From this survey of international literature, integrated with some unpublished material test data obtained by the author it can be seen that a substantial range of by-product wastes can be used, or are being considered for use, in brick manufacture.
l Careful investigation, testing and mix design is required before using mixes of various
waste additions or substitutions in order to obtain the optimum energy yield and product quality;
l Some wastes, such as clay off-cuts and crushed brick (grog) are an intrinsic part of the
everyday by-products arising from the brick manufacturing process. As in the past, they will continue to be recycled back to the beginning of the process-line;
l Sand is a common ingredient in brick making and has always been a relatively cheap
material to import into the factory. However, today there is increasing environmental con- cern about the quarrying of many sand deposits. The replacement of this virgin material by either, observing the necessary precautions, recycled foundry sands or, better be- cause of the lesser risks associated, ashes, is shown in literature;
l The availability of very large tonnages of fly and other combustion ashes from various
sources and their widespread geographical location internationally, offers considerable scope as a long-term accessible by- product. Their ceramic characteristics are covered extensively in the literature. Ceramic waste disposal processes of fly ash are not limited to brick: standardized (265) fly ash is used widely in the cement and concrete industry. Other processes, such as the only laboratory scale VitroArc processes described by Haugsten and Gustavson (266), are also available for the treatment and safe disposal. In general it can be said that in the papers reviewed so far, the general results are that the addition of ash, independent of the origin of the ash, leads to less dense product body but higher water absorption ratios whilst improving drying behavior and reducing shrink- age and with no major environmental or workers safety risks or hazards. The many re- ports examined, confirm their successful use in the brick industry worldwide. These by- products are especially recognized for their good fluxing characteristics and ability to be used in conjunction with clay to extend the life of remaining clay reserves;
l Waste glass has been shown to be able to achieve positive results as a flux, promoting
lower-temperature firing and decreased kiln emissions. The main obstacle stopping its wider adoption in brick making is its cost, much of which is due to the high level of pre- milling that is required to make it a suitable fineness to be added to bricks. Also contribut- ing to the high price is the competitive demand by other industries that are able to pay more for it. Understandably, brick manufacturers are unwilling to commit themselves widely to waste glass as the savings do not currently make a good financial case;
l Carbonaceous additives, such as coke breeze and coal fines, have been in use for a
considerable time in the brick industry. It is likely that they will continue to be used in in- creasing quantities, as the cost of premium fuel (gas and oil) might again rise in the fu- ture. Some problems have been encountered with the high concentration of sulphate sometimes associated with this material which can produce surface discoloration on the ware and also increase the level of sulfur dioxide in the kiln exhaust gases;
l This is also the case for many organic wastes produced by the food and beverage indus-
tries which have substantial calorific values albeit their use is due to the high volatile per- centage usually not contributing substantially to the overall energy balance of the firing process but can cause emission problems. However, the literature review has highlighted the potential problem of decomposition and the resulting odors associate with these by- products. Paper waste and paper ash are used increasingly in a similar role too, but problems with heavy metals (from the associated inks) have been recorded as producing
potential leaching problems. Wood in the form of sawdust is also well investigated, but non-continuity of supply and variability in quality have caused problems with efflores- cence;
l Paper waste and paper ash, are reported to be being used increasingly in a similar role,
but problems with heavy metals (from the associated inks) have been recorded as produ- cing potential leaching problems;
l Wood in the form of sawdust is also well investigated, but non-continuity of supply and
variability in quality has caused problems with efflorescence;
l The question of chromium and vanadium leaching must be addressed. From miscel-
laneous references in the surveyed literature, it would seem that the leachability of hexa- valent chromium is strongly influenced by the mass relationship of CaO:SiO2 and the al-
kali metal oxide concentration. Hinz and Press in their patent address the question of “encapsulation” of heavy metals (267). Again the main focus is on disposal but not on substitution of raw materials. Stube in (268) applies the same idea of encapsulation, most heavy metals are permanently bound within the ceramic matrix, principles as those ex- pressed in (197) but claims that by following the patented procedure there would be no risk from leaching.
The literature has also revealed a large number of ʻminorʼ wastes with a potential use in brick making such as pottery wastes, metal processing residues and textile sludge. In many instances there are limited quantities of a specific waste available and this will therefore restricts their use to those brick plant in reasonable proximity to the waste gen- erating industry.
What is certain is that, increasingly, new sources of potential usable wastes will be under investigation by the brick industry. This will be initiated both as source of additional rev- enue (via a gate fee) or because they can improve certain characteristics of the existing products. The waste inclusion work undertaken by the candidate at the Ziegel Gasser Mattoni GmbH S.r.l. brick plant is original as confirmed by the afore literature survey. It is consequently likely to be replicated more and more frequently, as the serious issues of energy conservation and raw materials sustainability, become even more crucial to the brick industry. This presents a demanding challenge in the attempt by the industry to sur- vive amid a growing number of alternative building material now available on the market, which in many cases are cheaper and possess improved specifications.
The unanswerable question is why this technology is not of greater interest to the brick sector as a whole. Usually, “universal” solutions such as the case of the use of paper sludge, are constantly being searched for. Individual solutions are not of interest. The answer may lie in the limited technical and analytical capacities of the single, smaller plants. But then, the large industry groups do not show any major interest either, so this cannot be the whole story.