Abstract:

The use of cemented paste backfill (CPB) in the mining industry has increased over the past two decades. A comprehensive understanding of the stiffness development of CPB during early binder hydration stages has great impacts on the safety and economy of mines. Ultrasonic wave velocity measurements were performed to characterize the effect of relative humidity (RH) conditions and binder content on the stiffness development of CPB. In this paper the evolution of degree of hydration (DOH) is investigated for CPB with binder contents of 3% and 5% at different RH conditions to study the impact of cement hydration on stiffness development. An insight is presented on the development of shear wave velocity (Vs) with the ongoing hydration process during the first week after mixing. Developing self-desiccation suction in sealed samples was measured. The lack of significant early restraint by the paste to volume change resulted in the delay of the evolution of significant suction forces until high DOH is reached. The study concluded that low cement concentrations results in delaying the formation of solid paths and stiffness development. Therefore, changes in shear stiffness of the CPB and self-desiccation suction are not proportional to changes in DOH.

Introduction

Cemented paste backfill (CPB) has been increasingly used in the mining industry over the past two decades. Confidence in cemented paste backfilling systems has been increasing as it has the advantage of easy transportation to underground openings, rapid rates of backfilling, increased ore recovery, less water drainage concerns, and the environmental benefits of diverting tailings from surface storage. A comprehensive understanding of the strength and stiffness development of CPB during early binder hydration stages has a great impact on the safety and economy of mines. In particular, it helps to achieve safe and economic barricade designs and to optimize the backfilling process which impacts the scheduling of mining activities. Strength and stiffness development of CPB are believed to be mainly influenced by two factors, the binder content and type; and the curing relative humidity (RH) conditions. One of RH bounding conditions is when free water is available to compensate for pore water consumption ensuing cement hydration. The other bound is when CPB is left to air-dry. Stiffness development in CPB at both bounding RH conditions is investigated by studying the ongoing changes in ultrasonic wave velocities during the first week of curing by Galaa et al. (2011). Wave velocity measurements to characterise stiffness development of CPB was practiced in several studies such as Ercikdi et al. (2014), Yilmaz et al. (2014) and Yilmaz and Ercikdi (2015). In the area of cement and concrete research, ultrasonic wave velocities have been used to assess the hydration process and strength development by many researchers (e.g. D'Angelo et al. 1995, Boumiz et al. 1996, Chotard et al. 2001, Akkaya et al. 2003, Abo-Qudais 2005). Moreover, the effect of RH curing conditions in the context of strength assessment and/or autogenous shrinkage prediction is extensively studied in the concrete area (Molina 1992, Jensen and Hansen 2001, Lura et al. 2002). However, the water to cement (w/c) ratio in CPB is extremely higher than what is normally used in concrete. Consequently, various factors are found to be of influence on cement hydration in concrete and may not have the same level of significance in the case of CPB. Galaa et al. (2011) concluded that RH has a more significant effect on CPB than that of the binder content. In the current study, the degree of hydration is measured for identical samples at the same RH conditions in Galaa et al. (2011) in order to further investigate the conclusion drawn by the mentioned study. In addition, another RH condition is studied where samples are sealed, evaporation is neither allowed, nor free water supply is available.

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