Sustainable Concrete

Sustainable Concrete
Name
University
Abstract
The research on sustainable materials has mainly been based on wastes from industrial processes and recyclable materials and how they can be used in construction. The primary objective has been to ensure the utilization of materials that ensure the preservation of the environment, save materials and ensure that the materials used are durable. Some of the cementitious components used are rice husk ash, silica fume and fly ash and have been shown to make concrete more durable. Sustainable concrete requires little energy, leads to minimize waste production, is made from materials that exist in abundance in nature, is made with recycled or recyclable materials, contains high thermal mass and should ensure the durability of structures. Structures made from sustainable concrete have minimal effects on the environment during its usage and its lifecycle. Sustainable concrete takes into account the short-term and long-term effects on the environment. Generally, the addition of fly ash to a certain degree increases the C-S-H crystals while reducing the porosity at the interface of the concrete
Keywords: Sustainable, Fly ash, Concrete, Microstructure
Sustainable Concrete
Increase in human population over the years has led to concurrent growth in physical and infrastructural developments. Concrete is the most popular building material that is primarily based on the use of cement and it has been one of the major components used in this development as it is one of the major construction materials During concrete’s manufacture, a lot of energy is used up thus making it an energy-intensive process.

In fact, the production of cement contributes 5% to the global carbon footprint, making it highly undesirable due to its negative effects on the environment. One of the components used to prepare concrete has traditionally been Portland cement extracted from limestone. This has had various impacts on the environment such as the release of CO2 and other greenhouse gases into the atmosphere leading to pollution. In addition, depletion of natural resources from which the limestone is extracted has led material researchers in the construction field to research on alternative and supplementary materials that can be used sustainably in concrete production. This has led to experimentations that have resulted in the use of fly ash as a sustainable means to increase the strength of the concrete as well as decreasing its effects on the environment. Research shows that the use of 1000 pounds of fly ash can save enough energy that can power the average person for 12 days (“Fly Ash Concrete”, 2018).
Effect of using fly ash on the concrete microstructure

Fly ash is a powdered substance that is a byproduct obtained from burning coal and is a pozzolan that forms a binder material when mixed with water. It can replace as much as 35% of the cementitious material in a variety of concrete mix designs. The process has been utilized in a plethora of large projects for many years and currently, smaller local suppliers of concrete are beginning to understand its importance. Various tests replacing cement at different percentages with fly ash are carried out to show how the concrete microstructure changes. Addition of fly ash to a certain degree increases the C-S-H crystals while reducing the porosity at the interface of the concrete. The number of pozzolanic C-S-H crystals increases as the amount of fly ash added is increased but the number decreases when the fly ash is used to replace more than 50 percent of the cement. In order to obtain the best result and get concrete of high strength, fly ash should be used to replace the cement to around 30 percent content but varies depending on the type and source of the fly ash.
In a sample of pure Portland cement concrete the cement paste between the aggregates mainly has big calcium hydroxide (CH) crystal existing as blocks, CH plates intertwined with calcium silicate hydrate(C-S-H) and thin plates of calcium hydroxide (CH) fill up the big porous spaces in the paste. When fly ash is added in the range of 15 to 30 percent replacement of cement little effect on the microstructure is observed — however the addition of large contents of the fly ash of around 50 to 70 percent results to the reduction of the size and the amount of CH crystals. The big CH crystals are replaced by hexagonal plates that are thin and are either stacked in small groups or exist as single plates and are separated by fly ash, C-S-H and ettringite. Addition of fly ash results in fly ash particles occupying the spaces between the clinker particles and the aggregates leaving little space for the calcium hydroxide crystals to grow.
The strength of concrete increases with the addition of fly ash with the compressive strength being highest with 30 percent fly ash cement replacement and tensile strength at 15 percent fly ash cement replacement. Tensile strength is highest at this level since not much pozzolanic reaction occurs and a large number of CH crystals exist at the interface region. Compressive strength was highest at this level since the number of C-H-S crystals increases. Also the volume porosity of the concrete decreases too as a result of filling of the pores and refinement of the grains and also the formation of more hydration products.

