Cement Concrete Res.: A New Strategy for Enhancing the Ultra-Early Strength of Shotcrete: Stage-wise Kinetic Control of Hydration Products

Novel strategies for ultra-early strengthening of shotcrete: Stage-wise kinetic control of hydration products

A New Strategy for Enhancing the Ultra-Early Strength of Shotcrete: Stage-wise Kinetic Control of Hydration Products

Shotcrete; Ultra-early strength; C-S-H nano-seeds; Stage-wise hydration kinetics;Ettringiteformation

Publication year: 2025

Source: Cement and Concrete Research

First author: Xie Hui, PhD student, School of Materials Science and Engineering, Southeast University

Corresponding authors: Liu Xin, Postdoctoral Fellow & Prof. Feng Pan, School of Materials Science and Engineering, Southeast University

Shotcrete is a special concrete material that can set and harden within minutes and reach specific strength targets within hours.The rapid setting and high early strength of shotcrete are two basic requirements for ensuring construction safety and improving construction efficiency.Achieving the above performance largely depends on accelerators (classified into alkaline liquid type and alkali-free liquid type). Among them, aluminum sulfate-based alkali-free accelerators (AS) are widely favored because of their significant accelerating effect and relatively small impact on the long-term strength of concrete. However,although the addition of AS helps achieve rapid setting of cement, it usually impairs the ultra-early strength of shotcrete.In recent years, with the increasing demand for underground construction, AS development faces increasingly prominent challenges: improving early strength while reducing setting time.

AS is mainly composed of Al₂(SO₄)₃and alkanolamines.The rapid formation of ettringite (AFt) in the early stage of cement hydration is one of the key factors controlling its rapid setting and early strength development.Studies have shown that AS can significantly shorten the initial and final setting times of cement paste (by 1.4 and 3.7 min, respectively). However, the rapid precipitation of AFt consumes a large amount of Ca²⁺, reducing the supersaturation of calcium silicate hydrate (C-S-H) gel and thereby delaying its precipitation. In addition, in a Ca²⁺-rich environment, AFt readily forms on the surface of tricalcium silicate (C₃S), thereby inhibiting C₃S hydration and further hindering strength development. Therefore,the inhibition of C₃S hydration by AS fundamentally determines the slow development of ultra-early strength of cement.Several strategies have been proposed to compensate for the inhibitory effect of AS on C₃S hydration. Some researchers introduced calcium bromide (CaBr₂) into accelerators and showed that CaBr₂promotes cement hydration by forming Br-AFm (calcium aluminate hydrate) phases, thereby increasing the 1 d compressive strength of mortar from 10 MPa to 13.25 MPa. Other researchers prepared an alkali-free accelerator using organic ethers and Al₂(SO₄)₃and found that the organic ethers produce numerous etch pits on the surface of cement particles through complexation during the action of the accelerator, destroying the surface AFt coating and thereby enhancing the compressive strength of shotcrete. The above modification methods have a certain improving effect on the early strength of shotcrete, but the introduction of elements such as Br may cause a corrosion risk to the concrete structure, thereby reducing its durability.

In recent years, C-S-H nano-seeds, as a mature commercial accelerator, have played an important role in promoting the early strength development of cement because their nucleation effect can significantly promote C₃S hydration. Some researchers found that incorporating 5 wt.% C-S-H nano-seeds into shotcrete can increase its 1 d compressive strength by 52%, and the compressive strength enhancement effect is even more significant at low temperatures (an increase of 273% at 5 °C). However,when it comes to ultra-early strength development, the nucleation effect of C-S-H nano-seeds is not significant.Studies have shown that incorporating 2 wt% C-S-H nano-seeds only increases the 6 h compressive strength of AS-containing mortar from 3.2 MPa to 3.5 MPa.

