On November 2, 2024, the China Building Materials Federation and the China Concrete and Cement-based Products Association organized a scientific and technological achievement appraisal meeting on the research and application of alkali free accelerator and high-performance shotcrete. This project was jointly completed by Beijing University of Civil Engineering and Architecture, China Railway 11th Bureau Group Co., Ltd., China Railway 11th Bureau Group Bridge Co., Ltd., and Shijiazhuang Tiedao University.

I. Project Background

In infrastructure construction, there is a significant increase in demand for shotcrete in projects such as tunnels, mines, and urban underground spaces, and the quality and construction efficiency of accelerators are crucial. However, traditional accelerators have many problems, such as large rebound of powdered accelerator, high dust content, large loss of strength in the later stage of alkaline liquid accelerator, poor impermeability, and easy injury. Traditional alkaline accelerators are also limited by environmental and health issues. Although alkali free liquid accelerators have received attention, there are still problems such as poor stability. In addition, the construction of high-speed rail passes through complex geology, which requires higher performance of sprayed concrete. Although nano reinforcement materials can improve the performance of concrete, they face technical bottlenecks such as high cost and difficult dispersion.

II. Research Contents

(1) Research and development of high-performance alkali free accelerator

A chlorine free and alkali free liquid accelerator was prepared through orthogonal experiments, which meets the specifications at low dosage and adapts well to various types of cement. On the basis of laboratory results, coagulation promoting materials were added, and industrial pilot tests and applications were completed. A high-performance fluorine free, chlorine free, and alkali free liquid accelerator was prepared using polycarboxylate superplasticizer mother liquor and other raw materials, with low rebound rate in the application of the Sichuan Tibet Railway project.

The influence of the dosage of accelerator on the initial and final setting time of different cements

(2) Enhance the research and development of admixtures

We studied the effects of sulfoaluminate cement, mineral powder, silica fume, etc. on the setting time and compressive strength of shotcrete, and subsequently developed powder nano thickening and reinforcing materials. Using techniques such as XRF, XRD, TG, and SEM, analyze the effects of alkali free liquid accelerators and nano viscosity enhancing materials on the hydration products and microstructure of cement, clarify the intrinsic reasons for improving the performance of shotcrete, and provide a basis for construction in special environments. This technological achievement reduces the amount of cement used in shotcrete, enhances concrete performance, and suppresses rebound.

The influence of the dosage of c nano thickening and reinforcing materials on the compressive and flexural strength of shotcrete

(3) Research on the application of high-performance shotcrete

Adopting alkali free liquid accelerators and nano viscosity enhancing materials to achieve high-performance sprayed concrete, providing adaptive solutions for different surrounding rock conditions, solving construction difficulties under different geological conditions, and ensuring the durability of concrete structures. The high workability, durability, and rapid solidification characteristics of high-performance shotcrete shorten the construction period, reduce maintenance costs, adapt to complex construction environments, and meet various engineering needs.

Initial support state of high-performance sprayed concrete after anchoring