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

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