The weak enhancement of C-S-H nano-seeds in the ultra-early strength development of cement (especially in the presence of AS) can be attributed to the competitive formation of AFt and C-S-H during the first few hours.During this period, the formation of both AFt and C-S-H requires the consumption of Ca²⁺. Compared with C-S-H gel, the rapid dissolution of C₃A in the first few minutes and the introduction of AS cause AFt to reach supersaturation earlier. AFt precipitation consumes Ca²⁺ in the solution, thereby inhibiting the formation of C-S-H gel. In summary, the effect of C-S-H nano-seeds on improving the early strength of cement is limited. In addition, AFt may rapidly form through the nucleation effect and cover the C-S-H nano-seeds, thereby reducing their effectiveness.Therefore, regulating the formation of AFt and C-S-H gel at different stages helps stagger the peak Ca²⁺ demand, thereby significantly improving ultra-early strength.

Adding AS and C-S-H into shotcrete in stages can effectively stagger the peak Ca²⁺ demand, thereby improving the mechanical properties of shotcrete without compromising its early strength.However, the effectiveness of this method has not yet been verified.

In this paper, AS and C-S-H nano-seeds were used as admixtures to improve the strength of shotcrete by controlling hydration products in stages.Specifically, various addition sequences of AS and C-S-H nano-seeds were designed, and the effects of these addition sequences on the mechanical properties of mortar were evaluated; the hydration kinetics and microstructure of shotcrete were characterized by isothermal calorimetry, X-ray diffraction (XRD), thermogravimetry (TG), scanning electron microscopy (SEM), and mercury intrusion porosimetry (MIP); the effects of different addition sequences on the pore solution and the assembly of hydration products were investigated in a model cement system to reveal the underlying mechanism.

This paper proposes a new method for improving the ultra-early mechanical properties of shotcrete, namely, controlling hydration products in stages by adjusting the addition sequence of aluminum sulfate-based accelerator (AS) and calcium silicate hydrate (C-S-H) nano-seeds. Various characterization techniques were used to comprehensively analyze the effects of the addition sequence on cement hydration kinetics, hydration products, microstructure, and pore structure, and model cement was further used to elucidate the underlying mechanism for the improved mechanical properties. The main conclusions of this paper are as follows:

(1)AS and C-S-H nano-seeds can effectively improve the mechanical properties of shotcrete, and the degree of modification strongly depends on the addition sequence of the two.Compared with the simultaneous addition of AS and C-S-H nano-seeds, delayed addition of AS reduced the compressive strength of shotcrete, whereas delayed addition of C-S-H nano-seeds significantly enhanced the strength.When 4 wt.% C-S-H nano-seeds were added 1 h after the addition of 8 wt.% AS, the modification effect on shotcrete was the best, with its 6 h compressive strength increased by 35 wt.%.As the addition time of C-S-H nano-seeds was extended, its modification effect weakened.

(2)Delayed addition of C-S-H nano-seeds by 1 h accelerated the hydration of tricalcium silicate (C₃S) and promoted the formation of calcium hydroxide (CH) and C-S-H gel after 6 h. Changing the addition sequence of AS and C-S-H nano-seeds had a significant effect on the chemical composition of the cement paste.Compared with delayed addition of C-S-H nano-seeds, delayed addition of AS resulted in a higher Al/Ca ratio and a lower Si/Ca ratio in the hardened cement paste. In addition,delayed addition of C-S-H nano-seeds optimized the pore structure of the hardened cement paste, resulting in an increase in gel pores and a decrease in large pores.

(3) Analysis of the pore solution and hydration products of the model cement system showed thatthere is significant calcium competition during the early formation of ettringite (AFt) and C-S-H gel.Delayed addition of C-S-H nano-seeds postponed the massive precipitation of C-S-H gel, effectively staggering the rapid formation stage of AFt, thereby mitigating Ca²⁺ competition. In addition, it is speculated thatdelayed addition of C-S-H nano-seeds facilitates the insertion of C-S-H gel into the pre-constructed porous AFt skeleton, thereby explaining the significant refinement of the microstructure of cement-based materials and the improvement of their ultra-early strength.