Characteristics And Mechanism Of Sulphate Effect On The Early Age Properties Of Cemented Paste Backfill

The early-age properties of cemented paste backfill(CPB) can be affected by sulphate contained in tailings and mine water. This effect caused by sulphate can lead to potential safety hazard in some project. To discover the change regularity and mechanism of sulphate effect on the early-age properties of CPB, in this paper, mechanical test, permeability test, capillary test, and suction and electrical conductivity monitoring and microstructural analysis techniques(X-ray diffraction, Differential thermal gravimetric analysis and Mercury intrusion porosimetry) were used to assess the effect of sulphate on the mechanical properties and microstructure(e.g., pore structure, binder hydration products) of CPB at the early ages. Qualitative analysis method, quantitative analysis method and statistical analysis method were used in this study.(1) Artificial silica tailings, binder(Portland cement, mixture of Portland cement and slag, mixture of Portland cement and fly-ash) and distilled water with different sulphate content were used to make CPB specimens. Mechanical tests were conducted on CPB samples with different sulphate content and binding material and cured for 1, 3, 7 and 28 days. Uniaxial compressive strength, elastic modulus and damage processes of the specimens were analyzed and early-age strength-curing time model, strength-elastic modulus model and CPB damage process model were built. It was found that sulphate can lead to significant effect on strength, elastic modulus and damage process of CPB at early age. There is a power correlated relationship between strength and curing time. The uniaxial compressive strength of CPB made by Portland cement or the mixture of Portland cement and fly-ash decreases as the initial sulphate content increases. However, negative or positive effect can be caused by different initial sulphate concentration in CPB made by the mixture of Portland cement and slag. It was discovered that there is a linear correlated relationship between strength and elastic modulus of CPB. For the certain strength value, 0.5% sulphate can reduce and higher sulphate concentration increase the elastic modulus of CPB made by Portland cement. However, the elastic modulus of CPB made by binder contains slag or fly-ash can be increase as the initial sulphate content increases. It was shown that the tenacity and fragility of CPB can be affected by sulphate. It can be concluded that:(i) measures should be taken to decrease the sulphate content in the condition of Portland cement is used as binder in the cemented paste backfill project;(ii) the mixture of Portland cement and slag should be priority selection as the binder in project;(iii) ferrous sulfate can be added in CPB to motivate the activity of slag and improve early strength.(2) Mercury intrusion porosimetry analyses were performed on the CPB samples with different binders and sulphate content and cured for 28 days. Fractal theory was used to classify pore diameter. Pore characteristic parameters, such as porosity, pore surface area and pore diameter distribution, were analyzed. Statistical analysis method was used to discover the relationship between porosity, pore diameter distribution and strength and the porosity-strength model was built. It was found that sulphate can lead to significant effect on the porosity and pore diameter distribution of CPB. In fact, 2.5% sulphate can increase the porosity and decrease the surface area of CPB made by Portland cement. However, porosity was reduced by sulphate in the CPBs contain slag or fly-ash. It was discovered that the surface of pore in CPB is mainly contributed by micro pore or small pore. It was found that the variation of pore structure is an important factor of strength change. From a general view, the strength of CPB decreases as the initial sulphate content increases. What’s more, the strength of CPB shows significant correlational relationship with the porosity of pore with diameter bigger than 0.2um. It can be concluded that:(i) pore structure variation caused by sulphate is a reason for strength change of CPB;(ii) moderate sulphate content can refine the pore structure of CPBs contain slag or fly-ash, thus improve its mechanical properties.