I have been watching a few videos demonstrating the mixing of mortar and cement and a constant theme is that you must not have the mix too runny.
I cannot fathom the reason... Does anyone care to elaborate?
My only guess is that a watery mix will have the layers settle in their different sedimentary layers with the fine cement settling to the bottom...
My only guess is that a watery mix will have the layers settle in their different sedimentary layers with the fine cement settling to the bottom...
One reason is that the aggregate can settle to the bottom much too easily, yes. This is called segregation and results in non-cohesiveness of concrete. Another reason too much water is bad is that it can result in porous concrete, especially at the surface.
"Concrete that is poured too wet will be weak regardless of how it is cured. Not all of the water that is added to the concrete mix is necessary for the hydration process. In fact, the amount of water required to completely hydrate the cement is only about one half to two thirds of what is usually added at the batch plant or on the job site. The rest is added strictly to make the mix more workable. Adding too much water, however, might save work during placement, but it will also result in very weak, porous concrete, even with proper curing." - source
Excerpt from a good article (read the whole article) describing this problem in good detail:
"When there is too much water in the concrete, there is greater shrinkage with the possibility for more cracks and reduced compressive strength. As a general rule, every additional inch of slump decreases strength by approximately 500 psi. So for example, if you ordered 5-inch slump concrete and received 7½ inches, a mix designed to be 4000 psi would end up being 2500 psi. This represents a serious loss in strength, especially if you were placing exterior concrete in a freeze/thaw climate where the standard requirement is 4000 psi for proper durability." - source
This is often the reason for the slump tests that are done on most large projects. It tests the workability, but also indirectly tests if there is too much water.
The reason is because of the hydration chemistry that takes place in the cement while it cures. When cement cures, it doesn't really "dry", the way a towel dries, but instead it reacts, the water ceases to be water and merges with the substances in the cement to form new molecules.
Having too much will disturb the chemical interactions taking place and will weaken the resulting crystal structures.
It's really not at all unlike baking. Too much or too little of certain ingredients will ruin the chemistry and ruin your bread.
Figure 5 shows the effect of porosity caused by excess water.
http://matse1.matse.illinois.edu/concrete/prin.html
I recommend reading the entire article.
"The strength of concrete is very much dependent upon the hydration reaction just discussed. Water plays a critical role, particularly the amount used. The strength of concrete increases when less water is used to make concrete. The hydration reaction itself consumes a specific amount of water. Concrete is actually mixed with more water than is needed for the hydration reactions. This extra water is added to give concrete sufficient workability. Flowing concrete is desired to achieve proper filling and composition of the forms. The water not consumed in the hydration reaction will remain in the microstructure pore space. These pores make the concrete weaker due to the lack of strength-forming calcium silicate hydrate bonds."
One of the measures used when specifying concrete is the water to cement ratio. If there is too much water, the chemical reaction (hydration) that "cures" the cement in the concrete is adversely affected if there is too much (or too little water) in the mix, leading to weaker concrete than if the optimum amount of water is used. Concrete companies measure the moisture content of their aggregates (the sand and stone) to ensure that they add just the right amount of water for each specified mix.
To adjust the workability (the slummp, measured in terms of the amount it "slumps" - or drops - when a cone shaped mound is removed) of concrete, water can be added (and this is often done "unofficially" on site to make life easier by site operatives), but to achieve the specified strength, the workability should be specified when ordereing concrete, as this allows the concrete company to compensate for the extra water by also adding more cement to the mix. For example concrete destined for pumping (which needs high worability to get through the pump) will have a lot higher slump than that used for a straight pour; to achieve the same compressive strength in the finished concrete, the pump mix will have a higher cement content than the mix used for a straight pour.
For concrete to be strong all gaps between the sand and stone must be filled with interlocking crystals of hydrated cement so as to lock the stones together.
Water takes up space, so pushes the sand and stone apart thus needing more hydrated cement to fill the gaps. Therefore in most mixes there will not be enough cement for the crystals to interlock strongly if extra water is added.
A superplasticizer can be used to make the concrete more workable without having to add a lot of water.
see http://www.waterproofconcrete.co.uk/what%20too%20much%20water%20does%20to%20concrete.pdf for lots more detail
