Material utilization of fibre cement waste

The term “fibre cement” stands for thin boards or tubes consisting of cement, additives and fibres, which, for example, are used in building construction as roof cladding, façade lining or in civil engineering. For many years fibre cement has been produced without asbestos. Thus, asbestos-free fibre cement waste is increasingly generated during the rehabilitation or demolition of buildings. This waste does not require special supervision, as opposed to products containing asbestos, the manufacture of which was prohibited for building construction in 1991 and for civil engineering in 1994. Therefore, it is possible to utilize them non-mixed or as component of recycling building materials. In the following is a brief survey of the features of fibre cement. The comminution in various types of crushers and the properties of the generated particulate material are investigated. Then the question is dealt with if the properties of RC materials of base layers are changed due to the addition of fibre cement.

Fibre cement is a composite building material consisting of hardened cement paste reinforced by fibres. It is produced from Portland cement, additives such as trass or limestone meal and fibres. The production process starts with making a viscous cement suspension containing fibre, with a water-cement ratio of 0.6, which is dehydrated in various steps via rotating cylindrical-shaped screens and felt pads. Then the desired board thickness is adjusted on the size roll. Then the raw board is processed by means of stamping and pressing to become small boards, plane slabs or corrugated sheets. During storage the cement is hydrated, which causes the hardening. Then follows a surface coating.

Fibre cement products are used in building construction. Striking products are corrugated sheets, small roof panels and façade panels, large panels for façades and for internal finishing as well as vent pipes. The amount of fibre cement products produced in Europe every year from 1995 can be estimated approximately based on Eurostat data. The amount produced every year is somewhat more than 2 million tons (Fig. 1).

The forerunner of today’s asbestos-free fibre cement is asbestos cement, which was produced for the first time more than 100 years ago. In the early 1980s a change to the material composition was initiated, while maintaining the technology based on the patent of Hatschek [2]. Today pulp and plastic fibres are used instead of asbestos fibres. The pulp fibres as process fibres have the task to retain the cement particles during the dehydration of the suspension. Synthetic, organic fibres of polyvinyl alcohol or polyacrylonitrile are used as reinforcing fibres. From 1991, at the latest, all products for construction building have been manufactured without asbestos. From 1994 asbestos has no longer been used for civil engineering products.

Figure 2 shows the volume composition of hardened fibre cement. Due to its embedded plastic fibres, the binder matrix consisting of the hardened cement paste and the filler is able to absorb high tensile stress and bending strain so that even large panels can be made with a lower thickness. The air voids ensure the frost resistance of the material because they are the expansion space for penetrated, freezing water. If the cement, the filler and the water are summed up as binder matrix, it follows that fibre cement consists of approx. 95 wt.-% of hardened cement paste including additive and of 5 wt.-% of fibres.

Fibre cement waste is generated during the manufacture, assembly and further processing at site as well as during the rehabilitation and demolition of buildings. The waste resulting from the manufacture and cutting to size is non-mixed. During rehabilitation and particularly during demolition it is usually mixed with other building materials or foreign materials.

According to the European Waste Catalogue it is not necessary that fibre cement waste be marked especially. It is covered by the waste code 17 01 01, which is also used to mark concrete waste [3, 4]. This classification assumes that it has clearly been proved that it is not asbestos cement. There are various analytical techniques to determine the proof [4]. Analyses can be waived if the date of manufacture is known, because it can be derived from this whether it is a procuct containing asbestos or an asbestos-free product. [5].

It is almost impossible to estimate the amounts of fibre cement waste resulting from rehabilitation and demolition. At present, appreciable amounts can only occur in demolition waste of relatively young buildings, which were built approximately 30 years ago after the introduction of asbestos-free products. This situation will change over the years. For the future an annual amount of 300 000 t to 400 000 t can be expected. This estimation is based on the amounts of gypsum waste [6, 7] and asbestos cement waste [4] included in the current waste statistics.              

Fibre cement waste without foreign matter can be recycled. After being ground it can be used as a raw meal component in cement making or as filler for the manufacture of fibre cement. So far there are no investigations as regards the utilization of fibre cement as a component of recycling products.

So far no experience has been gained as regards the comminution behaviour of fibre cement boards. The same applies to the properties of the resulting particulate material. Therefore, it was first tested in pilot plants which of the available crushers, i.e. impact crushers, hammer crushers, jaw crushers and roll crushers, are suitable for the comminution down to grain sizes below 8 mm. Unused roof panels of (200 x 200 x 4) mm³ of fibre cement were used as feed material (Fig. 3). 

