Use of Industrial By-Products as Cement Replacements

G. Hunt, M.Popham
Celtic Cement Technology, Wales, UK
K. P. Williams, B. I. G. Barr
School of Engineering, Cardiff University, Wales, UK

ABSTRACT.

It is becoming increasingly important to minimise the amount of waste going to Landfill and find alternative uses for these materials. 

In Europe no one appears to commit to the difference between by-products and waste, it as been quoted that a by-product must be a material that has been produced with a specific use and have a specific timescale for its use. We are of our own opinion that a waste is any material that if an alternative use can be found, with beneficial properties then this should be classed as a by-product.

This paper presents technical data to show that several industrial by-products which are currently being landfilled or stockpiled are capable, after a minimum of thermal treatment grinding and blending in pre-determined proportions are capable of replacing up to 50% of OPC when producing concrete. 

It has been found that modified cements can be formulated which exhibit better high early strengths (7-day) than comparable materials formed from Ground Granulated Blast Furnace Slag (GGBS) or Pulverised Fuel Ash (PFA).  These modified cements are shown to be acceptable in terms of commonly used characterisation parameters quoted in the BS EN196 series.

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INTRODUCTION

In the United Kingdom and many other countries landfill sites in general are becoming over crowded and expensive for waste producers, then wherever possible, material going to disposal should be minimised. If the production of the waste can not be prevented, then it is attractive to create an alternative use in another process before considering disposal. The key question that is always raised is what are waste and what a by-product is, a definition on this subject will be most welcome. We have spent many years on researching materials in the UK and Europe that have potential for use as cement replacements. The benefits of recycling are dependant on availability of landfill, local cost and government drivers. In Wales for example the cost of recycling aggregates tend be higher than natural aggregates, therefore, to justify recycling industrial by-products there must be technical, environmental and economical advantages.

The successful use of industrial by-products in concrete have been used for decades, such as Ground Granulated Blast Furnace Slag (GGBS), Pulverised Fuel Ash (PFA) and Silica Fume.
All the above are used as composite cements with Portland Cement (PC), at varying percentage replacements, it can be noted with all the above there is limited ability to control the chemical composition with percentage replacement.

Using modern technology such as X-Ray fluorescence (XRF) and X-Ray diffraction (XRD), we are able to compare new materials against historical data to establish what compounds are forming with time, also if new compounds are discovered identify how to treat these compounds to enhance the properties of concrete.

In order to maximise the use of industrial by-products in cement and concrete, you must first have an understanding of all the characteristics of each raw material. Secondly it is essential that you test each material in accordance with current standards to establish the short, medium and long term effects in concrete structures.

The purpose of this paper is to present data to show there are many industrial by-products that can be used as cement substitutes with minimal green house gas emissions and with low energy consumption. It is not only the mineralogical and chemical composition of the industrial by-products that are to be considered, but the method of treatment. You must take into consideration both chemical and physical properties, such as particle size distribution, strength development and an ability to modify cement replacements to local environmental conditions.

This paper also presents data to show that several industrial by-products which are currently being landfilled are capable, after a minimum of thermal treatment, grinding and blending, of replacing up to 50% of OPC in concrete. The tests carried out include chemical analyses for elemental composition using XRF, chemical compounds by XRD and particle size analysis (PSA), as well as measurements of compressive strength in mortar.

MATERIALS

The cement replacements presented in this paper have been composed of raw materials currently being landfilled or stockpiled and could be the subject of Waste Management regulations in the future. The materials used are from many industrial sectors outside the more common industries such as the Iron, Steel, Power Stations and Quarrying Industries. For this paper, the materials have been given ‘RM’ references due to confidentiality requirements. Cement and cement substitutes are referenced as ‘C’ these include PC, GGBS and PFA.

Table 1Table 1 (Chemical composition of a selection of PC’s) presents the chemical composition of a selection of Portland cements, during the research we have used these variations as upper and lower limits for acceptable chemical composition. We have also used the variations of composite cements but are not included as part of this paper. All the Portland cements have been tested as received from the cement suppliers.

