Effects of phosphoric acid-modified triethanolamine on properties of cement mortar

DING Xiangqun; LI Xingyu; LIU Xu; ZHI Xulu

doi:10.13732/j.issn.1008-5548.2025.06.015

Abstract：Objective To investigate the effects of phosphoric acid(H3PO4)-modified triethanolamine(TEA)on the mechanical properties and impermeability of cement mortar,with the aim of addressing the performance limitations of conventional TEA in cement-based systems. Methods H3PO4 was used as a modifying agent and modified TEA was synthesized by reacting H3PO4 with TEA at molar ratios of 0.8,1.0,and 1.2.The mechanical properties and impermeability of cement mortar were evaluated by measuring compres-sive strength,drying shrinkage,and chloride ion permeability after incorporating the modified TEA.The effects of modified TEA on the hydration products and microstructure of the cement matrix were analyzed using X-ray diffraction(XRD)and scan-ning electron microscopy(SEM).A semi-quantitative analysis of hydration product content was performed using Jade9 soft-ware,and mercury intrusion porosimetry(MIP)was employed to examine changes in pore structure. Results and Discussion Compared to the control group,all types of modified TEA improved the 3-day and 28-day flexural and compressive strengths of the cement mortar.The strengths exhibited a trend of first increasing and then decreasing with increas-ing dosage.For flexural strength,the optimal dosage was identified as 0.02％for all modifiers.At this dosage,the modifier syn-thesized at an H3PO4:TEA molar ratio of 1 exhibited the greatest improvement(15.7％increase at 3 days,7.5％at 28 days),followed by the modifier at a ratio of 0.8(11.0％at 3 days and 4.6％at 28 days),and then the modifier at a ratio of 1.2(9.2％at 3 days and 5.4％at 28 days).For compressive strength,the optimal dosage shifted to 0.04％.The modifier synthesized at an H3PO4:TEA molar ratio of 0.8 showed the most significant enhancement(21.4％increase at 3 days and 13.4％at 28 days),fol-lowed by the ratio 1 modifier(16.9％at 3 days and 8.5％at 28 days),and then the ratio 1.2 modifier(15.0％at 3 days and 7.4％at 28 days).Notably,the modifier with an H3PO4:TEA molar ratio of 0.8 achieved the highest strength values at its opti-mal dosage,effectively mitigating the long-term strength decline typically observed with unmodified TEA.All modified TEA sig-nificantly reduced chloride ion permeability.The modifier synthesized at an H3PO4:TEA molar ratio of 0.8 again showed the most pronounced effect.For this modifier,the chloride ion permeability first decreased and then increased with increasing dos-age,reaching its minimum value(25.3％reduction compared to the control)at a dosage of 0.04％.The modifiers at molar ratios of 1 and 1.2 also reduced permeability by 17.9％and 15.6％,respectively,at the same dosage.When the dosage exceeded 0.06％,the permeability of the modifiers at ratios 1 and 1.2 continued to decrease gradually.Compared to the con-trol,the incorporation of modified TEA reduced drying shrinkage at various ages,though shrinkage still increased over time.The modifier synthesized at an H3PO4:TEA molar ratio of 0.8 resulted in the lowest shrinkage,with a 12.1％reduction at a dos-age of 0.04％.The modifiers at molar ratios of 1 and 1.2 reduced shrinkage by 10.6％and 3.2％,respectively,at the same 0.04％dosage.This reduction in shrinkage was associated with a decrease in capillary porosity.XRD analysis confirmed that modified TEA did not produce new hydration products.However,semi-quantitative analysis of 28 days samples revealed that the modifier synthesized at an H3PO4:TEA molar ratio of 0.8 increased the contents of hydration products(monosulfate,ettring-ite,and calcium hydroxide)and reduced the contents of unhydrated clinker minerals compared to the control.Early-age(3 days)XRD patterns showed higher peak intensities of AFt and Ca(OH)2 in samples containing the 0.8-ratio modifier,indicat-ing accelerated hydration of both C3A and C3S.SEM micrographs illustrated microstructural refinement induced by the modifier synthesized with ratio of 0.8.At 3 days,the sample exhibited a denser structure with interwoven C-S-H gel,AFt,and embed-ded Ca(OH)2 crystals filling the pores,in contrast to the relatively loose and porous structure of the control.At 28 days,the sample displayed a tightly packed microstructure with minimal visible pores,in stark contrast to that of the control group.MIP analysis showed that the modifier synthesized at an H3PO4:TEA molar ratio of 0.8 significantly improved the pore structure at 0.04％dosage,total porosity was reduced by 15.9％,and average pore diameter was reduced by 21.5％.Crucially,the propor-tions of harmful pores(50～200 nm)and larger harmful pores(＞200 nm)decreased,while the proportions of less harmful pores(20～50 nm)and harmless pores(＜20 nm)increased.Similar improvements,though less pronounced,were also observed for modifiers synthesized at molar ratios of 1 and 1.2.This refinement in pore structure directly contributed to the enhancements in strength and impermeability. Conclusion The addition of H3PO4-modified TEA enhances the degree of cement hydration,promotes the growth of hydration products,regulates the morphology of hydration products,improves structural density,optimizes the pore structure,and ulti-mately increases the strength and impermeability of the cement mortar.