what happens if you put too much cement in concrete

What Happens if You Put too Much Cement in Concrete?

Concrete is a versatile and powerful tool in many forms of construction. Its strength comes from a fine balance in the different components that make it up. Understanding these ratios and the value behind each of them allows you to create the most effective form of concrete, but what if you include too much cement?

Putting too much cement in concrete can result in several disadvantages. If too much is added to the mix, the concrete’s workability will suffer, and some of the aggregates won’t properly bond to the cement. If too much is used versus the aggregate, the final product’s structural integrity will likely decrease.

We will focus on the risks of using the wrong ratios for making concrete and what each of them could lead to. Careful attention should always be placed on how much of each of these elements are involved.

The Relationship Between Concrete and Cement

Though you are likely familiar with both cement and concrete, a quick refresher can help before we dive further into it.

In basic terms, concrete is the final product, while cement is just a component of the mixture that makes concrete.

Concrete is typically made up of four specific parts. The largest portion, around 60%–75%, is the aggregates (source). These aggregates will vary in size, based on the task the concrete is being used for.

The aggregate usually consists of rocks, sand, or gravel. The finer the aggregate used, the more malleable the wet concrete will be, and the smoother the final product.

The next part of the construction of concrete involves what is known as the paste. This comprises three primary elements: water, cement, and air. The ratio of this mixture defines much of the strength and durability of concrete when it sets. This will be the primary focus of this article.

According to the Portland Cement Association, the typical ratios for the different components of concrete are 60–75% aggregate, 7–15% cement, 14–21% water, and up to 8% air.

Making Concrete

To make concrete, you must combine each of the above parts together. The first step is to mix together the cement and water to form the paste. At this point, air can be added to the cement and water mixture. The air can be entrained in a number of ways, even just through mixing, with the primary purpose of increasing the workability of the concrete.

Air voids added to the mixture will also increase resistance to freeze-thaw cycles that may occur. However, the entrained air will also reduce the strength of concrete, so this must be done with care and caution (source). 

Once the cement and water have been combined, the hydration process will begin. Hydration happens when cement is introduced to water. Each of the cement particles will form a node that allows them to bond to the particles adjacent. This includes other cement particles but, most importantly, the various pieces of aggregate.

This is why you must mix it quickly with the aggregate before this bonding occurs and the cement starts to harden. Wait too long, and you could be stuck with hardened bags of concrete, though there are still options available to you should this happen.

Mixing is an important part of the process because it allows the cement to get into the spaces in between all the aggregate and bond everything together.

Once all these parts are combined, the result is concrete. However, understanding the efficacy of this concrete requires a look at the disadvantages of excess used in the ratios between the cement, water, and aggregate.

white concrete surface

Image via Shivanshu Guar via Unsplash

Adding Too Much Cement to The Mix

Since the paste that makes up concrete is such an important part of the process, adding too much of a specific element can have some significant downsides. 

It is important to note at this point that, ideally, you want to add as little water as possible to the paste mixture. Generally speaking, the greater the ratio of cement to water, the better the resulting paste will be for the concrete.

That being said, there are, obviously, downsides to adding too much cement to the paste mixture. Fortunately, the primary one is easy to spot immediately.

This primary disadvantage of adding too much cement to the paste mixture is based on the final consistency. More cement means that the paste will be less liquid and will struggle to properly flow. This means that when you combine it with the aggregate, the bonding between these two components does not properly occur.

In a good mixture of concrete, the paste will sufficiently encompass all the necessary gaps in between the larger, more porous, aggregate. When the paste is too thick from too much cement, holes can develop in the final mixture. This is called honeycombing and can result in some notable side effects (source). 

These pockets of air can create spaces for water to seep into the concrete, which, in turn, leads to deterioration over time. This happens during freeze-thaw cycles that cause the moisture trapped inside to expand and contract. This can create cracks on the surface and directly affect the structural integrity of the concrete.

Porous concrete is also susceptible to acidic and corrosive substances from the surrounding soil. This reacts with the cement, which contains a significant amount of water, and can cause further degradation (source). 

Adding Too Little Cement to the Mix

While we are looking at the negative effects of too much cement in concrete, it is certainly worth noting that the reverse is also true. This is particularly the case when too little cement is added to the paste mixture.

When cement and water mix together, they undergo a process called hydration. This results in the solidifying of the cement and the hardening of the concrete. If the ratio of water to cement is done correctly, this process will leave the concrete strong and durable.

However, if too little cement and too much water is involved, the hydration process leaves behind some excess water. 

This water will then stay inside the concrete and result in cavities within the final product as the water evaporates. These holes weaken the structural integrity of concrete and reduce its compressive strength, which is one of the primary attributes sought when using concrete (source).

Another downside of too much water in the paste is shrinkage. Cement will always undergo some form of mild shrinkage as the hydration process occurs. If there is too much water, however, this shrinkage can be excessive as the remaining water evaporates. 

Significant shrinkage can lead to cracks forming in the concrete, weakening the tensile strength of the final product.

Excess water can also lead to a process known as dusting. This is when the paste allows the finer aggregate particles to float to the surface of the mixture. This will then settle there before the drying process is complete and leave a thin layer at the top. Again, this can affect the integrity of the end product.

As you can see, excess water has more disadvantages than excess cement in the paste mixture. This means that it is best to err on the side of cement when combining these two.

Too Much Paste in Proportion to Aggregate

Another consideration for the relative proportions of the components of concrete is paste versus aggregate. As you know, the paste consists of cement, water, and air. However, even if you get this ratio perfect, you must consider the amount of this paste you use for the aggregate you have.

When you use too much paste, the resulting mix will be easier to use and place. This is because it is more liquid and will flow into the necessary spaces required. While this is a benefit, there is also a significant drawback.

With less aggregate, there is less structural integrity, and the concrete is more likely to crack. This is especially the case under the extreme pressures concrete is often subjected to. These cracks can lead to further degradation and open spaces in the concrete that can fall victim to the freeze-thaw problems mentioned above.

Another, perhaps less serious factor to consider with this ratio is the costs involved. Cement is the most expensive component of the concrete mix, and using more than you have to will incur unnecessary costs. While this can be minimal, the more concrete you require for your project, the more this stacks up.

Creating and working with concrete is a balancing act and requires dedication and understanding. Utilize these warning signs to take note of potentially incorrect proportions in your concrete mix and adjust accordingly.

white and black marble floor

Image by Vic Shen via Unsplash

Final Thoughts

While there are not many components involved in the construction of concrete, each of them requires deliberation and understanding to get right. This is particularly the case with how much cement is used in the process, as this is the most reactive and important part of the entire operation.

Too much cement in the paste can lead to improper bonding of the aggregate and too little risks even worse due to the excess water. Too much paste can lead to a decrease in the integrity of the concrete and hike up the price of the project due to the relative costs of cement.

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