EXPERIMENTAL PROCEDURES AND RESULTS

The compressive strength results in mortar for the purpose of the research was by replacing 25% of Portland cement with either a raw material, a blend of raw materials, GGBS or PFA.
In the case for the raw materials alone there were different replacement levels from 5-25% replacement of PC. Many types of cement were used during the research but the cement used with all replacements has been labelled as C3.

We established upper and lower limits for strength determination by firstly reducing the total cement content from 100% down to 75% cement content as seen in Figure 1.

The cement was then replaced with 5, 10, 15 and 25% replacement of the raw materials, this was to establish the values of the replacement levels for individual raw materials. These results can be seen in figure 2.

Blends of raw materials, GGBS and PFA were all replaced at 25% replacement. Mortar prisms were made in accordance with BS:EN:196.

Figure 1

 

Figure 1. Portland cement results over 28 days at varying cement contents.

 

Figure 2

Figure 2 Compressive strength gain over 28 days of mortar composed of up to 25% of OPC

It can be seen that different raw materials give different strength performance, the performance of each material in most cases can be changed during the recycling process. The advantages here are the ability to modify the properties to suit the environmental conditions for the intended end use.

Figure 3

Figure 3 Comparison of strength development of a Celtic Cement blend against PC, GGBS and PFA at 25% replacement levels in mortar prisms.

It can be seen that the Celtic Cement blend can out perform GGBS and PFA over the first seven days and at 28 days the performance is between that of GGBS and PFA. These blends can be modified to match either GGBS or PFA depending on specification.

Particle size distribution can vary significantly between Portland cement suppliers and cement substitutes, this combined with the chemical composition can have significant affect on the concrete. The variations are illustrated below in figure 4.

Figure 4

Figure 4 Particle Size Distribution of Portland cement, GGBS and PFA

It can be seen from the above chart that there are variations between Portland cements and also GGBS and PFA. Theses differences have significant effect on the strength performance of concrete.

During the characterisation of industrial by-products we establish the correct particle size distribution to suit the requirements of the end use in concrete.

It is important to understand the reactivity of by-products for use as cement replacements, to do this you cannot rely on the chemical elements that are established using XRF. There is a need to establish what compounds are forming with time, this is carried using XRD.
Below in figure 5 we can see the compounds forming with Portland cement in the first 24 hours of hydration.

OPC Surface phases graphFigure 5. Observation of PC compounds as they change with time.

Using latest available XRD systems you can observe the compound changes with time, these are well documented in a historical database. During research we can monitor the compounds that form with time of PC and composite cements that include GGBS, PFA, Silica fume and any other raw materials that can be added to form new cement substitutes.

 

 

DISCUSSION AND CONCLUSION

It has been demonstrated during this paper that there are a wide range of performances between various cement producers, both in chemical structure and particle size distribution.
Therefore, when you add cement substitutes from a single source then you will also have a variation in performance between cement suppliers.

We have established a new technology that enables cement companies or manufacturers of cement substitutes to have the ability to modify their production to suit the needs of their end users.

The question of why these non-traditional substitutes are capable of exhibiting acceptable strength properties has recently been established during the research at the School of Engineering lat Cardiff University. The understanding of the mechanisms involved is still not complete, although the consensus seems to be that there could be a combination of chemical reaction and a possible micro filler effect with certain industrial by-products.

The enhanced strength effects are the results of synergy between the hydration of the OPC and the presence of pozzolans and/or micro filler materials. The effect of the latter is greatest when the particle size is at a minimum in some cases but not in all materials.

During the study we have found that materials identical in chemical elemental composition produce different strength performance. The effects of particle size distribution can give considerable variations between these materials. The result of this means that you can select the input materials to suit the requirements of the end user and the environment they are intended for,

You can also now make modifications not only to the percentage replacements of cement substitutes, but also to the chemical composition with percentage replacement.

The information provided during this paper as been the result of many years of research between Cardiff University and industrial partners partially funded by the Department of Trade and Industry and the Welsh European Funding Office.

ACKNOWLEDGEMENTS

We would like to acknowledge the support we have received in our research work at the School of Engineering Cardiff University form all cement manufacturers and substitute suppliers in the UK by providing us with regular samples of their products.

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Stormydown , Bridgend, Wales, UK, CF33 4RS

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Contact Gary Hunt

Recycle for Wales