The impact crusher proved to be the most suitable unit during preliminary tests. The fibre cement crushed on it had a maximum grain size of 16 mm. The portion of the fraction < 2 mm was very small with approx. 1 % (Fig. 4). As regards the comminution in the jaw crusher it was observed that in addition to the crushed material proper also slightly crushed roof panels occurred in the output. This effect was due to the little wall thickness of the feed material. However, it should be expected that this effect would be reduced if fibre cement panels are crushed together with broken concrete, and when the degree of material filling in the crushing chamber is sufficient. The roll crusher used without profiled rolls and with a minimum feed opening of 4 mm was not suitable for the comminution of the fibre cement panels.

The impact crusher selected for the further comminution tests has a rotor with a diameter of 340 mm and a width of 400 mm and is equipped with four impact bars (Fig. 5). The fibre cement panels (Fig. 4) are continuously fed to the impact crusher. ­During crushing a certain dust formation was observed, the extent of which, however, did not differ from the dust formation occurring during the comminution of other mineral materials in this crusher. The crushed product (Fig. 6) was sized by means of a screening machine to obtain the grain fractions needed for the following investigations. The oversize > 8 mm was fed again to the impact crusher and comminuted there.

The crushed roof panels were classified into 5 different grain sizes. These grain sizes were investigated as shown in Figure 7. The grain shape analyses were carried out to find out how far the flatness of the feed material was maintained in the crushed products. The length-width ratios and the sphericities were measured for each grain size over the entire particle-size range as characteristic values for the grain shape. Then weighted means were calculated taking into account the portions of the grain fractions. Based on the differential thermal analyses (DTA analyses) it should be found out if the fibres had increased or decreased in certain fractions. The densities of the materials and the water absorption were measured to detect differences as a function of the grain size and to be able to preliminarily assess the constructional properties. Then followed measurements of the resistance of the fibre cement grain sizes to freeze-thaw-cycling or to shattering, respectively, so as to be able to come to a conclusion as regards the suitability of the material as a component of RC materials for base layers.

The measured values of the fibre cement fractions were compared with the corresponding parameters of recycling building materials from practice that had been measured using the same method. For this purpose, a quality-controlled RC base layer material with the grain size 0/32 mm – referred to as RC 1 – was provided by a recycling firm and investigated parallel to the measurements of the fibre cements. In addition, results obtained from another project of recycling building materials in practice [8], referred to as RC 2, were included in the evaluation.

From the results of the grain shape analysis (Fig. 8) the following conclusions can be drawn:

– The measured length-width ratios as a degree for the flatness of the fibre cement particles reached values between 1.79 and 1.95. The length-width values, stated for comparison, of the two RC building materials produced in recycling plants from demolition concrete vary between 1.48 and 1.55. The difference is due to the “grain shape” of the fibre cement panels as feed material.

– The sphericities, which are a dimension for the deviation from the spherical form (sphericity = 1), vary between 1.50 and 2.20 for grain sizes of fibre cement. The sphericities of the recycling building materials amount to 1.13 and 1.16, respectively. The great differences of the sphericities cannot only be explained by the deviation from the spherical form. They may additionally be caused by surface roughness, which, in turn, is caused by fibres projecting from the fracture surface.

When comparing the fibre cement sam­ples with each other, the fraction 0/2 always shows the most favourable ­values. An increasing grain size results in an increase of the grain shape parameters. Up to the maximum grain size of 8 mm determined here, the influence of the small wall thicknesses of the feed ma­terials on the grain shape of the crushed products is not yet very distinct. A further increase can only be expected if the maximum grain size of the crushed material is clearly larger than the panel thickness.

The results of the differential thermal analyses are shown in ­Figure 9 as an example for the finest and the coarsest fibre cement grain size. Four essential conversions occur when fibre cement is heated up. First the free water and later a part of the bound water of the hardened cement paste is driven off. Then the pulp fibres and subsequently the reinforcing fibres are decomposed. The thermal decomposition of portlandite Ca(OH)₂ takes place at 420 °C. The decomposition of calcite takes place above 600 °C. There are hardly any differences between the examined fractions as regards the weight losses of the decomposition reactions. A pronounced enrichment of the fibres in one of the fractions could not be determined. So as to check this statement for the other samples as well, the weight losses in the relevant temperature ranges were compared with each other. Table 1 shows that there are hardly any differences as regards the weight losses of the fibre cement fractions among each other. Consequently, there is no distinct enrichment of fibres.

The true density and the bulk density as well as the water absorption of selected fibre cement samples were measured (­Table 2). The reference values again are based on the RC materials from practice.

