Rheological properties of cemented paste backfill with nano-silica: Link to curing temperature

Abstract

Current technical knowledge on the rheological behavior of cemented paste backfill (CPB) that contains nano-silica (nS) at different curing temperatures is insufficient. However, the assessment and an understanding of the yield stress and viscosity of CPB with nS added at the early ages are critical for using CPB technology in underground mines located in cold or warm regions and/or for deep mining. Therefore, the aim of this study is to investigate and develop a better understanding of the combined effects of temperature, addition of nS as an additive, and type of binder on the rheological properties of CPB. To this end, several CPB samples with different compositions are subjected to various temperatures (2 °C, 20 °C, and 35 °C) for up to 4 h. The yield stress and viscosity of the CPB samples are then measured at different time intervals of 0 min, 15 min, 1 h, 2 h, and 4 h. Moreover, electrical conductivity (EC) monitoring, and microstructural analyses (TG/DTG and XRD) are conducted on the samples. It is found that the yield stress and viscosity of the samples cured at 35 °C are much higher than the other samples. The enhancement of the rheological properties of CPB due to the coupled impacts of nS as an additive and higher temperature leads to reduced flowability of the paste. However, it is also found that the addition of a superplasticizer can partially compensate for the fluidity. Also, an increase in the water content reduces the yield stress and viscosity. The findings presented in this paper will contribute to the development of nano-CPB technology and its effective application in the backfilling operations of underground mines.

Introduction

The mining industry produces large volumes of solid waste through exploration, mining activities, and mineral processing which include waste rock and tailings. Thus, management of mine waste and large numbers of underground and/or surface voids which result from mining is an inevitable task for sustainable, more efficient, and prosperous mining activities. The mine waste on the earth surface occupies a vast amount of land and can also create serious geotechnical and environmental hazards, such as tailings dam failure, acid mine drainage, and ground water pollution [[1], [2], [3]]. For these reasons, tailings disposal has received tremendous attention from both the general public and government. For the past two decades, a large volume of mine waste has been returned to previously underground-mined voids (stopes) by using a common process that is known as cemented paste backfilling. Considered as a relatively modern technology, cemented paste backfilling can provide ground support and a safe work environment, and reduce surface subsidence. The technique is also an efficient and effective technique for mine waste disposal, which reduces pollution and other negative environmental impacts [[4], [5], [6], [7]]. Cemented paste backfill (CPB) is a mixture of dewatered tailings, water (fresh or processed), and binder (usually 2%–7% by weight). Binder consumption has the most significant influence on the economic performance of CPB. The cost of CPB can make up to 20% of the total mining costs, among which the binder can represent up to 75% of the cost of the CPB [[8], [9], [10]].

Portland cement (ASTM Type I; PCI) has been the most extensively used binding agent in CPB. The chemical composition of PCI consists of four major phases, alite (Ca₃SiO₅ or C₃S), belite (Ca₂SiO₄ or C₂S), tricalcium aluminate (Ca₃Al₂O₆ or C₃A), and aluminoferrite (Ca₂Al_xFe₂-xO₅). The weight percentage of C₃S, C₂S, C₃A, and Ca₂Al_xFe₂-xO₅ are approximately 50–70%, 15–30%, 5–10%, and 5–15%, respectively [11]. The reactions among these phases in an aqueous solution produce hydration products (see Equations (1)–(4)). The dominant hydrates are calcium silicate hydrate (C–S–H, 60–70 wt%), calcium hydroxide (CH) or portlandite (20–25 wt%), gypsum, ettringite and calcium aluminoferrite (C₄AF, 15–20 wt%) [11].

$$2C_{3}S+6H\to C_3{S}_2{H}_3+3CH$$

$$2C_{2}S+4H\to C_3{S}_2{H}_3+CH$$

$$3CaO.A{l}_2{O}_3+6H_{2}O\to 3CaO.A{l}_2{O}_3.6H_{2}O$$

$$4CaO.A{l}_2{O}_3.F{e}_2{O}_3+7H_{2}O\to 3CaO.A{l}_2{O}_3.6H_{2}O+CaO.F{e}_2{O}_3.H_{2}O$$

