Minefill 2020-2021

Proceedings of the 13th International Symposium on Mining with Backfill, 25-28 May 2021, Katowice, Poland

Ferri Hassani, Jan Palarski, Violetta Sokoła-Szewioła, Grzegorz Strozik, Ferri Hassani, Jan Palarski, Grzegorz Strozik, Violetta Sokoła-Szewioła

• Publisher: CRC Press
• Year: 2021
• Edition: 1
• eBook ISBN:
  9781000454222
• Topic: Technology & Engineering
• Subtopic: Civil Engineering
• Index: Technology & Engineering

The series of International Symposiums on Mining with Backfill explores both the theoretical and practical aspects of the application of mine fill, with many case studies from both underground and open-pit mines. Minefill attendees and the Proceedings book audience include mining practitioners, engineering students, operating and regulatory professionals, consultants, academics, researchers, and interested individuals and groups.
The papers presented at Minefill symposiums regularly offer the novelties and most modern technical solutions in technology, equipment, and research. In that way, the papers submitted for the Minefill Symposia represent the highest quality and level in the conference domain.
For the 2020-2021 edition organizers hope that the papers presented in this publication will also be received with interest by readers around the world, providing inspiration and valuable examples for industry and R&D research.

Paste backfill measurements and testing

The effect of curing conditions on the strength development of cemented backfill samples

Brendan Dennis

Laboratory Supervisor, Outotec SEAP

Matthew Helinski

Process Technology Manager, Outotec SEAP

DOI: 10.1201/9781003205906-7

SUMMARY: Historically, strength development in underground backfill is estimated based on the results of historic unconfined compressive strength (UCS) testing. During both preliminary study phases and operational phases are batched to specific solids and binder contents and placed in curing chambers, with the intention that these chambers replicate the curing conditions experienced underground.

The rate of strength development in mining backfill can be greatly affected by variations in environmental factors including temperature & humidity. Therefore accurate representation of in situ curing conditions for cemented specimens is crucial.

This paper investigates the effect that differing curing conditions can have on the strength development of cemented mine backfill specimens. This paper explores how variations expected to be encountered in typical fill masses impacts the resulting strengths and also investigates how aspects that may appear subtle can actually have a significant impact on the resulting measured strengths. Given the findings of this investigation a series of recommendations are provided for improving the way mine backfill curing chambers are managed.

Keywords: cemented backfill, curing conditions, temperature, quality control testing

1 Introduction

Historic evidence from temperature monitoring within fill masses during curing show a consistent trend of increasing temperatures during the initial stages of curing followed by a plateau in temperature, with most cases plateauing at temperatures well in excess of 35°C. The increased temperature is expected to be a result of the exothermic cement reaction, while the sustained higher temperatures are expected to be due to the fills thermal mass. Examples of in situ temperature monitoring during filling, at various sites throughout the world are presented in Figure 1.

Figure 1. In situ fill mass temperature data from various underground mines.

While it is widely recognised that curing temperature can significantly impact the strength of cementation materials, literature reviews show little record of controlled studies to quantify the impact of temperature on fill strengths in the context of how this may be relevant to onsite quality control testing.

This paper presents an investigation into the influence of curing temperature on the strength of different mine backfill types giving consideration to both; the magnitude of strength change that can occur as a result of temperature variations, and aspects that may appear subtle, but can actually have a significant impact on the quality control specimen curing temperatures, and therefore strengths measured in onsite quality control testing. The results from this testwork and analysis culminate in a series of recommendations for good practice quality control testing.

2 Curing Temperature Sensitivity

To investigate this influence of curing temperature on fill strength, a series of cemented hydraulic fill mixes were batched with a range of different binder types and specimens for each mix. The samples were cured at temperatures of 23°C and 35°C. The hydraulic fill adopted in this work was a copper tailings material that was predominantly Quartz and Chlorite minerals. All mixes were batched to 69% solids with 10% binder addition. Specimens batched for each mix were tested for unconfined compressive strength after hydration periods of 2, 7 and 28 days. The fill mix matrix is presented in Table 1.

Table 1. Experiment mix matrix.

Mix No	Binder	Binder portion (%)	Curing Temp (ºC)	Mix Solids Content (%)
1	General Purpose (GP) Cement	10	35	69
2	General Purpose (GP) Cement	10	23	69
3	GP 75% /Fly Ash 25%	10	35	69
4	GP 75% /Fly Ash 25%	10	23	69
5	GP 65% /Slag 35%	10	35	69
6	GP 65% /Slag 35%	10	23	69

The results from this testwork are presented in Figure 2, which shows the measured unconfined compressive strength (UCS) after 2,7 and 28 days hydration. Mixes cured at 35°C are presented as solids bars, while the equivalent mix cured at 23°C are presented as open bars.

Figure 2. Influence of curing temperature on strengths for fill batched with different binders at different curing temperatures.

Figure 2 shows a direct correlation between curing temperature and strength, with all mixes showing higher strengths when cured at higher temperatures. However, what is most interesting about these figures is that, when cured at 23°C the GP/Slag blend binder shows little strength gain. However, when the same mix is cured at the higher temperature (of 35°C) the resulting strengths are superior to all other mixes.

These results show that, compared with conventional concrete curing temperatures (of 23°C), not only does curing cemented backfill specimens at temperatures more representative of in situ yield higher strengths, but the elevated temperature also shows different binder types to be more efficient.

To investigate the sensitivity of more subtle temperature changes in curing temperature, over the range of temperatures measured in situ, a second testwork campaign was initiated. This campaign considered an Australian paste fill mix with high silica content tailings and a binder blend type that consists of 30% GP/70% Ground Granulated Blast Furnace Slag (Slag). The mix was cast at 70% solids with 4% binder addition. Immediately after casting, specimens from this mix were placed into curing chambers that were set to different temperatures. One chamber (Chamber 1) was set to 35°C, another (Chamber 2) set to 31°C and another (Chamber 3) set to a temperature of 43°C. Specimens were cas...

Table of contents

• Cover
• Half Title
• Title Page
• Copyright Page
• Table of Contents
• Preface
• Committees
• Sponsors
• Sponsor sessions
• Paste backfill measurements and testing
• Binders, admixtures, and other chemicals to improve fill performance
• Backfill reticulation: pumping, piping, hydraulic analyses
• Laboratory testing on static and dynamic behaviours of backfill
• Minefill geomechanics, numerical modeling, mitigation of subsidence
• Legal, safety, and environmental drivers for backfill
• Case studies
• Author Index