DING Xiangqun，WANG Yitong，WANG Yan，CUI Diansong

Abstract

Objective An accelerator is an admixture that significantly reduces the setting and hardening time of cement mortar or concretewhile ensuring sufficient early strength development. Therefore， accelerators are widely used in mining， tunneling， slope sup⁃port， and emergency repair projects. Among commercially available alkali-free liquid accelerators （AFA）， aluminum sulfate iscommonly used as the primary coagulant due to its high water solubility. However， Al³⁺ions tend to undergo hydrolysis， formingAl（OH）₃ colloids that can cause coagulation reactions. To enhance the stability of Al³⁺in the liquid phase， researchers generallyadd complexing agents such as fluorides， organic alcohol amines， or polyols. However， these agents can potentially undermine the early strength development of cement mortar. Current research on accelerator adhesiveness mainly focuses on the relationship between accelerator dosage and the rebound rate of shotcrete. Limited attention has been given to the optimized preparation of accelerators and the selection of suitable components to enhance their intrinsic adhesiveness. This paper explores the use of sulfate-based modifiers to improve the properties of AFA mother liquor. The effects of these modifiers on the early strength of cement mortar mixed with the accelerator and its bonding performance with existing substrates are evaluated， and the underlying modification mechanisms are analyzed.

Methods Different dosages （mass fraction， same below） of AFA mother liquor were added to cement mortar， and the optimal dosage was determined based on setting time. To address the issue of low early strength in the alcohol amine-aluminum sulfate system prepared by an organic-inorganic composite method， the compressive strength of cement mortar at 1 and 28 days and the interfacial flexural-tensile strength of bonded specimens at 1 and 28 days were tested under a constant accelerator dosage. The influence of sulfate modifier on the mechanical properties of cement mortar and its interfacial bonding performance with the existing substrate was also examined. To investigate the underlying mechanisms， hydration heat analysis， X-ray diffraction （XRD），and scanning electron microscopy （SEM） were conducted to study the effects of sulfate modifiers on the hydration rate， heat evolution， hydration products， and microstructure of cement at 1 day.

Results and Discussion The results indicated that the setting time of cement paste decreased with increasing dosage of AFA mother liquor. When the accelerator dosage ranged from 6% to 10%， both the initial and final setting times of cement met the national standards. Increasing the dosage from 8% to 10% further shortened the setting time， although the change was not significant. The AFA mother liquor improved the 1-day compressive strength of the cement mortar at a dosage of 8% but significantly reduced the 1-day interfacial flexural-tensile strength of the bonded specimens. The accelerator significantly enhanced the heat release rate during cement hydration， increased the two exothermic peak values， shortened the induction period，and advanced the onset of the acceleration period. The setting time， flexural strength， and compressive strength of the cement mortar and the interfacial flexural strength of the bonded specimens all initially increased and then decreased with increasing modifier content. When the modifier content was 2%， the setting time of the cement paste was minimized， and the compressive strengths of the cement mortar at 1 day and 28 days reached their maximum values， increasing by 61. 9% and 3. 6%， respectively， compared to the blank group， and increasing by 20. 3% and 3. 6%， respectively， compared to the 8% accelerator group. The 1-day interfacial flexural-tensile strength of the bonded specimens was 24. 3% higher than that of the blank group and 90. 6% higher than that of the 8% accelerator group. This improvement addressed the issue of reduced 1-day interfacial flexural-tensile strength caused by the sole addition of AFA mother liquor. The 28-day interfacial flexural-tensile strength of the bonded specimens was also significantly enhanced.Conclusion The addition of sulfate-based modifiers further shortens the setting time of cement paste when used in combinationwith the accelerator， significantly enhancing the compressive strength of the cement mortar at all ages. Additionally， itaddresses the issue of reduced interfacial flexural-tensile strength of the bonded specimens that occurs when only the acceleratoris used. The compressive strength of the cement mortar and the interfacial flexural strength of 

the bonded specimens initiallyincrease and then decrease as the modifier content increases. By consuming Ca（OH）₂， the modifier promotes the reaction of C₃A， leading to the formation of coarse， needle-like ettringite （AFt） crystals that densify the cement system. This improved morphology refined the microstructure， overcoming the problem of excessively fine crystals and a loose cement structure typically caused by the accelerator alone. As a result， the overall mechanical properties of the cement mortar are improved.