Effects of phosphoric acid and phosphates on magnesium oxysulfate cement

Advances in Cement Research

2016

High-strength modified magnesium oxysulfate cement is prepared using light-burned magnesia and industrial waste sulfuric acid resulting from titanium dioxide production. Amino trimethylene phosphonic acid (ATMP) is used as a modifier. Coal fly ash is added as a filler to the magnesium oxysulfate cement in order to further reduce its production cost. The compressive strength, setting time, phase compositions, micro-morphology and volume stability of the modified magnesium oxysulfate cement are comprehensively analysed. The results indicate that the compressive strength of the modified magnesium oxysulfate cement reaches 82·1 MPa when the molar ratio of the active magnesium oxide/sulfuric acid (MgO/H 2 SO 4 ) (M) is 6, the water/cement ratio (R) is 0·6 and the ATMP and coal fly ash dosages are 0·2% and 50%, respectively. Low ATMP doses markedly affect the microstructure, hydration and shrinkage of the magnesium oxysulfate cement. X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy analyses prove that the needle-like magnesium hydroxide sulfate hydrate5Mg(OH) 2 ·MgSO 4 ·7H 2 O crystal (5·1·7 phase) -is the predominant strength phase in this cement system.

Section: The Effect Of Atmp On Mos Cementsupporting

confidence: 89%

Preparation and properties of modified magnesium oxysulfate cement derived from waste sulfuric acid

Advances in Cement Research

2016

“…In our previous study, not the 3•1•8 phase, but Mg(OH) 2 was found in the hydrated MOS cement with the mole ratio of active MgO to MgSO 4 being more than 3. In recent years, many researchers have shown that the use of additives such as citric acid (CA) (Wu et al 2014;Wang et al 2018 ), tartaric acid (Wu et al 2017), phosphoric acid ( Wu et al 2015( Wu et al , 2016, and their acid salts can improve the strength of MOS cement. By controlling the process of hydration of active MgO in magnesium sulfate solution, the addition of additives, such as CA, favored the formation of large amounts of the 5Mg(OH) 2 •MgSO 4 •7H 2 O (5•1•7 phase), which resulted in MOS cements with high strength and high volume stability.…”

Section: Introductionmentioning

confidence: 99%

Effects of 5•1•7 Phase Seed Crystal on Performance of Magnesium Oxysulfate Cement

Luo

et al. 2019

ACT

Self Cite

Magnesium oxysulfate (MOS) cement is lightweight, offers fire resistance, and has good aesthetics. Although the addition of citric acid improves the cement's late strength, the concomitant retardation of hydration of MOS by citric acid results in slow setting and low early strength. This may in return decrease the production efficiency of MOS cement products. In this study, 5•1•7 phase (5Mg(OH) 2 •MgSO 4 •7H 2 O) seed crystal (SC) is used as a modifier that works as a seed nucleus and accelerates the hydration of MOS cement in the early stages, in the absence or presence of citric acid. The effect of SC on compressive strength, setting time, and volume change is investigated in detail. X-ray diffraction, scanning electronic microscopy, hydration-heat release rate, and mercury intrusion porosimeter are performed to explain the influence mechanism of SC on the performance of the MOS cement. The results revealed that the in situ formation and growth of the new 5•1•7 phase crystals on the SC surface by the addition of SC can significantly accelerate the setting, improve the 12 h and 28 d compressive strengths, and reduce shrinkage of MOS cement without or with 0.05% citric acid.

“…Electrostatic attraction is also an important factor in Pi removal in this system (Yan et al, 2015). Phosphate ions (PO 4 3− , H 2 PO 4 − , and HPO 4 2− ) would be easily and quickly adsorbed onto the charged MgO/CaO surface particles (Liu et al, 2015; Mukherjee et al, 2011; Wu et al, 2015). In addition, Ca‐ and Mg‐based oxides possess high Pi affinity, and this was reflected in the steeper slope with the CS sample (Xia et al, 2016; Yao et al, 2011b).…”

Section: Discussionmentioning

confidence: 99%

Using Recycled Concrete as an Adsorbent to Remove Phosphate from Polluted Water

Tang²,

et al. 2019

J of Env Quality

Phosphate pollution remains a significant hazard to terrestrial and aquatic ecosystems. We developed an economical and efficient method for phosphate adsorption on waste construction concrete modified with seawater. Compared with raw concrete materials, the phosphate adsorption capacity of seawater‐modified waste concrete was highly efficient, especially at low phosphate concentrations. The inflection point for seawater‐modified concrete was 0.66 and 1.22 mg L−1 for the raw material. The relative phosphate adsorption was 4.64 and 2.39 mg g−1, respectively. Phosphate removal was >90% over a pH range of 3 to 11 for the raw and modified materials. Chemical and physical analysis of the modified concrete indicated that Ca and Mg particles were uniformly sequestrated on the surface, and Ca was the determinant controlling phosphate uptake. Phosphate adsorption isotherms fit well using the Freundlich, Temkin, Elovich, Fowler–Guggenheim, and Hill–de Boer models and indicated that intermolecular forces in the concrete particles were enhanced by calcium oxides from seawater. This method can efficiently remove phosphate from polluted water and repurposes waste construction concrete. Core Ideas Phosphate adsorption capacity of modified waste concrete was highly efficient at low phosphate. Phosphate removal was >90% over a pH range of 3 to 11 for the raw and modified materials. Calcium was the determinant controlling phosphate uptake for seawater‐modified concrete. Intermolecular forces in the concrete particles were enhanced by calcium oxides from seawater.