Zero-cement vs. cementitious mortars: An experimental comparative study on engineering and environmental properties

Alkali-activated materials (AAMs) and geopolymers are identified as potential alternatives to conventional Portland cement due to their ability to convert wastes into useful materials and reducing the CO2 emissions. Furthermore, they offer a higher strength-to-weight ratio than comparable Portland cement binders, which makes them a suitable material for lightweight concrete/mortar applications. The present study aimed to examine the effect of precursor and lightweight aggregate type on workability, compressive strength, flexural strength as well as the environmental impact of lightweight mortars. For this purpose, precursors derived from waste aluminosilicate sources including ground granulated blast furnace slag (GGBFS), metakaolin (MK), waste ceramic powder (WCP), and waste clay brick powder (WCBP) were used for the synthesis of alkali-activated and geopolymer mortars. In addition, an equivalent Portland cement mortar was prepared for comparison purposes. Pumice and lightweight expanded clay aggregate (LECA) were used as the lightweight aggregates. Based on the test results, the WCP- and WCBP-based geopolymer mortars exhibited a higher workability than other mortar mixes. Generally, mortars prepared with pumice aggregate showed lower workability and higher mechanical strength than LECA incorporated mixes due to the higher density of pumice and its better bonding with paste. Alkali-activated and geopolymer mortars presented a higher initial strength and reached about 80% of their 28-day compressive strength in 1 day, whereas that of the cement mortar was only 33%. The maximum 28-day compressive strength (82.5 MPa) and flexural strength (9.28 MPa) were obtained for the GGBFS-based and MK-based mortars, respectively, which were about 29% and 41% higher than that of the counterpart Portland cement mortar. From an ecological point of view, geopolymer and alkali-activated mortars exhibited about 50% (on average) less carbon footprint and were 25% (on average) lower in energy demands. The findings highlight the superiority of alkali-activated and geopolymer mortars over Portland cement counterparts for lightweight applications due to their better workability, higher initial strength, and lower environmental impact.