Concrete setting time: chemical and physical models.
How long does the concrete set? What processes take place in it during the transformation of a semi-liquid solution into a solid material? How does the air temperature affect these processes? Let's figure it out.
Grasping: chemistry and physics
Для начала давайте выясним, что это за процесс - схватывание бетона, как он протекает с точки зрения химических реакций и физической модели (см.также статью «Бетонные столбики – важный элемент строительства»).
To clarify: all processes in concrete, strictly speaking, it is more correct to call cement hydration. For obvious reasons, far more chemically resistant quartz sand and crushed stone are not involved in the reaction.
Chemical model
When Portland cement comes into contact with water, several chemical processes start at the same time, resulting in the conversion of oxides and aluminum and calcium salts to hydroaluminates and hydrosilicates, respectively.
The main reactions are as follows:
- 3CaO*Al2O3+6H2O -> 3CaO*Al2O3*6H2O.
- Ca2SiO4+H2O -> Ca2SiO4*H2O.
- 3CaO*SiO2+H2O -> 3Ca2SiO4*H2O+Ca(OH)2.
All three reactions proceed with the release of a small amount of heat. Complete hydration of cement requires mixing it with water in a ratio of 1: 0.4; at the same time about 60% of the total amount of water is involved in the actual chemical processes. The remaining liquid remains locked in the cells of the future concrete, forming its porous structure; it will evaporate in a few weeks or even months.
In parallel with the hydration of the constituent cement minerals in the mass of concrete, several slower reactions take place, which gradually lead to a set of brand-strength concrete:
- Slaked lime Ca (OH) 2, in conjunction with carbon dioxide, gradually turns into strong and slightly soluble calcium carbonate CaCO3.
- The addition of silicon dioxide to this reaction produces not less durable and stable calcium silicates.
Physical model
How does the process look like in terms of physical processes occurring in the mass of concrete?
| Time from the beginning of hydration | Flowing processes |
| 5 minutes | The surface of the grains of cement is covered with needle-shaped crystals of calcium aluminate 3CaO * Al2O3. |
| 6 o'clock | The crystals grow to such an extent that they begin to form a common crystal lattice. The first spatial connections arise between the cement grains. |
| 8-10 hours | Лавинообразно ускоряющийся рост кристаллической структуры создает между зернами сплошную решетку. Прочность бетона стремительно растет. Одновременно внутри кристаллической структуры начинается рост более мелких кристаллов - своеобразного «ворса» из силикатов кальция. |
| 24 hours | Calcium silicates begin to displace the aluminate lattice, forming their own strong network between solid fillers. |
| 28 days | Cement stone is completely composed of calcium silicate crystals. Aluminates extruded. |
Reaction time
In general, how much time the concrete clutches is clear from the previous table. After 6 hours, its mass ceases to be mobile; after a day, it loses its plasticity, but remains fragile. Full brand strength is gained by the material in about a month.
Reference: brand strength corresponds to the maximum compressive force that concrete is able to withstand without destruction. Effort is measured in kilograms per square centimeter. Thus, concrete M200 must withstand 200 kgf / cm2, M300 - 300 kgf / cm2, M400 - 400 kgf / cm2 and so on.
However, the above table is true for the so-called normal conditions: + 18-22С at a relative humidity of about 60%. What happens when deviations in one direction or another?
| Deviation | Influenced by |
| Lowering the temperature | At low temperatures (from 0 to +18) the speed of the reactions slows down; setting may take up to 5-7 days. When the temperature of the concrete drops below 0 degrees, the water crystallizes, and the hydration of Portland cement stops completely. |
| Temperature rise | To a certain limit, reactions are accelerated; however, at 90 ° C and higher, water begins to evaporate from their growing crystalline structure faster than it reacts with it. As a result, the curing again stops. |
| Lower humidity | In dry air, water quickly evaporates from the surface layer of concrete, which leads to a drop in its strength and the appearance of cracks. |
| Increased humidity | При нормальной температуре оно приводит лишь к увеличению итоговой прочности бетона при отсутствии трещин. При повышенной до +80С температуре в насыщенной водяным паром атмосфере набор 60-70% прочности занимает всего 16 o'clock. |
How to level deviations from the normal mode?
- In the heat and dry weather, the structure in the formwork is covered with wet sacking, wet sawdust, or watered with water two or three times a day.
- At temperatures up to -5 ° C, it is enough to cover and, if possible, insulate young concrete. The heat released during hydration will not allow water to freeze.
- At lower temperatures, heating is required. The alternative is to use winter grades of concrete, which, thanks to special additives, are capable of setting at negative temperatures.
It is useful: thanks to antifreeze additives, the price of a cubic meter increases by only 100-300 rubles per cubic meter. Actually, winter concrete is easy to prepare with your own hands: the instruction is reduced to the addition of special anti-frost additives (for example, Antifreeze DS) in the amount of 1% by weight of cement.
Useful stuff
- Total strength may exceed the grade. Under normal conditions, the strength of a concrete structure in 3 months reaches 120%.
- It is possible to remove formwork and walk on the monolith in 3-4 daysI. Processing, building walls on concrete foundations, etc. maybe in 10-14 days.
- It is undesirable to use percussion instruments until reaching full strength. If machining is necessary, it is preferable to cut the reinforced concrete with diamond circles and diamond drill holes in the concrete: in this case, you can not be afraid of cracks and chips.
Conclusion
Надеемся, что предложенная вниманию читателя информация будет полезна ему при ремонте или строительстве (см.также статью «Фибропенобетон: особенности и область применения»).
As usual, the video in this article contains additional material. Successes!