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Eingeladener Vortrag
- nein (60)
Todays superplasticizers for self-compacting concrete and high Performance cementitious materials are very versatile. Typically superplasticizers are composed of a polycarboxylic backbone equipped with polyethylene oxide graft chains of variable length and grafting degree. The mode of operation of these admixtures is based on the steric repulsion of the particles upon adsorption. The adsorption is strongly depending upon the charge density of a superplasticizer and the time dependent Adsorption processes control the retention of the flow performance – an important issue particularly for ready-mix and construction site concrete. However, the properties of polycarboxylic superplasticizers typically remain a black box for users. The paper suggests a simple and rapid test method, which can be conducted without sophisticated equipment, to qualitatively distinguish between two types of superplasticizers based on the adsorption mechanism. Interpreting the results in the right way provides a powerful tool to choose the right admixture for individual time dependent flow specifications.
The existing cement and concrete standards are not capable of making full use of the current technology capacity due to strong focus on conventional concrete and thus they are not fit for the current and future challenges of construction industry. The paper highlights shortcomings with regard to the implementation of the existing standards. It can be seen that future-oriented standards are generally required to contribute to a lower-carbon footprint of the industry. These changes are significantly more relevant in sub-Saharan Africa, due to the rapidly increasing urbanisation challenge and the enormous potentials to develop lower-carbon technologies than elsewhere in the world.
Organic/inorganic mixtures were prepared from ordinary Portland cement (OPC), water (w/c 0.22), a fluorescent dye in aqueous solution (stable at alkaline pH; BAM-I), and two different comb shape polycarboxylates (PCEs), i.e., high charge (PCE-HC) and low charge (PCE-LC), respectively. Rheological and calorimetric measurements were performed prior to optical studies in order to select PCE concentrations. Absorption and fluorescence spectroscopy of the system OPC + BAM-I (CBAM-I) revealed maxima of dye BAM-I located at 645 nm and 663 nm, respectively. In presence of PCE-HC and PCE-LC, these mixtures displayed a small red shift in reflectance and a faster decrease in intensity compared to studies with CBAM-I; however, only slight differences were observed between the different PCEs. With time, all systems exhibited a decrease in intensity of BAM-I in absorption/reflectance and emission. This could be caused by dye adsorption and possibly decomposition when in contact with cement particles or hydration products.
Organic/inorganic mixtures were prepared from ordinary Portland cement (OPC), water (w/c 0.22), a fluorescent dye in aqueous solution (stable at alkaline pH; BAM-I), and two different comb shape polycarboxylates (PCEs), i.e., high charge (PCE-HC) and low charge (PCE-LC), respectively. Rheological and calorimetric measurements were performed prior to optical studies in order to select PCE concentrations. Absorption and fluorescence spectroscopy of the system OPC + BAM-I (CBAM-I) revealed maxima of dye BAM-I located at 645 nm and 663 nm, respectively. In presence of PCE-HC and PCE-LC, these mixtures displayed a small red shift in reflectance and a faster decrease in intensity compared to studies with CBAM-I; however, only slight differences were observed between the different PCEs. With time, all systems exhibited a decrease in intensity of BAM-I in absorption/reflectance and emission. This could be caused by dye adsorption and possibly decomposition when in contact with cement particles or hydration products.
Rwanda is a landlocked country in the East Africa. It is surrounded by Uganda, Tanzania, Congo and Burundi. Rwanda is a fast developing country and it spends most of its revenues to import fossil fuels from either through Mombasa Port in Kenya or Dar es Salaam port in Tanzania because the energy production in Rwanda is not sufficient for its development.
Transporting the fuels from these ports, add on to the cost of all materials, cement industry being no exception. There are three cement companies in Rwanda. The cement industries could not run in full production due to the shortage of fuel. Moreover, Rwanda is importing all the construction materials such as steel, roofing materials, etc from its neighbouring countries. This increases the cost of construction and the common man find difficult to own a house. In order to sustain the energy needs of Rwanda, different sources of energies should be focused. They are Peat, Geothermal, Methane gas, solar, wind, waste materials and Municipal wastes. Without affecting the environment, there is an urgent need to find a solution on sustainable energy in Rwanda. This paper discusses about the possible sources of energy in Rwanda which will improve the energy sustainability and turn the economy of Rwanda.
South Africa and recently Nigeria are the only net exporter of cement in Africa, yet cements are imported into the country mainly on the basis of price competitiveness. This poses potential for scatter in the properties of the cements in the South African market as pricing seems to be the only determining factor that affects Portland cements imported. A survey of cements of grade 42.5N in the South African cement market was carried out to identify major players in the industry. The identified cements, which included four locally produced and one imported cement were then analysed for both chemical composition and physical properties in line with the Eurocode EN 196. Tests carried out include loss on ignition, chloride content, sulphate content, specific surface (Blaine), standard consistence, initial setting time, final setting time, soundness and flexural and compressive strength at 2, 7 and 28 days. The testing program was initiated as part of on-going Africa-wide cement testing competency program in partnership with BAM and PTB in Germany. The performance of the identified cements from the different local manufacturers and the imported cement were analysed and are presented in this paper. These results reveal some similarities and differences in the properties. Particularly of interest is the marked difference in the 28 days compressive strength of the cements. This paper shows that there is need for further tests across the cement industry as some of the cements failed to meet certain requirement as set by EN 196. The paper also recommends the use of proficiency testing schemes in the cement industry of the country to ensure the cement laboratories are providing results of high quality and at the same time act as a check to make sure these laboratories are not failing to meet standard requirements.
In many regions of the world the urbanisation process is accelerating dramatically. This puts pressure on urban planners but also politics to develop strategies for sustainable city growth. With the rapidly increasing demand for living space in urban areas, cities typically grow vertically. This is largely driven by real estate markets and sometimes also by the desire for status symbols.
Certainly, vertical urban growth makes sense, when horizontal growth destroys important flora and Fauna (e.g. in rain forest regions), but in many cases vertical growth is result of real-estate business and Expansion limitation due to state or country borders. However, economics and borders are made by humans. They follow human-made rules. Gravity does not. Therefore, from a point of view of sustainable materials and resourceuse, the trending vertical growth of cities may come under scrutiny.
The following aspects should be considered, when a decision is taken between a new quarter with limited number of storeys or a new skyscraper.