I propose to discuss and highlight the advantages and features of cement-sand mortar (CSM). Now manufacturers produce a large number of dry mixtures in bags for all cases of creating screeds or leveling bases. At the same time, advantages are stated that, in general, the CPR also has. Perhaps these are just marketing ploys to sell CPR with additives at a higher price?
For my part, I consider the main advantage of the CPR to be its cost. Let's do a simple calculation. CPR = Cement + Sand + Water. The cost of a 50 kg bag of M500 cement today is 250 rubles. The cost of a 50 kg bag of sand is 50 rubles. We will take water for free. If the components Cement - Sand - Water are used in a ratio of 1: 3: 0.4 (water needs 40% by weight of cement), then the mechanical compressive strength of the screed after 28 days will be 30 N/mm2 (30 MPa). Without taking into account water, 200 kg of CPR will cost 250+3*50=400 rubles.
For comparison, let’s take a good floor leveler Litoliv Basis (manufactured by Litokol). The mechanical compressive strength after 28 days of a screed made from this product will be 30 N/mm2 (30 MPa), which does not exceed this value for the TsPR in the calculations above. The cost of a 25 kg bag of Litoliv Basis today is 450 rubles. 200 kg will cost 8*450=3600 rubles.
Thus, the cost of the CPR will be 3600/400 = 9 times less than the cost of a good Litoliv Basis floor leveler. Of course, you can find dry mixes for screed much cheaper, for example, 200 rubles for a 25 kg bag. 200 kg will cost 8*200=1600 rubles. In this case, the CPR will be 1600/400=4 times cheaper. However, the mechanical compressive strength of such screeds will not exceed 15 N/mm2 (15 MPa).
Here are the calculations. I'm waiting for your opinions.
Three weeks ago I worked with the sand concrete M500 dry mix. The cost of a 50 kg bag is now 250 rubles. Continuing Grachik’s calculations, we get: 200 kg will cost 250 * 4 = 1000 rubles. The CPR will be 1000/400=2.5 times cheaper. The mechanical compressive strength after 28 days should be 30 N/mm2 (30 MPa). Like CPR.
It turns out that the price and mechanical compressive strength of the CPR are unrivaled.
The calculations in the topic are interesting and seem fair. I'll contribute to the discussion.
I think the downside of CPR is the long drying time. Those who have worked with such screeds advise carrying out subsequent work on surface cladding after about a month. Otherwise there will be problems in the future. For manufacturers of dry construction mixtures, this figure varies from a day to 7 weeks. Depends both on the specific product and on the thickness of the screed layer.
The time required for the mortar to harden (crystallization of cement), dry (relative humidity equilibrium), and then until dimensional stability (end of shrinkage) of the sand-cement mortar is obtained is 1 week (7 days) for each centimeter of thickness . For a 4-centimeter screed, this turns out to be 28 days (about a month).
Thus, the first minus emerged. Perhaps in some cases this point is fundamental, but in my practice, when the thickness of the screed is from 2 centimeters, I use CPR. I use dry mixes in bags only as a finishing coating of thin thickness (self-leveling).
Another negative point for CPR is shrinkage. As a result, cracks form on the surface of the screed that need to be repaired (additional time and work), and for further use of the floor you will have to use additional finishing materials.
Dry mixes in bags have practically no problems with shrinkage (as stated by the manufacturer in the technical data sheet).
If you follow the mixing proportions of the components Cement - Sand - Water in the ratio 1: 3: 0.4 (water is needed 40% by weight of cement), and during the maturation process, cover the screed with a film or periodically moisten the surface, then there will be no cracks. I use such actions in my practice, and the results are excellent. In case of large areas, we must make expansion joints.
Thus, the second disadvantage can be minimized by observing the proportions of mixing the components and controlling the maturation process of the screed.
By the way, what about the lifetime of the cement-sand mortar (viability)? I guess it's not big, I don't know exactly how much. In addition, the mixture needs to be stirred regularly, otherwise it will “sag”.
For comparison, the lifetime of the Litoliv Basis leveler, which was mentioned above, is 120 minutes. This is indicated in the technical data sheet.
This indicator largely depends on the air (ambient) temperature.
For example, if work takes place at a temperature of +20 degrees Celsius, then the cement begins to set within two hours after mixing the CPR. It is logical that the solution should already be in the screed by this time. An increase in ambient temperature shortens the time interval, and a decrease in temperature increases it. In practice, two hours is enough to knead the CPR and place it in the screed, even when doing the work manually.
This is mainly due to non-compliance with the proportions of mixing the components. They pour a lot of water.
Thus, in terms of life time, CPR is almost as good as ready-made dry mixtures.
According to the main technical characteristics, cement-sand mortar and dry mixture in bags are materials of the same order. Of course, subject to the technology of performing the work. In my opinion, the significant difference in the cost of these materials is due to the “comfort” in working with them. That is, the higher the price, the greater the comfort. Here we can draw an analogy with motor transport. You can drive a distance of 100 km in a Matiz, or you can drive a Bentley. However, the comfort level of the trip varies.
Let's return to the screeds. To feel comfortable, you need to work independently with both the CPR and the dry mixture in bags. I will highlight several factors for dry mixtures in bags that affect comfort :
I'll express my opinion. If we are talking about such objects as apartments (especially small-sized ones), then here an ordinary internship “rests”. The customer will not even consider the difference in cost when he imagines what he will need to supply to the site: cement, sand, mixing container. That every brigade has a concrete mixer? But every team has a mixer!!! And to pour a cement screed, you need at least two people, and even more if loading and unloading work is involved. You will have to mix in portions, in some places there is too much water, in others there is more cement... And with ready-made levelers it’s a nice thing! The customer bought it, delivered it, and picked it up. One master is quite capable of diluting it, mixing it with a mixer, pouring it and leveling it.
Larger objects are a different matter, of course there is a screed there. But there are more people and different tools.
As a rule, it is recommended to carry out screed pouring work at temperatures from +5 to +35 degrees Celsius. Therefore, the question arises: how does temperature affect the rate of hardening of Portland cements. Here I deliberately write “Portland cement” because... the following reasoning is valid for both CPR and dry mixtures in bags.
So, temperature has a great influence on the hardening of cement. The hardening speed at low temperatures from 0 to +8 degrees Celsius is 2-3 times less than at ordinary temperatures from +15 to +25 degrees Celsius. At temperatures below 0 degrees Celsius, hardening processes almost completely stop. An increase in the temperature of hardening solutions is accompanied by an acceleration in the growth of strength. It becomes noticeable at a mixture temperature of 30 - 40 degrees Celsius when they harden during warm periods of the year. However, the intense effect of temperature on the hardening of cements manifests itself only in the presence of liquid water in them. The lack of water during hardening at elevated temperatures not only slows down the hydration process, but also reduces the strength and durability of the screed. When the water completely evaporates, the hardening process stops.
Draw your own conclusions.