Because fully-hardened concrete is impenetrable, any surplus water trapped inside the mortar or concrete will not be able to escape once the cement paste has hardened. As a result, a hardened bag of cement will always be significantly heavier than a fresh bag. The amount of weight increase depends on the age and condition of the concrete as well as its density.
Concrete's average density varies depending on how it is made. Concrete that is mixed in a truck with aggregate (gravel or sand) added later for casting needs more air space between the grains of gravel or sand to allow for breathing room for the cement to dry. This type of concrete has a higher porosity (volume of voids within the material) and thus less dense than ordinary concrete. Concrete cast at a manufacturing plant without airspace is very dense, usually no greater than 2% below maximum strength.
The weight of concrete changes as the mixture hardens and the internal water becomes locked down into the cement paste. The amount of water required to make a given volume of concrete depends on its density. Concrete used in forms for structures needs enough water to produce a strong but workable product, while concretes used as roadbeds or playground surfaces can be dried out slightly to reduce their weight.
The weight differential between the fresh bag of cement and the hardened bag is a measure of the quantity of water that has interacted with the cement to cause it to set. The difference in weight can be as much as 2 pounds (0.9 kg) for every 1 inch (2.5 cm) of height.
When the cement sets, its volume increases by about 30 percent, so you would expect the weight to increase by about 20 percent if there's no other material absorbed or bound up inside the cement mass. However, this is only true if the cement stays completely dry; if it gets wet, it loses some of its strength and weight. Cement that has dried out but remains somewhat moist will feel spongy under your foot. If it's very dry, it may even crack when you step on it. But if it's fully hydrated, it won't feel any different from when it was fresh.
As long as the cement is still damp, you can add more water to it and create a stronger bond with the surrounding materials. Once it's hardened, however, you can't make it weaker by adding more water.
Hardened cement also loses weight over time as some of its water content is lost through evaporation. The rate of loss depends on many factors such as temperature and humidity.
Concrete weighs heavier than water, and when placed into any container or shape, it will displace the water rather than mix with it. Concrete hardens as a result of a chemical reaction and does not need to "dry" to harden. Water is essential for the chemical interaction with the cementitious material to occur. However, if you want your concrete to be more durable, then it is recommended to allow it to dry before adding another layer of concrete.
If you use too much water, the concrete will be ruined. To make bagged concrete stronger, add additional Portland cement. Hydrated lime can also be used. Sand extracted from volcanic lava with a high silica concentration should be used to build the strongest concrete. This type of sand is very expensive though.
The more Portland cement, the stronger the concrete will be. Concrete that contains 20 percent or more cement by weight requires special treatment for air pollution control purposes.
Concrete that is strong enough to be used without further strengthening does not contain enough cement to resist chemical attack. Concrete that is used in areas where it will come in contact with chemicals such as acids and alkalis needs to be resistant to chemical attack. This type of concrete usually contains 30 percent or more cement by weight.
Adding aluminum powder to concrete makes it harder and gives it a silver color. This property is useful when making sculptures out of concrete or using it as decorative fencing.
Silica fume is created when glass factories burn their waste gases in order to remove any harmful substances they contain. These materials are then collected and sold as a premium ingredient for concrete.