Effect of calcium lignosulfonate on the deformation and failure characteristics of cementing stone and its modification mechanism

In the coalbed methane extraction of surface wells in the mining area, the mining of coal seam in working face will cause a significant disturbance to overlying rock layer, which in turn will lead to the deformation and damage of surface wells. The coalbed methane extraction of surface wells in the mining-disturbed areas cannot be effectively performed. Cementing technology can effectively elongate the life span of mining-disturbed coalbed methane surface wells. As a critical factor of the cementing quality of surface wells, the early strength of the cement needs to meet the high cementing requirements. Cement stone is formed after cement consolidation. The mechanical properties of cement stone are of great importance to maintain the stability of mining-disturbed coalbed methane surface wells. It is an effective way to improve the cementing effect by adding dispersants to enhance the mechanical properties of cement stone. There are few researches on the deformation and failure characteristics of calcium lignosulfonate modified cement under the influence of mining disturbance. Uniaxial compression tests of calcium lignosulfonate modified cement with different mass fractions under cyclic loading were carried out to investigate the effect of calcium lignosulfonate on the deformation and failure characteristics of cement stone in mining-disturbed coalbed methane surface wells and its modification mechanism. The results show that the P-wave velocity and peak stress of cement stone increase first and then decrease with the increase of calcium lignosulfonate mass fraction, while the total acoustic emission ringing counts of cement decreases first and then increases with the increase of calcium lignosulfonate mass fraction. With the increase of the uniaxial cycle steps, the deformation modulus of cement stone shows a strengthening phenomenon, and the initial loading and unloading cycle has the most significant strengthening effect on the deformation modulus. With the addition of calcium lignosulfonate, the modified cement stone shows a tensile-dominated → shear-dominated → tensile-dominated combined damage modes. Besides, the fractal dimension of modified cement stone shows a trend of decrease before increase, indicating that the addition of the appropriate amount of calcium lignosulfonate can effectively improve the damage resistance of cement stone under uniaxial cyclic loading. With the increase of the mass fraction of calcium lignosulfonate, the pores between cement hydration products show a trend of decrease first and then increase. The addition of an appropriate amount of calcium lignosulfonate can promote the formation of abundant C−S−H gel and ettringite in cement, and the precipitates interweave on the surface of cement particles, which can significantly improve the peak stress of cement stone, and play a positive role in improving the mechanical properties of cement stone. Additionally, the porosity of cement stone decreases, which leads to the increase in the longitudinal wave velocity of cement stone and the decrease of cumulative acoustic emission ringing counts of the cement stone during cyclic loading. However, when the calcium lignosulfonate is excessively added, the air entraining and electrical repulsion of calcium lignosulfonate play a dominant role in the hydration process, which will introduce more bubbles, resulting in the occurrence of the gap between the cement particles, and the inhibition of the early formation of C−S−H gel and ettringite, which has a negative impact on the mechanical properties of cement stone. Furthermore, the porosity of cement stone increases, which leads to the decrease of the longitudinal wave velocity of cement stone and the increase of the cumulative acoustic emission ringing count of cement stone in the process of cyclic loading. Therefore, the influence of calcium lignosulfonate on the mechanical properties of cement stone has a double effect.