Sustainable management of natural stone waste.udA proposed re-use for the production of mortars and the assessment of their potential CO2 sequestration capacity

摘要

Quarrying activity usually generates significant amounts of waste. Generally, the processedudquarrying production accounts for 30% of total stone extracted, whilst waste (quarrying waste andudprocessing waste) amounts to about 70% of total stone extracted.udNot only could coping with waste be environmentally damaging, but it is also economicallyudexpensive for the stone industry, which in addition has to deal with the economic loss caused by theudlower efficiency of quarrying. Some re-uses of natural stone waste, such as the production ofudaggregates, have already been investigated, but they are not profitable for most quarryingudcompanies. Other re-uses of stone waste are related to economic fields which are too distant fromudquarrying companies to foster their commitment on their re-use (such as agriculture; paper industry;udetc.). In fact, they are encouraged to sell waste at increasingly lower prices so as to get rid of it,udreducing landfill costs. This research identifies the re-use of stone waste in the construction industryudas the most profitable use, since quarrying activity is strictly related to the building industry. Hence,udquarrying companies have the necessary skills and expertise to evaluate the economic risks, thusudthey are more eager to undertake the production of new by-products.udTherefore, the generic goal of this work is to convert natural stone waste into some by-productsudwith a renewed environmental and economic value. To this purpose, the production of cementbasedudand lime-based materials such as mortars (mixtures of a binder, fine aggregates and water)udwas identified amongst the possible re-uses as the most suitable one, since stone waste can be reusedudto some extent as a substitute of the binder (cement or lime) fraction and for the production ofudthe aggregate fraction, thus achieving a higher re-use rate. In fact, since mortars have no structuraluduse, their requirements are far more flexible than materials such as concrete and it is possible to reuseudhigher percentages of waste in their manufacture.udA further goal of this dissertation was to enhance the environmental advantages of re-using stoneudwaste for the production of mortars by investigating their CO2 sequestration capacity, since audpercentage of CO2 emissions from the production of mortars is reabsorbed as the mortar hardens,udowing to carbonation. Indeed, Portland cement production is responsible for 7% of annual CO2udemissions, due to the calcination reaction. Nonetheless, a secondary effect of carbonation is theuduptake of atmospheric CO2, which reacts with calcium hydroxide in mortars and precipitates asudcalcium carbonate. Although CO2 uptake figures are far from the performance of other materialsud(such as coal ash and industrial residues), it should be pointed out that mortars have great potential,udsince their use in all built-up environments is impressively widespread.ududInitially, the assessment of the CO2 uptake of some selected commercial mortars by means ofudaccelerated carbonation tests was undertaken through an experimental procedure, in order that audstandard methodology and a set of operative parameters could be established. Then, some mortarudmixtures constituted by stone waste were manufactured and henceforth called “ecological mortars”.udThese mixtures underwent the same carbonation tests as the commercial mortars, so as to permit audcomparison between their performance.udThe obtained experimental results showed that both the commercial and the ecological mortarsudare capable to take up CO2. In particular, despite showing fairly lower figures, the “ecologicaludmortars” reported a carbon absorption during the first 28 days of curing exceeding 6% of thatudemitted in the calcination reaction occurring during the production process.udLow though they may seem, this value could represent a significant amount considering theudperiod of service life, and after the service life (i.e. after demolition) of mortars.