Application of Opalized Tuff as an Aggregate in Lightweight Concrete

Lightweight concretes have gained great momentum in construction in the last decade, due to the large number of sustainable characteristics and construction advantages associated with them. The sustainability of lightweight concrete depends mainly on the application of sustainable aggregates, such as the amorphous opalized tuff, found in large quantities in Eastern Macedonia. It is economically viable, easy to extract from surface mines, and easy to process. The physical, chemical, and mechanical properties, porosity, and water absorption of the tuff as a stone aggregate were examined as the aim of this study, with the objective of assessing its potential application in lightweight concrete. The tuff showed an average bulk density 87.2% lower than that of limestone. The compressive strength of the tested opalized tuff samples was 41.16 MPa, or 48.5% of the average strength of limestone rock (84.88 MPa). Furthermore, three concrete mixes with different aggregates were tested: with 100% limestone, with 50% tuff and 50% limestone, and with 100% tuff. The increase in the amount of tuff in the concrete mix required a larger amount of water, due to the high porosity of the tuff; the high water absorption of the tuff aggregate reduced the consistency of the concrete mix, so the bulk density decreased significantly with increasing tuff content. The concrete with 100% tuff aggregate was 44% lighter than concrete with 100% limestone aggregate, which means that concrete–tuff mixes can be classified as lightweight concrete. Our results further showed that by increasing the amount of opalized tuff aggregate in the concrete, the compressive strength of the hardened concrete decreased. The 50:50 mix showed an average compressive strength of 25.68 MPa at 28 days, i.e., 42% lower than the average compressive strength for limestone concrete (44.27 MPa). The tuff-only mix exhibited a compressive strength of 10.46 MPa that was 76.4% lower than limestone-only concrete. The increase in the amount of tuff in the concrete was shown to reduce the thermal conductivity; i.e., concrete with tuff aggregate showed a thermal conductivity coefficient of 0.3585 W/m·K, which is 5.58 times lower than that of conventional concrete with limestone aggregate. The results from the laboratory analyses provide guidance for the application of the local amorphous opalized tuff as a natural stone and as a filler for producing lightweight mortars and concretes. Every alternative and possibility for its application would contribute to reducing waste, reducing energy consumption in buildings, and thus creating an ecologically safe environment. The application of opalized tuff in lightweight concrete will support green jobs and the circular economy using locally available, alternative material, through reducing transportation emissions and decreasing waste.