Abstract
The world is moving towards more sustainable ways of doing things, including the construction industry. Most construction methods are carried out using Ordinary Portland Cement (OPC) as a binder in mortars and concretes. Cement has proven to be a great binder over hundreds of years in developing structures. However, its production pose serious effects on the environment. The production of cement uses a considerable amount of fuel well as raw materials such as clay and limestone. The main concern with the usage of OPC cement is its contibution to the global carbon emissions, which is a whooping 8%. This number may continue rising as the use of cement is increasing in demand due to the rapid rate of urbanisation. Urbanisation is happening at a rapid rate in South Africa and there is a need for construction methods to keep up with the demand. Additionally, there is national housing backlog where the low income cannot acquire adequate housing as part of a human being’s basic needs. Traditional construction methods do not produce infrastructure at the rate at which housing is in demand.
Geopolymer cement have been introduced as one of the ways to limit or reduce the use of OPC. 3D printing technology was developed as it allows for complex geometry and reduces the impact of structural environment and also allows for faster construction. Thus, the explore on ways to minimise the use of cement, which in turn reduce carbon emission, and ways to overcome housing backlog, are currently on the rise all over the world and in South Africa, respectively. The use of Geopolymer concrete and 3D printing technique, may be one of the ways to achieve sustainability, as they are both environmentally friendly.
The current dissertation represents the study on fly ash geopolymer mortar (fresh, hardened and durability properties), and the effect of material composition on the 3D printing properties. Material compositions were optimised by varying different material mixes to study the printable properties. Studied 3D printing properties were extrudability, buildability and open time. This study carried out the following tests; fresh properties were flowability and setting time, hardened properties was compressive strength, and durability studies were drying shrinkage, alkali-silica reaction and resistance to sulphate attack. The tests were undertaken by varying different material contents. Fly ash was replaced with 10% OPC to control the setting time, reduce workability and enhance compressive strength. Activator to binder ratios (alkaline activator content) of 0.4, 0.45 and 0.5 were adopted, as well as sodium silicate to sodium hydroxide ratios of 1, 1.5, 2 and 2.5. An effort to use waste products as an aggregate replacement was considered, where glass waste was used to replace 100% silica sand as an aggregate in the FA-based geopolymer mortar. The studied parameters that were considered to analyse the chosen properties were; effect of activator content, effect of alkaline solutions ratio, effect of OPC and effect of using glass waste aggregate as an aggregate.
The results showed that glass waste aggregate peformed poorly in workability, compressive strength, ASR and sulphate attach resistance. Glass waste however showed great performance with respect to drying shrinkage. The obtained results showed an effect of studied parameters on the studied properties. Workability of the geopolymer specimens increased with an increase in activator to binder ratio and decreased with an inrease in sodium silicate solution to sodium hydroxide solutio ratio, replacing silica sand with glass waste and using 10% OPC. Specimens prepared with a sodium silicate to sodium hydroxide ratio of 1 experienced flash set and were thus withdrawn from the remainder of the test properties. Setting time undertaken on geopolymer sampleas prepared with silica sand showed an increase with an increase in activator to binder ratio and soidum silicate to sodium hydroxide solution ratio. The setting time was also observed to decrease with an inclusion of 10% OPC.
The compressive strength of the fly ash geopolymer specimens showed a decreasing trend as the activator to binder ratio increased. Replacing silica sand with glass waste sand as an aggregate reduced compressive by up to 55%. However, a 10% OPC inclusion increased compressive strength in both aggregate systems. A maximum compressive strength of 49.6 MPa was recorded for geopolymer specimen prepared with fly ash, 10% OPC, silica sand as an aggregate, activator to binder ratio of 0.4 and sodium silicate solution to sodium hydroxide solution of 1.5. Durability tests undertaken show that glass waste sand specimens exhibited up to 30% lower drying shrinkage compared to those prepared with silica sand. However, glass waste sand specimens had very poor resistance to alkali silica reaction and sulphate attack resistance. In the final analysis, results showed that geopolymer mortar with silica sand can attain compressive strength that is comperable to that of cement morta, whereas glass waste specimens reported very low results of about 55% compared to those of silica sand specimens.
Material optimisation of 3D printable geopolymer materials showed that different properties affect the printability of the mix. A parametric study was done on the printability of the geopolymer mortars. The results showed that a mix with excellent extrudability had weak buildability and vice versa. Another important determining factor was the 3DP pump system used, which was believed to have played a bigger role in the printabilioty of the mix. A suitable geopolymer mix needs to allow for printed layers to support subsequent layers, as the geopolymer mortar mix is quite viscous due to the alklaine solution. Results showed that further experiments are required to obtain a 3D pritabe mix, and most importantly a suitable 3D pumping system suitable for geopolymer mortar mixes.