Rheological and Thermal Characterization of 3D Printable Lightweight Cementitious Composites with Fly Ash Cenoshperes

The thermally poor design of residential and commercial buildings considerably increases heating and cooling demands which lead to an increase in energy consumption and carbon emissions. Recent advances in cement-based composites offer improved thermal properties and lower embodied energy and emission using functional fillers and supplemental cementitious materials. 3D printing of cement-based composites also allows structural members to be produced in complex shapes that are not possible to achieve with traditional methods. In 3D printed concrete structures, the inner and outer geometries of building walls can be optimized to receive, store and dissipate thermal energy depending on the needs of the building’s climate. Combining advantages offered by advanced materials and digital fabrication can improve energy use during the construction phase as well as the life cycle of a building.
Towards this goal, this thesis investigates the development of fly ash cenoshperes (FAC) based lightweight cementitious composites that can be used as both structural and nonstructural material in digital construction. FACs are hollow alumino-silicate spheric particles obtained as a by-product of a thermal power plant’s coal combustion. The hollow and air-filled nature of FACs can lead to a considerable reduction in thermal conductivity and density of cementitious composites. Portland cement, fine sand, silica fume, water, FACs, water reducer and viscosity modifying agent (VMA) are used to fabricate 3D printable lightweight cementitious composites. The effect of FACs to sand volume ratio and VMA to binder ratio on rheological, mechanical and thermal properties of developed composites are studied. Rheological tests are conducted using a shear rheometer. Yield stress and apparent viscosity of the FACs-based cement mixtures with four different FACs to sand volume ratios and four different VMA to binder ratios are determined by stress growth tests. Cubic specimens are prepared to determine the compressive strength of the developed composites. A heat flow meter technique is employed to determine the thermal conductivity of the developed composites with different FACs volume ratios. Prismatic specimens are cast and cured for 28 days for thermal tests. Printability of the most promising mixture proportion is assessed using a 3D concrete printer equipped with a screw pump and two different nozzles. Results show that thermal conductivity is inversely correlated with FACs content. By replacing 60% of sand with FACs and using VMA at 0.3% by weight of binder in a mortar composite, a printable lightweight cementitious composite with good thermal and mechanical properties can be obtained.