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Urbanisation, habitat, environment, infrastructure and sustainability are major global challenges of the 21st century. By planning, exploiting of resources, building, and maintaining, civil engineers and relevant adjacent disciplines have been carrying a large responsibility for the existing environmental problems. Civil engineers are responsible for 70% of all material uses in the world, and civil construction has been dominating the growth of the developing world for the next decades, with enormous impact on the global climate as well as the distribution of wealth and quality of living in the world.
Today a variety of sustainable construction concepts have been developed and discussed. Recently a UNEP report was published, which provides a comprehensive overview of the challenges and potentials in the future from a scientific and industrial point of view. Sustainable solutions based on abundantly available resources (pozzolana and clay) or on renewable instead of limited industrial by-products (e.g. agricultural waste ashes) and new construction echnologies do exist. By applying, promoting and developing the implementation of the existing knowledge into real life constructions, civil engineers also hold the key for the mitigation of the global challenges. Using best practice sustainable construction solutions is particularly of highest importance in the developing world, since the implementation at an early stage creates the highest leverage for positive effects.
Why Africa can spearhead innovative and sustainable cement and concrete technologies globally
(2016)
The perception of concrete in the society as well as in the politics is rather negative. This becomes obvious in the fact that the phrase “concrete jungle” has become synonym for hapless living with no perspectives. In politics and research funding, it is also not easy to create a broader audience, since concrete is falsely considered as old-fashioned material that is sufficiently understood today and does not need further considerations, particularly compared to allegedly newer materials. However, particularly since the last two decades the technology has completely changed. Binders of today are no more the same binders as used before, and concrete mixture compositions of today diverge quite significantly from compositions in the past. There is little understanding world-wide about that. This causes that potentials the concrete technology bears are wasted.
In the broadly found opinion that concrete is old-fashioned and ugly, it is ignored that architectural sins are not inherent to the material, which actually is extremely versatile and CO2-friendly compared to all other construction materials available. It is also ignored that 98% of the outer Earth’s crust are made of the elements cement and concrete are made from, and therefore it will be an illusion to believe that the complementary 2% can create materials to develop regions and infrastructures in less developed areas in the world.
For betterment in Africa the infrastructural development should have highest priority, since poor connections between settlements are responsible for enormous Price increases [2], and urban traffic congestion is responsible for an incredible loss of productivity. It is not unrealistic to assume that earners that are dependent on a car get stuck in traffic about 3-4 hours per day in cities like Lagos, Nairobi or Dar es Salaam. However, the traffic congestions do not only affect the car owners negatively but the living of the entire urban population every day.
Besides infrastructure, housing should be the other priority, since a large part of the African population does not live in adequate condition. This is a societal problem, since unequal Distribution of wealth is a major driving force for instability in societies. The latter has a global impact, since 8 many phenomena that can be observed all over the world such as political radicalism, xenophobia, terrorism, and migration can often be linked to instable societies.
However, the importance of infrastructure has an even wider range. Most African countries go through a change process recently. In order to strengthen very positive perspectives, the focus in politics and research funding is put on issues such as agriculture, energy, and health, which are without doubt extremely important issues. However, it is typically overlooked that all enhancements in these areas can only become effective, when an infrastructure is created to support the implementation of better concepts. Mobility is the key to a prosperous future, and mobility can only be granted by infrastructural construction activities. Hence, compared to many other regions in the world, cement and concrete technologies have a significantly higher relevance in Africa.
Concrete technology was exposed to a rapid development during the last three decades. For the longest time in its history, concrete was considered as a three component System consisting of aggregates, which are bound by the hardened cement paste consisting of hydrated cement. Traditionally, the only way of adjusting the consistency of concrete was using well adjusted aggregates and grading curves and adding excess water to the concrete, accepting that the latter in return reduces strength and durability. During the last three decades, however, concrete has developed further from a three component System towards an (at least) five component system, since the use of mineral additions and Chemical admixtures has become state of the art. Both components are able to enhance the workability, the compactability, and the density of the microstructure with effects on strength, ductility and durability, while cement can be saved in parallel. Due to reasonable use of admixtures and additions, concrete can be designed to match mechanically high performance specifications. Traditionally, cement paste was considered the weakest component in concrete. Flowever, in modern concrete a good paste composition can yield highest performance, passing the role of the mechanical bottleneck towards the aggregates.
As soon as cement is mixed with water, the hydration reaction starts to set of. The progression of the hydration process is governed by the amount and the availability of water and its possibility to access the unhydrated cement grain surfaces. For a total hydration cement theoretically requires water amounting approximately 0.38 of its own mass. Nevertheless, due to the cement’s highly hygroscopic nature, already the moisture in the ambient air can cause first hydration reactions to set in upon the cement grain surfaces. Such pre-hydration processes have an effect on the cement's properties. Dpending on the specific conditions during the storage the significance of the impact on certain properties can vary.
If cement is consumed soon after production and is transported only short distances in a silo wagon to its final destination for immediate use without further stock transfer, the effects are rather negligible. However, if cement is delivered in bags, transported over long distances, shifted serval times and stored for longer time periods until it is finally consumend, these effects can be quite severe. Due to the actual logistical situation with comparably few cement plants, large delivery distances and partially less developed infrastructures, the latter scenario pictures the situation for the most cases in Africa quite well.
