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Effect of Amino Trimethylene Phosphonic Acid and Tartaric Acid on Compressive Strength and Water Resistance of Magnesium Oxysulfate Cement

Yutong Zhou,Zheng Zhou,Lvchao Qiu,Kuangda Lu,Dongmei Xu,Shiyuan Zhang,Shixuan Zhang,Shouwei Jian,Hongbo Tan

Organic acids could act as retarders in magnesium oxysulfide (MOS) systems, not only delaying setting and improving fluidity but also enhancing compressive strength and water resistance. These effects are generally attributed to both the presence of H+ions and anion chelation. However, the enhancement efficiency of different organic acids in MOS systems varies significantly due to differences in their molecular structures. To determine the underlying mechanism, this study comparatively investigated the effects of amino trimethylene phosphonic acid (ATMP) and tartaric acid (TA) on the setting time, fluidity, compressive strength, and water resistance of the MOS system, with the two additives incorporated at mole ratios to MgO ranging from 0.002 to 0.006. The mechanism behind it was revealed by discussion on the hydration heat, hydrates, and pH value. Results showed that both ATMP and TA could effectively improve the fluidity, delay the setting process, and enhance the mechanical properties, including strength and water resistance. At a mole ratio of 0.006, the incorporation of ATMP increased the 28 d compressive strength and the softening coefficient by 214.12% and 37.29%, respectively, compared with the blank group. In contrast, under the same dosage, TA led to an increase of 55.13% in the 28 d strength and 22.03% in the softening coefficient. Furthermore, hydration heat, product analysis, and pH measurements indicated that both ATMP and TA inhibited hydration during the initial hours but promoted hydration at later stages. The potential reason could be divided into two aspects: (1) H+ions from ATMP and TA suppressing the formation of Mg(OH)2; (2) anion chelation with Mg2+in the liquid phase, leading to a supersaturated solution with higher saturation, which further hindered Mg(OH)2formation and facilitated the later development of 5Mg(OH)2·MgSO4·7H2O (517 phase). By contrast, under the same mole dosage of H+or anions, the enhancement in compressive strength as well as the water resistance is superior when using ATMP. This was owing to its stronger chelating ability of ATMP, which more effectively inhibited Mg(OH)2formation and then promoted the formation of the 517 phase. These findings confirm that the chelating ability of anions exerts an important impact on the retarding effect as well as the enhancement of strength in MOS systems.