The Improvement Effects of NaH₂PO₄ and KH₂PO₄ on the Properties of Magnesium Oxysulfate Cement

Abstract

Sodium dihydrogen phosphate (NaH₂PO₄) and potassium dihydrogen phosphate (KH₂PO₄) were selected as additives for magnesium oxysulfate (MOS) cement. The phase composition and the microstructure of MOS cement were characterized using X-ray diffraction (XRD), thermogravimetric analysis (TG-DSC), Flourier transform infrared spectroscopy (FT-IR), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM). It is found that both NaH₂PO₄ and KH₂PO₄ lead to an increase in the compressive strength and an improvement in the volume stability of MOS cement. The XRD, MIP and SEM results show that the addition of NaH₂PO₄ or KH₂PO₄ does not change the phase composition of MOS cement but promotes the formation of strength phase of 5Mg(OH)₂·MgSO₄·7H₂O (5·1·7 phase). This phase brings a considerable improvement in the microstructure of MOS cement, which has a positive effect on the properties of MOS cement.

Keywords

magnesium oxysulfate cement / phosphates / compressive strength / volume stability / microstructure

References

[1] Demediuk T, Cole W F, Hueber H V. Studies on Magnesium and Calcium Oxychlorides[J]. Aust. J. Chem., 1955, 8(2): 215-233.

[2] Beaudoin J J, Ramachandran V S. Strength Development in Magnesium Oxysulfate Cement[J]. Cem. Concr. Res., 1978, 8(1): 103-112.

[3] Wu C Y, Yu H F, Dong J M, et al. Effects of Material Ratio, Fly Ash, and Citric Acid on Magnesium Oxysulfate Cement[J]. ACI Mater. J., 2014, 111(3): 291-297.

[4] Qin L, Gao X, Chen T F. Recycling of Raw Rice Husk to Manufacture Magnesium Oxysulfate Cement Based Lightweight Building Materials [J]. J. Clean. Prod., 2018, 191(8): 220-232.

[5] Kahle K. Mechanism of Formation of Magnesium-sulfate Cements[J]. Silikatechnik, 1972, 23(5): 148-151.

[6] Walling S A, Provis J L. Magnesium-based Cements: A Journey of 150 Years, and Cements for the Future?[J]. Chem. Rev., 2016, 116(7): 4 170-4 204.

[7] Demediuk T, Cole WF. A Study of Magnesium Oxysulfates[J]. Aust. J. Chem., 1957, 10(3): 287-294.

[8] Tan Y N, Liu Y, Grover L. Effect of Phosphoric Acid on the Properties of Magnesium Oxychloride Cement as A Biomaterial[J]. Cem. Concr. Res., 2014, 56(2): 69-74.

[9] Deng D H. The Mechanism for Soluble Phosphates to Improve the Water Resistance of Magnesium Oxychloride Cement[J]. Cem. Concr. Res., 2003, 33(9): 1 311-1 317.

[10] Urwong L, Sorrell C A. Phase Relations in Magnesium Oxysulfate Cements[J]. J. Am. Ceram. Soc., 1980, 63(9–10): 523-526.

[11] Dinnebier E R, Pannach M, Freyer D. 3Mg(OH)₂·MgSO₄·8H₂O: A Metastable Phase in the System Mg(OH)₂-MgSO₄-H₂O[J]. Zeitschrift für Anorganische und Allgemeine Chemie, 2013, 639(10): 1 827-1 833.

[12] Wu C Y, Zhang H F, Yu H F. Preparation and Properties of Modified Magnesium Oxysulfate Cement Derived from Waste Sulfuric Acid[J]. Adv. Cem. Res., 2015, 28(3): 1-10.

[13] Runcevski T, Wu CY, Yu HF, et al. Structural Characterization of A New Magnesium Oxysulfate Hydrate Cement Phase and Its Surface Reactions with Atmospheric Carbon Dioxide[J]. J. Am. Ceram. Soc., 2013, 96(11): 3 609-3 616.

[14] Guo T, Wang H F, Yang H J, et al. The Mechanical Strength Properties of Magnesium Oxysulfate Cement Enhanced with 517 Phase Magnesium Oxysulfate Whiskers[J]. Constr. Build. Mater., 2017, 150: 844-850.

[15] Wang N, Yu H F, Bi W L, et al. Effects of Sodium Citrate and Citric Acid on the Properties of Magnesium Oxysulfate Cement[J]. Constr. Build. Mater., 2018, 169: 697-704.

[16] National Technical Committee of Standardization for Cement. Method of Testing Cement-determination of Strength[S]. GB/T 17671–1999, 1999

[17] National Technical Committee of Standardization for Cement Products. Standard Test Methods for Drying Shrinkage Stress and Cracking Possibility of Cement Mortar and Concrete[S]. GB/T 29417–2012, 2012

[18] Wu C Y, Yu H F, Zhang H F, et al. Effects of Phosphoric Acid and Phosphates on Magnesium Oxysulfate Cement[J]. Mater. Struct., 2015, 48(4): 907-917.

[19] Wu C Y, Chen W H, Zhang H F, et al. The Hydration Mechanism and Performance of Modified Magnesium Oxysulfate Cement by Tartaric Acid[J]. Constr Build. Mater., 2017, 144: 516-524.

[20] Qin L, Gao X J, Li W G, et al. Modification of Magnesium Oxysulfate Cement by Incorporating Weak Acids[J]. J. Mater. Civil Eng., 2018, 30(9): 04 018 209

[21] Wu C Y, Chen C, Zhang H F, et al. Preparation of Magnesium Oxysulfate Cement Using Magnesium-rich by Products from the Production of Lithium Carbonate from Salt Lakes[J]. Constr. Build. Mater., 2018, 172(5): 597-607.

[22] Wu C Y, Yu H F, Dong J M, et al. Effects of Material Ratio, Fly Ash, and Citric Acid on Magnesium Oxysulfate Cement[J]. ACI Mater. J., 2014, 111(3): 291-297.

[23] Tan Y S, Yu H F, Bi W L, et al. Hydration Behavior of Magnesium Oxysulfate Cement with Fly Ash via Electrochemical Impedance Spectroscopy[J]. J. Mater. Civ. Eng., 2019, 31(10): 04 019 237

[24] Zhang N, Yu H F, Gong W, et al. Effects of Low- and High-calcium Fly ash on the Water Resistance of Magnesium Oxysulfate Cement[J]. Constr. Build. Mater., 2020, 230: 162-170.

[25] Giesche H. Mercury Porosimetry: A General (Practical) Overview[J]. Part. Part Syst. Char., 2006, 23(6): 9-19.

[26] Zhang N, Yu H F, Wanga N, et al. Effects of Low- and High-calcium Fly Ash on Magnesium Oxysulfate Cement[J]. Constr. Build. Mater., 2019, 169: 697-707.