Fresh or Salty
If sea water is added to cement instead of freshwater then the resulting concrete will still retain its usability because the salt will only make the concrete slightly denser and will not have any noticealbe affect on the concrete.
What is Concrete?
Concrete is a substance that when initially made is very malleable and will fit into almost any form but when dry will be hard and solid. The density of concrete is approximately 2400 kilograms per cubic meter or 150 pounds per cubic foot. It is made with three main ingredients: water, cement, and aggregate which is sand, gravel or rock. Aggregate makes up 60% to 80% of the concrete mix and is used because it reduces the cost since cement is the most expensive and important ingredient within a concrete mixture. Aggregate is divided into two categories: coarse and fine and is typically created by crushing rocks to the corresponding size. Coarse aggregates are larger than 5mm and are used when making larger structures because it reduces cost due to it having to be processed less. Fine aggregates are normally used when making a smaller structure and you want a smooth finish however one of the drawbacks is that it is more expensive. A second ingredient that you can not make concrete without is water. There is not a specific amount of water that is used in a mixture. This is due to the fact that the less water you use, the stronger the concrete will be because excess water creates voids in the structure making it weaker. However consumers do need to use enough to affect all the cement powder and for the wet mixture to go into the shape that is required, as workability is one of concrete’s greatest strengths. After being added, water turns the cement into a paste that then attaches to the aggregate and hardens when it dries. It usually takes 24 to 48 hours to set and takes about 28 days for it to be mostly cured which is about 90% of its strength as concrete will continue to harden forever. This is the concrete that is a staple of the modern world but in different forms it has actually existed for a long time.
What is Cement’s History?
Different civilizations have made very primitive versions of concrete with burned limestone making up the cement powder component. Water and sand, as an aggregate, were added to it and the paste was used to form a protective coating on buildings. However it wasn't until the year 1793 that the first version of concrete, as we think of it today, was invented. John Smeaton used limestone containing clay that he then fired until it turned into clinker, which is the unground version of cement powder. He then crushed it into powder to make cement before adding water and aggregates in order to create modern concrete. Eventually, in the year 1824, a man named Joseph Aspdin invented Portland cement by burning finely ground chalk and clay in a kiln until the calcium carbonate decomposed to form calcium oxide and carbon dioxide gas. It was named “Portland” cement because he thought the concrete made out of it resembled the high-quality building stones found within Portland, England. This is just how some of the first concrete was made as the techniques have become much more refined over time.
CaCO₃ → CaO + CO₂ Chemical equation of calcium carbonate decomposing
How is Cement Made Nowadays?
Before cement powder can be created specific ingredients have to be found in order to produce high quality cement powder. Cement powder is made from naturally occurring rocks and minerals such as limestone, shells of dead animals, chalk and sand which are all then refined. These rocks and minerals contain calcium, silicon, aluminum, and iron in various proportions. They are then crushed and fed into what is usually a very large kiln consisting of a long cylinder about 20 meters long and 2 meters high and wide. Once placed inside, the kiln is heated to around 1400C so that the material releases a large amount of carbon dioxide that was inside of the minerals. After that it is known as clinker and when it is crushed, it is mixed with small amounts of gypsum and limestone to prevent particles of the powder from sticking together and adhering to machinery. However, this process to make cement powder comes at a cost to our environment. The process releases a large amount of carbon dioxide into the environment and accounts for 5% of all carbon dioxide released to the atmosphere from the world. The carbon dioxide comes from two different parts of the operation of creating cement. When the kilns are heated it requires huge amounts of energy to get them up to the required temperature and when the materials are heated they release a lot of carbon dioxide. This cement powder is made so when transformed into concrete it can hold up to many different kinds of stress.
What Should Concrete Be Able To Do?