The true density of the fibre cement is a little below to clearly below that of the RC materials. At the same time it is clearly higher than the typical values for hydrated hardened cement paste, which vary between 1.64 g/cm³ and 1.89 g/cm³ according to our own measurements [9]. One of the reasons for this could be the relatively strong carbonation proved by means of the DTA. The bulk densities are clearly below those of the RC materials, which is due to the high porosity of the fibre cement. The total porosities calculated on the basis of the true and bulk density are approximately in accordance with the data of the volume composition of fibre cement (Fig. 2). The water absorption of the fibre cements after two hours is ten times higher than that of the RC material for base layers. The water absorption of the fraction 2/4 is higher than that of the fraction 4/8. Fibre cement has a relatively high porosity and contains major volume portions of pulp fibres. Possibly both factors could influence the frost resistance and the mechanical resistance. These influences should be investigated in experiments. The results are summarized in the Tables 3 and 4:  

– The resistance of crushed fibre cement to freeze-thaw-­cycling is at least as good as that of the RC materials for base layers. The high porosity of the particulate material of fibre cement acts, as in the feed material itself, as a buffer concerning the increase of the water volume during freezing.

– The resistance of crushed fibre cement to abrasion, ­measured according to the Los Angeles test method, was higher than that of the RC material for base layers. However, the stress intensity of the fibre cements according to the LA tests was reduced as opposed to the tests with the RC materials for base layers. This, however, is admissible according to the valid testing guideline.

The properties of fibre cement and the fact that fibre cement waste is registered in the European Waste Catalogue under the code number of concrete suggest the idea to utilize fibre cement as a constituent of RC building materials for base layers. It was tested in experiments how the addition of different quantities and grain sizes affects the properties of the RC building material. For this purpose, mixtures of fibre cement and RC material were prepared. Their content of fibre cement varied between 0 and 10 wt.-% (fibre cement fraction 0/8) and the grain size varied between the fractions 0/2 and 2/8 (fibre cement content 5 %).

The resistance to freeze-thaw-cycling and to abrasion of the mixes with various fibre cement contents was measured. When testing the mixtures, the ball charge of the LA drum corresponded to the specifications of DIN 1097-2. Furthermore, the Proctor densities and the California Bearing Ration (CBR value) of all mixes were determined and compared with the values of the RC materials for base layers to be able to assess the material behaviour under placement conditions. The CBR values are obtained from a load test and refer to the relative load-bearing capacity.

The resistance values (Fig. 10), measured in mixtures with varied fibre cement contents, show that there are no changes compared to the pure RC material for base layers up to a content of 5 % (freeze-thaw-cycling) and 10 % (abrasion), respectively. Under this aspect, a content of fibre cement of up to 10 % in the base layer material would be entirely unproblematic. The Proctor densities of the mixes, irrespective of the content of the added fibre cement, are slightly higher than the value of the reference material (Fig. 11). Consequently, a higher packing density is achieved. The required amount of water of the building material mix is increased, which is due to the higher water absorption of the fibre cement as opposed to the RC material and to the increase of the specific surface caused by the addition of the fibre cement.

The results of CBR tests (Fig. 12) show that the CBR value is increased due to the admixture of fibre cement, i.e. the load-bearing capacity of the building material mix is increased. This increase is continuous up to the maximum admixture of fibre cement of 10 wt.-% investigated in this case.

The increase of the CBR value is confirmed by the investigations into the influence of the grain size of the fibre cement. The highest CBR values were obtained with the fraction 0/4 of the fibre cement. Thus, the placement properties of the RC material are improved by the addition of fibre cement.

Investigations were carried out into particulate materials of crushed fibre cement and into mixes of fibre cement and RC material for base layers. The pure fibre cement, in spite of its relatively high porosity and water absorption, exhibited a high resistance to freeze-thaw-cycling and a sufficient resistance to abrasion. These properties were confirmed by the investigations of the mixtures. Positive effects on the Proctor density and the CBR value were observed. The reasons for this need more profound research to be further confirmed.

As regards the utilization of fibre cement waste, the con­clusion can be drawn from the investigations that crushed fibre cement can be utilized as a constituent of RC materials for base layers. With the maximum content of 10 wt.‑% investigated here, the Proctor density is increased and the load-bearing capacity is improved. Negative effects on the physical properties, such as the resistance to freeze-thaw-cycling and to abrasion do not occur. Should the occasion arise, a field of application for selectively demolished fibre cement could be developed based on the observed property improvements, e.g. as addition for soil stabilization or to improve the load-bearing capacity of RC material.