In order to verify the relevance of the pre-hydation for practical application, a series of tests was conducted at the German Federal Institute for Materials Research and Testing (BAM). The influence of the cement storage on gresh and hardened concrete properties was investigated for different concrete and mortar types. The investigations show that the impact of the storage conditions is more pronounced for specialized concretes with high sophisticated optimized mixture compositions containing admixtures. Nevertheless, the effects also occur for ordinary concrete and should not be ignored.
Chemical admixtures allow to the create a variety of fresh and hardened state properties in cementitious materials. In the case of self-compacting concrete, plasticizers or superplasticizers are introduced with the aim to decrease the yield stress and the viscosity of the materials. However, in order to prevent segregation and bleeding, and to improve the water retention of cement-based system, stabilizing agents or viscosity agents are often introduced in addition. Among these admixtures, polysaccharides are most commonly used.
The aim of this study is to provide an understanding of competitive or synergetic effects induced by the combination of hydroxypropyl guar stabilizing agents (HPG) and superplasticizers on cement pastes. Two polycarboxylate superplasticizers (PCE) exhibiting different charge densities and one HPG were studied. It was found that the combination of HPG with PCE superplasticizer strongly affects the rheological behavior of cement pastes. Despite the presence of HPG, the viscosity of the pastes strongly decreased with increasing dosage of PCE until it became close to the viscosity of a cement paste with PCE only. However, the use of HPG in combination with PCE allows maintaining a significant yield stress in the cement paste compared to PCE alone. The increase in the charge density of the PCE seemed to amplify the drop of the viscosity and to reduce the gain on the yield stress induced by HPG.
The results also highlight a delay in the setting-time of the cement paste by adding HPG and PCE. The delay induced by HPG is significantly lower than that generated by PCEs. However, the combination of the both kind of admixtures leads to a slightly shorter setting-time compared to the PCE alone.
Potentials for sustainable cement and concrete technologies - Comparison between Africa and Europe
(2017)
The fundamental knowledge about cement and concrete has made enormous progress over the last decades, and today it would be possible to find optimised sustainable concrete solutions tailored for every given boundary framework and raw material supply. However, this knowledge barely finds implementation into practice despite the urgent global need to minimise carbon emissions and energy consumption. A major reason is that most concrete developments were historically made in the northern hemisphere, where today over-regulations and stagnating market perspectives slow down innovation drive towards higher sustainability. In most African countries, however, sustainable building is simply an urgent real-life problem. The demand for building is enormous, Standard solutions are not an option, and the pool of innovative local raw materials and concrete concepts is enormous. The paper provides a comprehensive comparison between the boundary frameworks of Europe and Africa, and it is explained why local African solutions shall be given priority over imported solutions. Examples of local African concrete solutions are given, and ideas for a rapid implementation are developed. Most of the potentially useful materials such as agricultural ashes, natural and calcined pozzolans, polysaccharides, etc. have not yet been subject to intensive research to date. Therefore, it is not unlikely to assume that with an open mind for non-Standard solutions, combined with creativity and particularly knowledge and awareness, the next generation of innovative and sustainable concretes will be developed and applied on the African continent. Therefore, the conclusion is that particularly the African continent provides the best starting position to develop better and more sustainable concrete solutions than anywhere else in the world. Hence, Africa can become a global pioneer in green cement and concrete technology with impact to the entire world.
The paper showed that the STA behave completely different in pastes with LSF and cement, respectively, as well as with or without PCE. While STA can affect plastic viscosity and yield stress in LSF and cement pastes, the addition of PCE causes that the STA have mainly an impact on plastic viscosity, except DGUM and ST-low. The performance of STA in LSF and cementitious systems can vary greatly due to different ionic strengths of the liquid phase. The charges due to modification of the starches have an intense influence on the rheological properties in pastes. The combined use of PCE and STA can lead to a significant increase of the plastic viscosity without a strong effect to the yield stress.
For the evaluation of the effectivity of STA the ionic strength of the liquid phase and the solid volume fraction of the binder have to be taken into account. Further influences can arise from grain size and aggregate content as well as the temperature, which were not discussed in this paper.
Today, chemical admixtures like superplasticisers and stabilising agents are extremely important for modern concrete technology. These agents have meanwhile become common practice in concrete technology, but the understanding within the entire system lags far behind their application. The macroscopic rheology of concrete in the presence of superplasticizers strongly depends upon effects on a much smaller scale such as the hydration of the cement, the adsorption of superplasticizers, and the pore solution chemistry.
Chemical admixtures like superplasticisers or stabilising agents are of ever increasing importance for modern concrete technology. They liberate the workability of concrete from its dependency on water content, and thus, open the gate towards innovative and future oriented concrete technologies such as self-compacting concrete. Today, admixture addition has become common practice in concrete technology, but the understanding of their highly complex mode of operation is extremely difficult and demands for understanding of processes within the range between nanometres and centimetres. Due to its complex time-dependent, multi-phase and multi-scale behaviour, flowable concrete systems are highly complicated and cannot be described comprehensively by simple models. It is therefore extremely challenging to identify the relevant parameters that predominantly control flow phenomena on different size scales, since these may occur on any scale between the nano scale (e.g. superplasticizer adsorption) and macro scale (e.g. grading of the aggregates). The present study discusses fundamental mechanisms at the interface between particle or hydrate surfaces and the fluid phase at a very early stage of concrete formation, and links these effects to macroscopic flow phenomena. Methods are discussed that appear promising interdisciplinary tools for enhancement of the understanding of the relevant interactions that are responsible for the macroscopic flow of flowable concrete.