Using Geopolymerization Technology to Develop High Performance Pumpable Roof Support Cementitious Material and to Recycle Mine Tailings

Abstract

This research aims to develop high performance pumpable roof support cementitious material and to recycle mine tailings (MT) through geopolymerization technology. First, a systematic study on the incorporation of ordinary Portland cement (OPC) to adjust/improve the workability, setting time, and compressive strength of Class F fly ash (FA)–based geopolymer binder was performed. The geopolymer binder specimens were produced by mixing OPC with FA at a dosage of 0%, 5%, 10%, 15%, and 20% by weight of FA, respectively, and then mixing the mixture with a blended sodium silicate (SS) and sodium hydroxide (SH) solution at a SH concentration of 5 M and a SS/SH ratio of 1. A water-to-solid (W/S) ratio of 0.35, 0.40, 0.45, and 0.50, respectively, was used in preparing the specimens. The viscosity and setting time of the fresh geopolymer binder were measured by using a coaxial cylinder viscometer and a Vicat apparatus, respectively. The specimens were cured at 35°C in an oven for 7 days before tested to measure the unconfined compressive strength (UCS). The results show that the viscosity of the geopolymer binder increases with higher OPC content and lower W/S ratio. The addition of OPC reduces the setting time. The shortest initial and final setting times of 16 and 46 min, respectively, were obtained at W/S = 0.35 and 20% by weight OPC. The incorporation of OPC increases the UCS of the geopolymer binder. The highest UCS of 42.4 MPa was obtained at W/S = 0.35 and 20% by weight OPC. Microstructural and chemical analyses including scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD) were also carried out and the results indicate that the addition of OPC produces a denser microstructure by the formation of calcium silicate hydrate (CSH) gel along with geopolymer gel.Second, a study was conducted to examine the feasibility of using class F fly ash (FA) with added cement kiln dust (CKD) to synthesize a geopolymer cementitious material (GCM) that will address the limitations of current cementitious materials (CMs) for building pumpable roof supports. The GCM is designed to be a combination of two individual pumpable grout streams: stream 1 being a slurry composed of FA, CKD, superplasticizer (SP) and water, and stream 2 being an alkaline solution prepared with sodium silicate (SS) and sodium hydroxide (SH). When the two streams stay alone, they remain as a slurry and a solution, respectively, and can be easily handled and transported. When they are mixed together, a GCM is formed. The study systematically analyzed the influence of different factors on pumpability, setting time, and mechanical performance of the GCM. The results show that stream 1 can be designed to be pumpable for a long distance within a certain period of time simply by adjusting the water to solid ratio and superplasticizer content. Stream 2 is an alkaline solution containing SH and SS and can be easily pumped. The setting time can be effectively adjusted by including CKD. Furthermore, the GCM shows much higher peak and residual strength than the pumpable CMs currently used in practice. Third, the utilization of biopolymer (BP) to improve the ductility of class F fly ash based geopolymer cementitious material (GCM) developed for pumpable roof support has been investigated. Specifically, two biopolymers, kappa-carrageenan (CAR) and gellan gum (GEL), at different dosages, were used to prepare the GCM specimens at various conditions and systematic tests were performed to measure the peak unconfined compressive strength (UCS), Young’s modulus, residual UCS, and tensile strength of the hybrid geopolymer-biopolymer cementitious material (HGBCM). The results show that depending on the specific water to solid (W/S) ratio, superplasticizer (SP) dosage and BP content, increasing the amount of CAR or GEL up to 0.5 wt.% only slightly increases or decreases the peak UCS and Young’s modulus of the HGBCM. This is good because the main purpose of using the BP is to increase the residual UCS. Depending on the specific W/S ratio, SP dosage and BP content, increasing the amount of BP also results in an increase or decrease of the residual UCS; but the maximum residual UCS is obtained at 0.3 wt.% CAR or GEL at nearly all W/S ratios and SP dosages, and those values are much higher than the corresponding values at 0 wt.% BP. Furthermore, the incorporation of BP slightly decreases the tensile strength of the HGBCM, with the HGBCM containing CAR showing higher tensile strength than that containing GEL. Compared with the cementitious material currently used in mining operations, the HGBCM developed in this study shows superior performance. Forth, the feasibility of using low-reactive copper mine tailings (MT) with slag (SG) as a supplemental cementitious material to produce green bricks based on the geopolymerization technology was studied. To this end, the effects of several parameters including NaOH molarity (10 and 15 M), Na2SiO3/NaOH ratio (0, 0.5, 1, 1.5, 2, 2.5, and 3), slag (SG) content (0, 10, and 20 wt.% of MT+SG), forming pressure (0, 5, 10, 15, 20, and 30 MPa), water-to-solid ratio (0.12, 0.14, and 0.16), and curing temperature (60, 75, 90, 105, and 120 ℃) on the physical and mechanical performance of geopolymer brick specimens are investigated through unconfined compression and water absorption tests. In addition, scanning electron microscopy (SEM), energy dispersive X-ray (EDX) and X-ray diffraction (XRD) analyses are conducted to understand the changes in microstructure and phase composition after geopolymerization. The results show that using only MT does not lead to a durable geopolymer product because of the low leaching of Si and Al species from the MT. However, with the incorporation of SG as a supplementary cementitious material, geopolymer bricks that satisfy the ASTM requirements can be produced. Fifth, the durability and leaching behavior of geopolymer bricks produced from low-reactive copper MT as the primary aluminosilicate precursor and slag as the supplementary cementitious material (SCM) was studied. Durability tests include fourteen (14) wet and dry cycles and fifty (50) freeze and thaw cycles, respectively. After the durability test, the weight loss and unconfined compressive strength (UCS) of the geopolymer brick specimens were measured. The leaching study was conducted following TCLP guidelines by immersing the geopolymer brick specimens in solutions with pH = 4 and 7, and measuring the concentration of contaminants in the solution after 120 days. After the leaching test, the wet and dry UCS of the geopolymer specimens were measured. In addition, microscopic studies including SEM/EDX and XRD were performed to study the microstructural changes and the phase composition. The results show a substantial loss in UCS after the durability tests. However, the weight loss is small compared with the conventional cementitious materials. Most importantly, the contaminants in the MT are successfully stabilized within the geopolymer framework. Finally, the feasibility of using low-reactive copper MTs to produce geopolymer concrete (GC) was investigated. The effect of different parameters including sodium silicate to sodium hydroxide ratios (0, 1, 2, and 3), class C fly ash (CFA) contents (0, 10, 30, and 50 wt.% of binder), water to binder ratios (0.22, 0.26, and 0.30), binder to aggregate ratios (0.18, 0.20, 0.22, 0.24, and 0.26), fine aggregate to coarse aggregate ratios (0.30, 0.35, and 0.40), curing temperature (25, 60, 75, 90, and 105 ℃), and curing time (1, 3, 7, 14, 21, and 28 days) on the mechanical performance of the GC has been studied. The results showed that by proper tuning of the concrete parameters, the GC providing a higher mechanical performance could be produced. This research paves the way for utilization of waste-based construction material which significantly contributes to the reduction of CO2 emissions and consequently alleviation of global warming.