- What are the additives in concrete
- Means for increasing the mobility of the mixture
- Lamination stabilizers
- Air entraining agents and seals
- Increased frost resistance and hydrophobicity
- Strength additives
The concrete solution gradually ceases to be just a mixture of stone and cement milk. The modern chemical industry offers a number of compounds that can improve the working and performance properties of concrete, which expands the scope of its application. Which of them can only be purchased, and which you can make yourself – you will find out today.
What are the additives in concrete
The classification of concrete additives is quite extensive, but all varieties are divided into two main groups. The first includes additives that improve the working qualities of the mixture: setting time, mobility, tendency to delamination, and others. The second group includes impurities that help to optimize the performance of concrete: frost resistance, water absorption, strength, corrosion rate. We note in advance that many supplements have a combined effect.
A distinction can be made on the nature of the action of the additives. Some of them are chemical, some are mechanical. The first type of additives includes plasticizers, hydration regulators and many others, almost all of their varieties, the principle of action and scope are described in the construction catalog of Gosstroy SK-4.4.3 and GOST 24211-91. Mechanical additives are all kinds of microfibers, porous fillers and finely ground particles, the effect of which on the concrete structure is most obvious and predictable..
In this review, we will consider options for replacing popular combined additives with those chemical compounds that are available on the open market and are produced by non-construction brands. They are quite suitable for improving the most important characteristics of self-prepared concrete, but without overpaying for a well-known product brand..
Means for increasing the mobility of the mixture
The concrete mobility determines its ability to take the form of the formwork without the formation of voids. To improve the mobility of the mixture, surfactants (surfactants) of the hydrophilic type are used. These are mainly sodium oleate and stearate, which form the basis of household detergents, as well as sulphite-yeast mash (lignosulfonate) – a waste of the cellulose industry, widely used in the production of dry building mixtures..
It is possible to add to concrete both mixtures containing the indicated substances (liquid or laundry soap) and liquid / solid concentrates. In the latter case, the question of the correct dosage of additives is well resolved. For reference, the content of surfactants in household cleaning chemicals is from 35 to 70%, while you should always proceed from the calculation of the maximum concentration so as not to exceed the dosage. Both of the described types of plasticizers are added to concrete in an amount of 0.2-0.35% by weight of cement.
The side effects of plasticizers are mostly positive. This is a slight slowdown in the setting of the mixture, a decrease in the water-cement ratio by 10–15%, and a slight increase in porosity. Correct use of lignosulfonate allows, while maintaining the volume of water used, to reduce the content of cement in the mixture by 7-10% while maintaining the brand strength.
Stratification of a concrete mix consists in the deposition of solid particles of cement and filler with the rise of water to the surface, resulting in a lack of moisture for hydration. Basically, concrete delaminates due to excessive vibration or when dropped from a great height. Almost all surfactant-based plasticizers improve the uniformity of the colloidal system of the concrete mixture, however, sometimes additional stabilization is required, for example, when casting massive monolithic structures.
One way to protect the concrete from delamination is to add high surface area milled solids, which makes the cement dust more bound to water. Examples of such substances are soot, tripoli, kaolin, and metallurgical ash. It is important that the materials used are precisely finely ground, otherwise they will not be of much use. Such additives are used in amounts up to 10-15% of the cement mass.
Otherwise, it is possible to achieve high-quality stabilization of the concrete mixture by introducing small portions of methylcellulose (MC) – up to 0.5% of the cement mass. When using cement from plasticized clinker, the MC content is halved, and this additive can also be introduced in smaller portions when using a high-grade binder.
Air entraining agents and seals
Surfactant-based plasticizers entrain tiny air bubbles into the concrete mixture, thereby increasing the porosity of the concrete. This action is secondary and has a weakly pronounced character, if necessary, the porosity of concrete can be significantly increased or, conversely, made it denser.
