Development of sustainable substitutes for Pulverised Fly Ash in Cement and Concrete

Academic Institution: University of Strathclyde

Academic Supervisor: Professor Joanna Renshaw

In Collaboration with: Low Level Waste Repository Ltd (LLWR)

PhD Student: Andrea Kozlowski

Summary

This project examines the leaching behaviour of elements from cementitious materials incorporating supplementary cementitious materials (SCMs), focusing on their environmental and functional impacts.

Cement production is a major contributor to global CO₂ emissions, yet it remains essential for critical applications such as the encapsulation of low-level radioactive waste (LLW), with using  an ordinary Portland cement (OPC) to SCM ratio of 1:3. Traditionally, the industry has relied on pulverised fly ash (PFA) from coal burning as an SCM, but this resource is both limited and unsustainable.

To address these challenges, this research tested a range of alternative materials, including both anthropogenic (biomass fly ash, beneficiated pulverised fly ash, and wastepaper fly ash) and natural (metakaolin, olivine, pozzolanic sand, and volcanic ash) sources, to evaluate their suitability for maintaining grout performance replacing PFA. The results were compared to the currently used PFA grout and a 100% ordinary Portland cement grout.  

By investigating the leaching of key elements in deionised water and artificial groundwater for 24 hours, 11 days, and 1 year, the project seeks to identify materials that minimise toxin release while maintaining the required grout performance and contributing to sustainable cement formulations. Through a combination of experimental leaching tests and chemical analysis, the project seeks to provide insights into developing more sustainable and environmentally friendly cement formulations.

This work ultimately supports the nuclear industry's efforts to reduce its environmental impact while maintaining material performance and waste safety.

Abstract

As of April 2022, approximately 94% of all nuclear waste in the UK is classified as low-level waste (LLW) or very low-level waste. This waste is immobilised by the UK’s Nuclear Waste Services (NWS) using grout composed of a 3:1 mixture of pulverised fly ash (PFA) and ordinary Portland cement (OPC). Historically, PFA has been used for LLW encapsulation. Produced as a fine by-product of coal-fired power stations, PFA serves as a supplementary cementitious material (SCM). However, with the UK’s Net Zero legislation requiring the closure of coal-fired power plants by, a suitable alternative SCM is needed to maintain or improve the grout’s mechanical and chemical properties.

Local sources of PFA for radioactive waste conditioning are already depleted, and UK companies now rely on imported PFA. Key properties of the PFA grout in use for the NWS include maintaining a pH of approximately 11, avoiding the release of hazardous pollutants, and providing consistent physical and chemical performance. With coal-fired power station closures planned globally, either LLW cement formulations must change or PFA must be replaced with alternative SCMs that sustain the 3:1 SCM-to-OPC ratio. Such alternatives must be scalable for long-term use, widely available, and low in CO₂ emissions.

This project evaluated potential SCMs, including biomass fly ash (BMFA), beneficiated PFA (BPFA), metakaolin (MK), olivine (OL), pozzolanic sand (PS), volcanic ash (VA), and wastepaper fly ash (WPFA). Previous studies have examined how SCM percentages affect grout compressive strength and shown that the variability in material sources, such as BMFA and WPFA, can influence performance. Investigations into leaching behaviour highlight the importance of leachant properties, with element release from OPC, PFA, WPFA, BMFA, and MK being affected by fluid composition and acidity. Notable elements include calcium, along with iron, copper, and manganese.

Certain elements leached from SCMs (e.g., boron, chromium, selenium, and tungsten) may pose environmental toxicity risks, while others (e.g., aluminium, iron, magnesium, and titanium) could compromise long-term grout durability. This research focuses on grout performance, particularly pH stability and element release, tested via three main leaching experiments: static leaching (24 hours), dynamic leaching (11 days), and semi-dynamic leaching (1 year), all tested in deionised water and artificial groundwater.

The element release was measured with ICP-OES and IC. In the static experiment, it was determined which elements are freely available in the grout matrix and are directly flushed out on contact with water. The dynamic experiment consists of a series of static experiments with full leachant exchange after 24 hours, and represents a high-water flow rate. The long-term effects were investigated using the semi-dynamic experiment, where only parts of the leachant were exchanged. Physical tests of the raw materials were carried out to characterise the SCMs. This included tests on particle size distribution, grout flow, density, porosity, compressive strength, microstructure, and mineral phases. These tests enabled the classification of the SCMs and basic suitability for use in the 3:1 SCM:OPC grout.

Key Results/Outcomes:

  • WPFA and MK are not fulfilling the strength requirements needed (400MPa)

  • MK does not fulfil the pH requirements (pH11) in all leaching experiments

  • PS had higher particle sizes than any other SCM

  • BMFA does not meet the grout flow requirement (250mm)

  • All other SCMs (BPFA, OL, PS, VA) fulfilled the requirements for strength, grout flow, and pH

  • All SCMs leach mostly similar elemental concentrations to 100% OPC/3:1 PFA:OPC grout in all experiments, with highest concentrations found for S, Na, and K – typical leaching elements in cement grouts

  • Only very minor concentrations (<1ppm) of toxic elements (As, Be, Cd, Cr, Hg, Ni, Sb, Se) were leached, except Al, with concentrations <10ppm

  • BPFA does not meet the widely availability, as being PFA with lower quality

  • PS is of specific unknown origin and availability

  • The most promising to replace PFA are of natural origin: OL and VA

Journal publications/conference presentations/awards

A. Kozlowski, T. Peshkur, J. C. Renshaw, K. J. Dobson, and F. Taylor, F. (2024): Comparing the Short-Term Leaching Behaviour of Cements Containing Supplementary Cementitious Materials in different Leachants for Low-Level Waste Encapsulation, Geoenergy 1(2), doi:10.1144/geoenergy2024-004

Roy G. Post Scholarship for WM Symposium 2024

Various conference presentations such as WM Symposium 2025, EGU 2023, NUWCEM 2023, ERA14 2023, FISA/EURADWASTE 2022