions suggesting a low degree of hydration of the unreacted MgO particles (likely to be adsorbed on the surface of magnesium oxide).
Author Contributions
Funding
Institutional Review Board Statement
Informed Consent Statement
Data Availability Statement
Acknowledgments
Conflicts of Interest
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L-MgO | Periclase (MgO) | Magnesite (MgCO3) | Dolomite (CaCO3) | Brucite (Mg(OH)2) | Quartz (SiO2) | Anhidrite (CaSO4) | Calcite (CaCO3) | RWp |
---|---|---|---|---|---|---|---|---|
(%) | 57.62 | 25.77 | 5.41 | 3.12 | 1.96 | 3.97 | 0.70 | 7.62 |
Compound * | Concentration (g/L) |
---|---|
NaCl | 24.53 |
MgCl2 | 5.2 |
Na2SO4 | 4.09 |
CaCl2 | 1.16 |
KCl | 0.69 |
NaHCO3 | 0.201 |
KBr | 0.101 |
H2BO3 | 0.027 |
SrCl2 | 0.025 |
NaF | 0.003 |
Mass Loss (%) | ||||
---|---|---|---|---|
Leaching Solutions | Interval 30–200 °C (1) | Interval from 300 to 380 °C (2) | Total Loss | |
Reference | 23.35 | 0.8 | 35.67 | |
MKPC 28 days | H2O | 23.32 | 1.13 | 35.84 |
Na2SO4 | 23.88 | 1.18 | 36.34 | |
Seawater | 23.09 | 0.93 | 36.01 | |
MKPC 1 year | H2O | 24.17 | 1.45 | 36.27 |
Na2SO4 | 24.69 | 1.3 | 36.87 | |
Seawater | 23.11 | 2.0 | 36.58 |
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Chhaiba, S.; Martinez-Sanchez, S.; Husillos-Rodriguez, N.; Palomo, Á.; Kinoshita, H.; Garcia-Lodeiro, I. Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack. Materials 2024, 17, 4252. https://doi.org/10.3390/ma17174252
Chhaiba S, Martinez-Sanchez S, Husillos-Rodriguez N, Palomo Á, Kinoshita H, Garcia-Lodeiro I. Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack. Materials. 2024; 17(17):4252. https://doi.org/10.3390/ma17174252
Chicago/Turabian StyleChhaiba, Salma, Sergio Martinez-Sanchez, Nuria Husillos-Rodriguez, Ángel Palomo, Hajime Kinoshita, and Inés Garcia-Lodeiro. 2024. "Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack" Materials 17, no. 17: 4252. https://doi.org/10.3390/ma17174252
APA StyleChhaiba, S., Martinez-Sanchez, S., Husillos-Rodriguez, N., Palomo, Á., Kinoshita, H., & Garcia-Lodeiro, I. (2024). Durability of Magnesium Potassium Phosphate Cements (MKPCs) under Chemical Attack. Materials, 17(17), 4252. https://doi.org/10.3390/ma17174252