Silver powder is widely used as a gas-forming agent in very small doses, of the order of 0.02–0.05% of the mass of cement. If desired, you can use an organosilicate water repellent called GKZH-94. In order to qualitatively prepare an additive for concrete based on it, the concentrated liquid should be diluted and thoroughly mixed with water in a ratio of 1: 3 until a stable emulsion is formed, and then with this composition, the mixture should be brought to the desired consistency. The final content of concentrated GKZh-94 in concrete is about 2–3% of the volume of water used.
If it is necessary to make concrete more dense, ferric chloride is added to it during mixing at a concentration of about 0.1% of the mass of cement. It is one of the most common and widely available chemicals used in PCB etching. Otherwise, it is possible to increase the density of concrete with the help of less common ferrous sulfate or calcium nitrate, but their content in the mixture strongly depends on the qualities of the cement and mineral filler..
Almost all additives that increase the porosity and plasticity of concrete slow down the setting, and the sealing additives contribute to a faster flow of hydration. The more time remains for the mixture at the initial stage of hardening, the higher the final strength of the structure. In addition, set retarders are indicated for the preparation of large portions of concrete mix, especially in hot weather, as well as for the stage-by-stage pouring of bulk structures to eliminate cold joints..
Various forms of sugar are the main means of slowing down the setting of the mixture, but this additive should be used with extreme caution. Normal set retardation occurs at a concentration of about 0.3–0.5 grams per liter of mixing water. In higher doses, sugar can disrupt the course of hydration, or even make the hardening process incomplete. For these reasons, instead of pure sugar, molasses with its content is used, which facilitates the calculation of the dosage..
Sometimes the combined effect of numerous additives makes the setting of the mixture too slow, which requires faster hydration. To shorten the setting time, a mixture of potash and sodium aluminate or baking soda is used. It is necessary to mix these substances in a ratio of 4–6: 1, the resulting mixture is added to dry cement in an amount of 0.5–1% by weight. The setting accelerators must also be used with caution, as they can negatively affect the strength of the concrete..
Increased frost resistance and hydrophobicity
It is widely believed that frost-resistant concrete must necessarily be dense, because the destruction of the structure occurs mainly due to the expansion of water in the pores. However, the closed pore structure does not cause such vulnerability, quite the opposite: the presence of microscopic cavities helps to relieve internal stresses caused by linear thermal deformations.
It can be argued that most air-entraining plasticizers and stabilizers have a beneficial effect on the frost resistance of concrete. Otherwise, the required resistance to low temperatures can be achieved by mixing the mixture in water with a 2–2.5% content of calcium water glass. This additive reliably closes the pores and prevents the formation of microcracks, due to which the water absorption of the concrete structure is significantly reduced..
In order to be able to carry out concrete work at negative temperatures, the cement paste is mixed in a mixture of water with nitrite-nitrate-calcium chloride (NNHK). Such a compound cannot be prepared by yourself, it is toxic and can only be used for non-hydrophobized mixtures. However, there are practically no other alternatives for winter concreting. Concrete with the use of this additive retains increased frost resistance also during operation..
To strengthen the structure of concrete, it is compacted using the methods described above, or mechanical reinforcing admixtures are introduced. The classic material for dispersed reinforcement is mineral, steel or polymer fiber. Its amount in the concrete mix can be up to 30% of the filler volume. Fiber is introduced either by dry mixing with cement before mixing, or in small portions into the ready-made mixture with thorough mechanized mixing.
Also, an increase in strength almost always occurs with the addition of plasticizers and stabilizers. Even air-entraining agents have an increase in compressive strength as a secondary effect, the increased porosity is compensated for by more optimal cement hardening conditions..
The industrial development of concrete modifiers is a rather complex and painstaking process. The ratio of added chemical compounds is not determined by universal rules, but by the type, composition and shelf life of the cement used. The type of mineral filler and dust impurities contained in it are also taken into account separately..
Collecting a “cocktail” of numerous components cannot be guaranteed that their combined effect on concrete will not be negative. Even at enterprises producing concrete products, the amount and composition of additives to concrete are determined empirically through a series of intermediate tests. This all suggests that it is better to use modifiers in quantities that are obviously less than recommended, not to strive to prepare a universal multicomponent additive, but, on the contrary, to improve only reasonably required qualities..