Abstract
The low early strength of materials for paste filling in mines caused by low early strength of composite cementing material has been a severe issue. In this study, the effects of sulphoaluminate cement and gypsum on strengths of composite cementing material were investigated experimentally by employing the constrained formulation uniform design. With the content of the sulphoaluminate cement below 14% and the content of the gypsum below 16%, the compressive strengths of composite cementing materials increased, especially early strength. However, the initial and final setting time does not meet the engineering requirements in this case. Optimization tests of composite additives demonstrated that H2BO3(0.3%) + Na2SO4(0.1%) and H2BO3(0.3%) + NaNO2(0.1%) were ideal setting retarding and early strengthening composite additives as they can both reduce the initial and final setting time and enhance compressive strengths of composite cementing material. Investigations by XRD and SEM revealed that the hydration products of composite cementing material were dominated by AFt (ettringite) at the early stage and by C-S-H (hydrated calcium silicate) gel + CH (calcium hydroxide) gel at the middle and late stages. The hydration products of ratio-optimized composite cementing material do not restrain each other due to the generation sequence. Instead, they grew interactively and were coupled, thus facilitating the growth of the hardened body. This study can provide references for optimization of composite cementing material for paste filling in coal mines.
Highlights
In cementing filling, cementing agents were added into the filling materials, which were pumped to the underground to generate fillers with moderate strengths. e cement filler does not collapse even after partial or full relievation of its limiting conditions, supporting surrounding rocks and pillars
As the hydration rate and duration of composites are directly related to chemical compositions of the raw materials, the couplings of the hardened body produced by hydrations inevitably affect the compressive strength of the composite samples
The compressive strengths of composite cementing material may shrink after 3 d in this case
Summary
In cementing filling, cementing agents (e.g., cement, red mud, and gypsum) were added into the filling materials, which were pumped to the underground to generate fillers with moderate strengths. e cement filler does not collapse even after partial or full relievation of its limiting conditions, supporting surrounding rocks and pillars. Owing to its long setting time and slow growth of early strength, the early performance of Portland cement must be enhanced before being used as dominant cementing fillers in paste filling. Based on the practical requirements on performances (e.g., compressive strength and setting time) of filling paste cementing material for coal mining, Portland cement was used as the dominant material, with sulphoaluminate cement and gypsum as additives to enhance its early strength. The hydration process was identified by investigating the hydration mechanism of composite cementing material to obtain that it meets the requirements of paste filling in coal mines. According to the results of mechanical performance tests, optimized formulations of composite cementing material with adequate early compressive strength were identified. Sample blocks at different ages were dried at 70°C for 2 h, sprayed with gold in vacuum, and characterized to investigate hydration products and micromorphologies
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