Concrete has a very high compressive strength which is a material’s strength against being pushed together but a low tensile strength which is being pulled apart. This is one of the main reasons metal reinforcements are added to concrete because metal has a high tensile strength but a low compressive strength (Practical.Engineering August 1, 2018). One of the most useful places for concrete is roads because if the ground under the road is solid, concrete will have to use little tensile strength. However concrete is used just about anywhere because of it’s high workability so salt water concrete should be able to withstand many different types of stress with the salt inside of it. These stresses include but are not limited to erosion by elements, freezing and thawing, tensile strength, compressive strength, and erosion by people and vehicles.
What is Salt?
Salt is made of the two elements sodium and chloride and it can be represented as NaCl. It has a pH of 7 meaning that it is not acidic or basic and it has 1.2g/cm³ as it’s density. The two most familiar types of salt are pure sodium chloride and sea salt which contains sodium chloride plus other minerals from the ocean. Some of the more common ones include Magnesium 1272 ppm, Sulfur 884 ppm and Calcium 400 ppm (ppm stands for parts per million). Salt is naturally found as a solid because it must be heated up to 801°C for its melting point and 1,465 °C for its boiling point. Salt is also very soluble in many different liquids and attracts moisture. That is why so much is dissolved in ocean water, for the majority of ocean water contains 3.5% salt on average. This salt comes from two main sources: runoff from rain water and openings in the sea floor. Rainwater is slightly acidic so it can erode rock over long periods of time and then carry the minerals it gets from the rock into rivers that flow into oceans. The second main way that salt gets into the ocean is from underwater eruptions that spew out minerals including salt. Once salt is in the ocean it can have many effects on different aspects of the world.
What are the Effects of Salt?
When salt is dissolved in water it makes the water denser because the salt goes in between the water molecules making it have a greater mass. In addition when adding salt to water it can change the melting and freezing point of the water. The reason this happens is because when the salt goes inside the water it makes it harder for the water to form the right shape for it to freeze and the salt particles block the water particles from coming together as easily and so salt lowers the freezing point of water. Additionally salt water will erode metal much faster than regular water because salt solution acts as an electrolyte which is any substance containing free ions that allows the substance to conduct electricity. This allows iron to lose electrons more easily and so greatly speeds up the rusting process allowing salt water to make metal corrode five times faster than water would make it corrode normally. All these factors could affect concrete because the salt could attract more moisture and in some weather regions it could make the freeze thaw cycle of water on top of concrete more sporadic. In addition any metal reinforcement inside salt water concrete will rust more easily than normal. Despite these disadvantages salt water concrete could still be very useful in the world.
Salt dissolving into water
Manipulated Variable: sea water or tap water in cement mixture
Responding Variables: compressive strength, tensile strength, permeability, freeze-thaw resistance and corrosion of metal in contact with the concrete samples
Controlled Variables: Amount of cement powder, amount of water, time for the concrete to set, size of the samples
Cement powder with sand included, Water, Aquarium salt,
Ice cube molds, Cookie sheets, non-coated nails,
Freezer, scale, oil spray, plastic wrap,
Containers, piece of wood, hammer, chisel,
Bricks, ruler, hand weights and voltmeter
Procedure - Making The Concrete Samples
Measure out 1500g of cement powder in a container and 225g of water in a separate container. If using salt in the mixture add 7g to the water and stir until there are no salt particles visible in the water. Then add the water mixture into the cement mixture and mix until there is no dry powder left. Take a cookie sheet that is 18 inches by 13 inches and take a piece of wood that is 18 inches long so that you can create an area that is 18 inches by 6.5 inches in the sheet. After that, line the area with plastic wrap, making sure that there are no wrinkles in it. Pour the mixture into the lined area and smooth it out so there are no bumps and it is all level. Let it sit for 48 hours before taking it out and then let it sit for an additional 28 days.
To make ice cubes spray ice cube tray with oil spray, use the same ratios as above but do 1000g of cement powder and add the cement mixture to a ice cube tray, ensuring each section is full. Scrape off any additional mixture. If making samples for the corrosion test add vertical nails into the center of 10 of the samples making sure that the head of the nail is 1 cm above the sample surface. Let the samples sit for 48 hours before turning it upside down and gently taping to get them out. Let the samples sit for 28 days before using them.