Image for the microstructure of concrete having fly ash
The microstructure of cement paste

In the hydration process of cement, the hydration products are intertwined with one another but the calcium silicate hydrate(C-S-H) phase is formed on the boundaries of the initial cement grains(Thomas & Jennings, n.d.). The cement paste microstructure contains three phases:
Hydrated cement particles: They contain C-S-H of high density and in some instances unhydrated cement as the interior core. They act as single solid particles in a continuous matrix. The features are also referred to as phonograms since they are clearly visible in a microscope.
Outer hydration product: It is a continuous phase and occurs within the capillary pore space and acts as a binder to the cement. It contains solid C-H-S gel, calcium hydroxide (CH), gel pores and calcium sulfoaluminate phases. In an electron or optical microscope the phase is seen as a shade of grey and is at times called “groundmass”. The low-density C-S-H gel and its gel pore is the most crucial phase since it has a big surface area that gives the phase strength.
Large pores: they contain true capillary pores, air voids and entrained air system. In a microscope they appear as distinct black voids. The pore system can either be continuous or discontinuous and this is dependent on the extent of hydration and the starting water-cement ratio although this is not visible in a microscope.

lefttopBackscattered SEM image of a mature cement paste showing the main microstructural features. (Image courtesy of Paul Stutzmann, Concrete Microscopy Library).
Effect of using fly ash on cement paste microstructure
Addition of fly ash to cement produces a different microstructure of the cement paste as compared to the Portland cement paste microstructure. Addition of small amounts of fly ash has minimal changes on original Portland cement paste which is characterized by very big Ca(OH)2 (CH) crystals and porous mass of calcium silicate hydrate (CSH) and monosulphate(Papadakis, Pedersen & Lindgreen, 1999). However, an addition of high amounts of fly ash produces a microstructure with smaller and low amounts of CH. This is since the fly ash occupies the space that the CH crystals would grow. A dense structure of CSH is formed and few amounts of unreacted fly ash. The fly ash cement paste is characterized by smooth particles and their pore spaces are filled. However, both the pure cement paste and the cement fly ash paste are characterized by the presence of pores along the particle boundaries.

Low calcium fly ash cement paste high calcium fly ash cement paste
Predicting the microstructure of fly ash paste when cement is 100 percent replaced with fly ash
It can be predicted that 100 percent replacement of cement with fly ash to form fly ash paste would result in a fly ash paste with a microstructure comprising of spherical particles of fly ash and little particles of the CH hydroxide and the C-H-S crystals. This is because fly ash as a binder material gains strength slowly and its strength only increases as the age increases, hence its particles which are spherical in shape would be readily available in its microstructure. The CH and C-H-S crystals would be little since the hydration process occurs slowly and the fly ash particle occupies the space that would be occupied by the CH and C-H-S crystals. The paste would also be quite porous due to the voids left by the spherical fly ash particles.

Image for the microstructure of fly ash
The use of various recycled and recyclable materials to produce sustainable concrete is quite useful in reducing environmental pollution and ensuring durability (Naik, 2008) of resources to guarantee continued infrastructural development with the ever-increasing human population. More research needs to be carried out in order to obtain more materials that can be used to replace Portland cement in the construction process. Also the right proportions in which the researched materials can be used to replace cement need to be determined so as to obtain concrete having similar properties to concrete prepared using pure cement or even produce concrete having superior properties. This would ensure sustainability in the construction industry with both short-term and long-term effects on the environment accounted for in the design processes.

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References
Fly Ash Concrete. (2018). Greeneducationfoundation.org. Retrieved 10 December 2018, from http://www.greeneducationfoundation.org/green-building-program-sub/learn-about-green-building/1229-fly-ash-concrete.htmlNaik, T. (2008). Retrieved from https://ascelibrary.org/doi/10.1061/%28ASCE%291084-0680%282008%2913%3A2%2898%29Papadakis, V., Pedersen, E., & Lindgreen, H. (1999). Journal Of Materials Science, 34(4), 683-690. doi: 10.1023/a:1004500324744
Thomas, J., & Jennings, H. (N.d). 5.5.1 – General Features of the Cement Paste Microstructure. Retrieved from http://iti.northwestern.edu/cement/monograph/Monograph5_5_1.html