(3) Constant head method was used in the hydraulic conductivity test on CPB with different binder, sulphate content and curing time. The relationship between hydraulic conductivity and porosity was analyzed. It was found that sulphate can lead to different effect on hydraulic conductivity and capillary property of CPB samples on the basis of binder. In the condition of same binder, permeability and water absorption capability decreased in 0.5% sulphate content and increased in higher sulphate content. The hydraulic conductivity of CPB shows significant correlational relationship with the porosity of pore with diameter bigger than 2um. It can be concluded that:(i) pore structure variation caused by sulphate is a reason for permeability and water absorption capability change of CPB;(ii) moderate sulphate content can reduce the hydraulic conductivity and water absorption capability, thus contribute be beneficial to control heavy metal pollution in backfill project.(4) To study the sulphate effect on the evolution of the self-desiccation of CPB at early age suction monitoring was performed on the CPB samples. According principle of effective stress and the shear strength theory of unsaturated soil, the relationship between suction and uniaxial compressive strength was deduced and suction-strength model of CPB was built. The feasibility of Strength development forecast based on suction was discussed. It was found that suction can be contributed to the strength of CPB and there is a significant linear dependence relationship between suction value and early-age strength. What’s more, suction value is affected by pore diameter and there is a negative correlation existed between suction value and pore diameter. It can be concluded that:(i) sulphate can lead to significant effect on the self-desiccation process of CPB at the early age;(ii) suction development can be representative of strength development;(iii) Predicting strength development by suction monitoring may be a feasible method in backfill mine practice.(5) Main hydration reactions of Portland cement, slag and fly-ash were summarized. The reactions between sulphate and binder were enumerated. Suction and electrical conductivity monitoring are also performed on the CPB specimens with different sulphate contents while differential thermal gravimetric and x-ray diffraction analyses were performed on the cement paste samples. According to these results, the effect caused by sulphate on hydration reaction can be discovered.It was found that sulphate can have negative effects, i.e., leads to a decrease in the CPB strength and reduction in the intensity and rate of self-desiccation within the CPB. The magnitude of these effects depends on the initial sulphate concentration. The inhibition of cement hydration by sulphate ions is a key reason for the observed decrease in CPB strength and self-desiccation intensity or rate. Ettringite formation, changes in the pore structure are found as additional negative factors that affect CPB strength. It can be observed that compared to the 28 day sulphate-free sample, the 28 day-strength of the specimens with a sulphate content of 0.5%, 1.5% and 2.5% is reduced by 31%, 46% and 50%, respectively.The results show that depending on initial sulphate content and curing time, sulphate can lead to inhibition on cement hydration or activation effect on active Al2O3 in slag. Ettringite can be produced by the reaction of active Al2O3, sulphate anion and calcium hydroxide. It can be observed that activation effect held a dominant position in the specimens with 0.5% sulphate content. Compared to the 28 day-sulphate-free sample, the 28 day-strength of the 0.5% specimens was increased by 20% and the 28 day-porosity was decreased by 8.9%, in other words, the early-age properties of CPB were improved. Inhibition effect held a dominant position in the specimens with 1.5% and 2.5% sulphate content. Compared to the 28 day-sulphate-free samples, the 28 day-strength of this two group specimens was decreased by 45% and 72%, respectively and the 28 day-hydraulic conductivity was increased by 61% and 62%, respectively, in other words, the early-age properties of CPB were deteriorated.The results reveal that sulphate has inhibition on cement hydration or activation effect on active Al2O3 in fly-ash. However, due to the low activity of fly-ash, inhibition was found as the dominant effect in the specimens with sulphate. Compared to the 28 day-sulphate-free samples, the 28 day-strength of 0.5%, 1.5% and 2.5% specimens was decreased by 19.4%, 45.2% and 74.2%, respectively and the 28 day-hydraulic conductivity was increased by 66.4%, 683% and 712%, respectively, it means that, the early-age properties of CPB were deteriorated.It can be concluded that the change of hydration process caused by sulphate is the root reason for the variation of CPB early-age performance. The effect caused by sulphate on different binder hydration is mainly shown in two aspects:(i) inhibition on cement hydration;(ii) activation effect on activated aluminium oxide in slag and fly-ash. The behavior of this two impact changes along with binder type and sulphate concentration.