Outdoor/Indoor Corrosion test (using ice cube samples)
First measure the weight of all the samples that you are going to use in the test. Put eight samples on a plate with an additional two samples without nails for both salt and no salt as well as four nails that are not in concrete. Put these samples outside and keep the remaining samples inside with four nails not in concrete. Check frequently on these samples and record all your observations. When four months is reached bring the samples inside and weigh them all, as well as recording observations. Then using a hammer and a chisel crack the sample open down the middle recording anything that you find inside.
Compression strengh test (using cookie sheet samples)
To do the compressive test tape a measuring tape to a post that is straight up and down. Make sure 0 is at the very bottom of the post and the measuring tape is completely vertical. Take the piece of concrete that you plan to test and place the center of it under the tape measure. Then take something that weighs 2.25 kg and has a circular impact area with a diameter of 2.5cm. Place the weight at the 4cm mark and drop it onto the concrete holding it straight up and down. Record all observations and repeat the test until the sample breaks.
Tensile strengh test (using cookie sheet samples)
Set up two bricks that are the same size and space them apart so that the concrete sample is hanging on to each side by 5cm. Then fill a bucket that has a circular area with a diameter 20.5cm with 13.5 kg of weight. Start a timer as the bucket is placed onto the middle of the sample and at the 10 seconds mark place 2 kg of additional weight into the bucket. At each 10 s interval, add 2.25 kg of weight , and then at 60 s add another 1.1 kg if the sample has not yet broke. If the sample has not cracked at 70 s, add 6.8kg of weight and record the time the sample breaks at. If the sample has not broken at 15 minutes, end test as the sample did not break within the time limit.
Permeability and Freeze and Thaw test (using ice cube samples)
Take a styrofoam cup for every sample that you are using, label them and then fill them with 100g of water. Take the corresponding sample and place it in the cup recording all observations. After 24 hours soaking in water, take the samples out, weigh them and record observations. Test for salt in the water using a voltmeter set on ohms. Repeat this process at 48 hours and 72 hours. Then put the cups in the freezer for 24 hours, take them out for 6 hours, put them back in for 24 hours, take out for 6 hours, and then put them in the freezer for 24 hours again, recording all observations . Afterwards crack them open in the middle with a chisel and hammer and record all observations.
Once the test was finished both types of samples displayed large amounts of rust on any part of a nail that was exposed to the elements. The salt concrete samples were much more smooth than the no salt counterparts, which were full of small holes. When broken open they were both damp (dark) on the inside but displayed no signs of ice or cracks anywhere. The part of the nails that were unexposed had no rust.
There was only the slightest amount of rust found on any of the nail heads. However when broken open neither of the sample types seemed damp on the inside (lighter color) but on the salt samples there was a small amount of rust on the nail where it did not touch the air.
Tensile strength test
The samples remained stable and then would suddenly break with no warning down the middle into two pieces.
Absorption test/ Freeze thaw test
At the start there were many little bubbles clinging to the concrete and the cup. By hour 24 most of the bubbles were gone and by hour 48 all of them were gone. The concrete samples appeared to be coming apart a little in the water because there were what looked like very small concrete bits floating in the water. After the freezing you could see there was ice clinging to the side of all the samples but after breaking them open you could see that they were wet on the inside but there was only very small amounts of ice within. Additionally there was nothing different about the two different types of samples and there were no cracks or structural problems.
Before breaking the samples would show a thin crack and then break one or two times later on average. Typically when breaking the sample would split down the center into two pieces but sometimes it would break into multiple pieces, like a spider web pattern. The samples that did not do as well were from the bottom of the sample pile
Results - Compressive Testing
The salt water samples withstood more impacts on average than the fresh water samples before breaking.
