Abstract

Objective 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 compressive 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 scanning electron microscopy （SEM）. A semi-quantitative analysis of hydration product content was performed using Jade9 software， 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 increasing dosage. For flexural strength， the optimal dosage was identified as 0.02% for all modifiers. At this dosage， the modifier synthesized at an H₃PO₄：TEA molar ratio of 1 exhibited the greatest improvement （a 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 （a 21.4% increase at 3 days and 13.4% at 28 days）， followed 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 optimal dosage， effectively mitigating the long-term strength decline typically observed with unmodified TEA. All modified TEA significantly 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 dosage， reaching its minimum value （a 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 control， 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 dosage 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-day samples revealed that the modifier synthesized at an H₃PO₄：TEA molar ratio of 0.8 increased the contents of hydration products （monosulfate， ettringite， and calcium hydroxide） and reduced the contents of unhydrated clinker minerals compared to the control. Early-age （3 day） XRD patterns showed higher peak intensities of AFt and Ca（OH）₂ in samples containing the 0.8-ratio modifier， indicating accelerated hydration of both C₃A and C₃S. SEM micrographs illustrated microstructural refinement induced by the modifier synthesized with a ratio of 0.8. At 3 days， the sample exhibited a denser structure with interwoven C-S-H gel， AFt， and embedded 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 a 0.04% dosage， total porosity was reduced by 15.9%， and average pore diameter was reduced by 21.5%. Crucially， the proportions 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 Phosphoric acid modification effectively enhances the performance of TEA as an admixture for cement mortar. At a molar ratio of 0.8， the modified TEA significantly improved both the early-age （3 day） and later-age （28 day） compressive strength of cement mortar， and also enhanced flexural strength at their respective optimal dosages （0.02% for flexural and 0.04% for compressive strength）. The modified TEA， particularly at the H₃PO₄：TEA molar ratio of 0.8， markedly improved the mortar’s impermeability by significantly reducing chloride ion permeability at a 0.04% dosage. It also effectively reduced drying shrinkage， which was associated with a reduction in capillary porosity. The morphology of hydration products was modified， promoting the formation of a denser， more interconnected microstructure with fewer visible pores. A significant refinement of the pore structure was achieved， characterized by reductions in total porosity and average pore diameter， particularly through a decrease in the proportion of harmful pores （>50 nm） and an increase in the proportions of less harmful and harmless pores （<50 nm）.

Keywords： modified triethanolamine； cement mortar； compressive strength； impermeability

Get Citation: DING Xiangqun， LI Xingyu， LIU Xu， et al. Effects of phosphoric acid-modified triethanolamine on the properties of cement mortar［J］. China Powder Science and Technology， 2025， 31（6）： 1-10.

Received: 2024-01-26 .Revised: 2025-04-26,Online: 2025-09-24.

Funding Project:国家自然科学基金项目，编号：52108235。

First Author: 丁向群（1970—），男，教授，博士，硕士生导师，研究方向为高性能混凝土。E-mail：xiangqunding@126.com

DOI：10.13732/j.issn.1008-5548.2025.06.015

CLC No:TB4； TU525 Type Code: A

Serial No:1008-5548（2025）06-0001-10