However, the increasingly high manufacturing cost of PCI along with the greenhouse gas emission during its production process has raised environmental concerns. During the last decade, supplementary cementitious materials (SCMs) have been widely used to partially substitute for PCI. Blast-furnace slag (Slag), a byproduct of iron production, and fly ash (FA) produced from coal combustion, are well known as common types of SCMs and their use leads to a significant reduction in both the cost of the CPB and amount of carbon dioxide (CO₂) emissions [[12], [13], [14], [15]]. The use of SCMs is also a means of utilizing the byproducts of industrial manufacturing processes and providing economic, ecological, and engineering benefits for the industry [16].

During the last decade, a large number of research studies have been carried out to gain a better understanding of the impact of nano-silica (nS) on the development of the strength of cementitious materials and the changes in their microstructure [[17], [18], [19], [20], [21], [22], [23]]. Sonebi et al. [24] examined the effect of the dosage of colloidal nS and temperature on the rheological behavior and strength of grout [24]. Their results showed that an increase in the dosage of nS from 0.5% to 3.5% increases the yield stress and plastic viscosity. Also, an increase in the temperature from 6 °C to 26 °C accelerates the rate of hydration and increases the viscosity. Del Bosque et al. [25] investigated the effect of temperature and nS on the hydration of C₃S. Their research showed that increasing the temperature from 25 °C to 65 °C accelerates the hydration of C₃S and changes the structure of the C–S–H. The effect of elevated temperatures on the properties of cement, concrete, and mortar that contain nS was studied by Heikal et al. [26], Bastami et al. [27], and Horszczaruk et al. [28], respectively. They showed that nS reacts with cement to form greater volumes of C–S–H, which results in an enhanced microstructure and improved mechanical properties of the cementitious materials.

However, CPB differs from concrete, mortar, grout, and cement. The water to cement (w/c) ratio of CPB is typically higher than 6 [[29], [30], [31]]. The reason for a high-water content in CPB is to enhance its rheological performance, and thus its flowability [[32], [33], [34]]. The rheological properties of CPB are used to show its behavior under flow. CPB is a non-Newtonian fluid that can maintain a constant flow under high shear stresses while low stresses inhibit its flow. The flowability or workability of a CPB mixture entirely governs the ease of the pipe transport of fresh CPB [35]. Thus, there is a significant disparity in the material properties of CPB in comparison to those of concrete, grout, and mortar which have a w/c ratio that is less than 0.5 and the obtained results are therefore not compatible with CPB.

Despite the tremendous strides made to understand the rheological and mechanical properties of cementitious material with PCI/SCMs and nS, a fundamental understanding of the effect of nS on CPB is still far from obtained and no research has been performed to examine the effect of curing temperature on the rheological properties of CPB with nS. Note that each underground mine or each type of backfill material is unique with regards to its temperature conditions. These temperatures can vary with the depth of a mine and the geological conditions, geographical location of the mine, heat produced by hydration, self-heating of the surrounding rocks of the mine, and human activities [3]. Therefore, there is a lack of knowledge on the rheological and microstructural responses of CPB with nS as an additive to different curing temperatures.

Current knowledge on the important relationships between the rheological and microstructural properties of CPB systems is insufficient and some fundamental questions have been left unanswered. For example, what is the effect of temperature on the yield stress of CPB with nS? How would nS affect the viscosity of fresh CPB in cold versus warm regions? How would the yield stress of CPB with nS change with curing/transport time with respect to temperature? How would the nS particles change the microstructure of CPB at the early ages at different temperatures and what is the impact of these changes on CPB flowability? What are the combined effects of curing temperature, w/c, and nS content on the rheological properties of CPB? How much superplasticizer or mineral admixture should be present to change the yield stress and/or viscosity of CPB with nS?

To date, there is no study that addresses the aforementioned key design issues. Therefore, it is timely to address this research gap in the existing literature.