Results - Tensile Testing
The salt water samples held more weight for longer periods of time than the fresh water samples.
Results - Permeability and Freeze/Thaw Cycle Testing
The salt water samples are less permeable than the fresh water samples. Both types of samples withstood the freeze thaw cycles.
In conclusion, the hypothesis was correct because the salt water concrete maintained the same strength as regular concrete so it would be practical to use in many places. In some of the tests, salt concrete even displayed more strength than the regular variants of concrete. This is most likely because the saltwater goes into the concrete and when it is curing, forms crystals that help it hold up to forces. The only experiment that salt proved to be worse in was the erosion/corrosion test in which it had more rust on it’s nails than the no salt samples. However this was only on some salt samples that were inside and that was believed to be because the nail was wet while the concrete was curing and the salt sped up the process of corrosion. This may make salt water concrete less practical when building structures such as bridges that need high tensile strength. Due to the fact that concrete does not normally have much tensile strength, it is typically reinforced with steel reinforcement which would rust too easily if using salt water concrete. However there could be many other applications in which the concrete could be useful, such as building roads or floors of houses that do not need high tensile strength. Something else that salt water concrete could have an advantage over no salt concrete is with the fact that the Salt concrete absorbed less water. This is significant because if water goes into concrete it will erode faster than concrete that absorbs less water, though this can be combated with a protective coating. Testing concrete is a hard process and there could have been errors with in the testing of the experiment.
Pros of Salt Water Concrete
Cons of Salt Water Concrete
-Stronger in tensile and compressive strength test
-Saves fresh water for other uses
-Can rust metal more quickly
Why is This Experiment Helpful in the World?
According to (concretehelper.com n.d section 1) “about 10 billion tons of concrete are produced every year” world wide. If taking into account that it takes about 20% water to create concrete there is around 2 trillion liters of fresh water used each year. There are several problems with having to use that much water to build with concrete. When workers first start on a project using concrete they must first get the water from somewhere. However if this project was in a third world country or in a drought they would not have a lot of water to work with and so they might not be able to do the project. If the workers did have enough water to use concrete in places with little water they would probably be taking the water from drinking water. You might be able to take water from rivers to do concrete but it has to be fairly clean and not all places have rivers close to them. Once the water is used it will take a long time for it to get back to the place it started at and some places that have a small supply of water might not be able to give up their water for a period of time as the water will have to be used for more important things. Even if it did go through the water cycle quickly there is no guarantee that the water will end up in the same place that needs water. If the water does come back to the place it started at, it still will need to be filtered before it can be used again. This takes both time and energy that some places don’t have, and it will further pollute our environment. If salt water was to be used in places near the coast where it is readily available then this has the potential to greatly help third world countries and places with little water in general. Ocean water will not deplete water used for drinking or other necessities and it could save both money and time with the fact that it will not have to be filtered to be used or after it is used. Overall salt water has the potential to help solve some major problems in society today.
Sources Of Error
There was multiple issues with in the testing process. One of the more prevalent problems was that the scale is not completely accurate so some of the weight tests could have been off. Additionally not all the concrete samples were made exactly the same way despite best efforts. When the concrete samples were taken out of the cookie sheets they were stacked on top of each other when not at their maximum strength, possibly making them lose strength as they cured and this could have led to the inconsistent results in the compressive strength test. In the erosion test the samples could not be placed in exactly the same spot and this could have led to slightly different results. This is seen in the fact that one inside nail out of all the other ones had rust on its head but none of the other ones had rust on the head of their nails. When doing the compression test it could not be guaranteed that all the drops were from exactly the same height. And the concrete was not completely flat so the impact area could have varied depending on how smooth the samples were. For the freeze test they should have been left in the freezer longer to see if ice would have developed. However despite all these inaccuracies, the evidence shows clearly that salt water concrete can be very beneficial in certain situations.
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Thank you to RT Alderman for giving me this opportunity to go to the CYSF as well as my parents who continue to support me with my projects.