Modifying the Properties of Construction Mixtures Containing Crushed Concrete Waste for 3D Printing

The paper presents the results of a study focused on technological parameters that ensure the effectiveness of using concrete powders obtained from crushed concrete waste as an active mineral additive in construction mixtures used for 3D printing. The efficiency of using a complex polyfunctional additive to cement pastes and mortars based on it is experimentally demonstrated. This additive includes a naphthalene–formaldehyde-based superplasticizer and a water-retaining additive, hydroxyethyl methylcellulose. Using the mathematical experiment planning methodology, polynomial models of the cement pastes and mortars’ mechanical properties were obtained. The models showed a positive effect of the complex additive on the cement pastes’ normal consistency and viscosity. Additionally, the results of the study demonstrate the possibility of regulating the cement pastes’ setting time and the plastic strength of mortars based on them using a complex additive. Analysis of the experimental–statistical models shows that using the complex additive allows regulation of water separation as well as the compressive and flexural strength of cement–sand mortars based on the investigated cement pastes within the required limits. Improving the key properties of building mixtures containing crushed concrete waste for 3D printing using complex polyfunctional modifier additives opens up new opportunities for increasing their economic and environmental efficiency.