Application of Amoras filter cakes in infrastructures

Application of AMORAS Filter Cakes in Infrastructures

1          Introduction

AMORAS is a sustainable and innovative plant, located in the port of Antwerp and exploited by 'SeReAnt'. Flemish authorities and the Antwerp Port authority are investing in sustainable solutions for the storage and processing of maintenance sludge from the docks.

Up to now, the filter cakes with a capacity of 500.000 tons dry matter per year are being deemed as waste material. The ambition of this project is to reuse and revalue the filter cakes as a renewable raw material. [1]

2          Research objectives

Because of the large annual amount of filter cakes processed and the limited storage space (13 million tons ~ 26 years), there is need for an application with a significant market. This way the product filter cakes are reused and upgraded to secondary materials, allowing the amount of storage to be reduced.

3          Research methodology

To start off, a method is required to reduce the filter cakes in the desired fraction. Subsequently the fine fractions will, for the first time, be mixed with bitumen in different forms, respectively hot or cold and wet or dry.

Stage 1 is the classification of filter cakes as filler, described in 'SB250 v3.1' and the related standards.

In the second stage, the effect of filter cakes on hot mix asphalt in comparison with reference filler is defined. During this study, a reference SMA-C2 asphalt mixture will be used with a fixed quantity of bitumen and aggregates.

To ensure the commercial viability of a new product on the market, several minimal requirements need to be fulfilled.

A trade-off between processing into filler and deposit must be made by checking whether filter cakes can be an alternative for the current primary fillers.

4          Filler classification

Before new filler can be used in infrastructures, the product needs to be submitted to the European and Belgian standards.

4.1           Voids

By using the 'Rigden apparatus', the percentage of voids is determined. The ratio of voids defines certain characteristics of the filler, more specifically particle size, shape, and texture. It is a way to state the stiffening. [3]

It is worth mentioning that both batches of filter cakes produce comparable results, V44/55. However, in comparison with the other fillers, filter cakes show a remarkable increase in voids. This results in increased bitumen absorption and much stiffer mastic.

4.2           Delta ring and ball

On the basis of 'Delta ring and ball', the softening point is determined, a characteristic for the stiffening effect of filler.

These results show a slight difference between the two batches filter cakes, the difference with Duras is much bigger. The very high softening point of filter cakes predicts a very stiff mastic, which indicates that high temperatures are required to actually produce asphalt mixtures. [4]

4.3           Bitumen number          NBN EN 13179-2

The bitumen number is a measure of the absorbability of bitumen by filler.

Figure 6 shows clearly the difference in water absorption. This will result in a higher amount of bitumen required in asphalt compositions, which result in a higher cost. NBN EN 13043 classifies fillers with a maximum bitumen number of 62, filter cakes exceeds this value.

4.4           Methylene blue                NBN EN 933-9

This test defines the amount of clay particles present in the filler. Clay will swell in contact with water, leading to an unstable mixture of asphalt.

The results obtained for the different batches filter cakes are in general fairly similar, however the result of batch 1 stands out. In comparison with Duras, filter cakes contain a high amount of clay though.

5          Hot mix asphalt

5.1           Experimental design

The results from the investigation of filter cakes as filler are clearly conclusive, as during the production of the different asphalt mixtures certain characteristics were instantly noticed. Firstly the compositions with filter cakes were stiffer, resulting in a much more difficult mixing procedure. Secondly, it was immediately clear that filter cakes absorbed a lot more bitumen than the reference filler, which increases the production cost.

5.1.1        Voids                            NBN EN 12697-8

Percentage of voids is determined by two different procedures in this research. On the one hand with the gyratory compactor and on the other hand calculated from the maximum and bulk density.

Results show a clear increase in voids as more filter cakes are added. However, when only 50 % of the filler is replaced, the percentage of voids decreases slightly when calculated according to the standards.

5.1.2        ITS                              NBN EN 12697-23

ITS or indirect tensile strength is tested on two different sets of test cylinders, one set in dry form (before conditioning) and one set in conditioned form.

The rupture surface is examined and is a way to visually determine the weakest link in the mixture.

As for indirect tensile strength, results before conditioning (BC) show (Figure 6) a slight decrease when all of the filler is replaced by filter cakes and even a slight increase when only 50 %. All of these values can be interpreted as acceptable for an SMA-C asphalt mixture.

Results after conditioning (AC), however, show a significant bigger decrease, especially for the compositions with all of the filler replaced.

ITS before conditioning is also examined for mixtures were filter cakes are added in pure, wet conditions. Results are very poor though. As for mixtures F1 and F2 respectively an ITS-value is defined of 0,81 and 0,84 MPa, the test cylinders with wet filter cakes only reach 0,25-0,28 MPa.

6          Economic analysis

Finally, an economic analysis of the application as filler is examined. It is determined whether or not it is economically viable, from a social point of view, to process filter cakes into filler. Quantities which could be processed are defined and at what price. The balance between processing as filler and deposit should be evaluated, just as the determination of the competitive position. The ultimate cost for the refining, drying and grinding into filler should not exceed the price of alternative products plus the deposit costs. Grants can ensure the economic position of filter cakes to minimise deposit and maximise sales as filler.