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High strength gypsum and modifying additives

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09.12.2019

The main components of gypsum dry mixes are: gypsum binders, fillers, functional additives.

In gypsum mixtures, building gypsum based on beta-or alpha-hemihydrate of calcium sulfate is used as a binder. Less commonly, they are used in a mixture with anhydrite (especially for plastering) to provide the necessary setting time.

The compositions of self-leveling mixtures use high-strength gypsum (alpha hemihydrate) and anhydrite. The use of high-strength gypsum binders in dry mortar is explained by the special requirements for hardened mortars - the necessary survivability and high early strength.

  • The filler is introduced to reduce the cost of the product, but most importantly - to prevent cracking during operation.

  • In addition, the processability during application is improved.

As fillers in gypsum mixtures use:

  • Quartz sand

  • Limestone flour

  • Dolomite flour

  • a piece of chalk

  • Ash

In the composition of light plasters, perlite is used. In certain types of filler mixtures, the use of mica is recommended.

The dispersion of the aggregates is determined by the type of gypsum mixture: quartz sand and limestone sand are used with a dispersion of up to 0.8 - 1.0 mm.

When selecting aggregates, special attention is paid to particle size distribution - there should be approximately the same ratio of aggregate fractions.

Hydrated lime Ca (OH) 2 is introduced into the composition of gypsum mixtures to improve plastic properties, reduce shrinkage deformation and slow down setting.

Functional supplements carry out:

  • gypsum setting retardation

  • increase water retention

  • mobility

  • plastic

  • grip strength

  • create a special pore structure

  • reduce the risk of cracking.

Setting retarders are important functional additives that increase the survivability of mortar gypsum. It is known that gypsum binders quickly set, and the solution to the problem of increasing survivability is the right choice of special additives - setting retarders.

Water-retaining additives are introduced into gypsum mixtures in order to increase water retention, adherence to the base, improve mixing, give the mortar mixture the necessary viscosity and plasticity. For these purposes, various cellulose ethers are used.

In addition to water-retaining additives, rheological additives, so-called thickeners, are added to some compositions of gypsum mixtures, which improve the consistency of the mortar mixture, its workability and reduce stickiness to the tool. In this capacity, starch ethers are used.

Redispersible polymer powders (RPPs) are introduced into gypsum mixtures to increase adhesion to the substrate, bending strength, water resistance and improve the manufacturability of the preparation and application of the mortar mixture.

Pore-forming agents or air-entraining agents are additives used in stucco mixtures to create a special pore structure of hardened material. They are surfactants that reduce the surface tension of water, and thereby improve the envelopment of the smallest particles of the mortar mixture. In this case, micropores are formed, evenly distributed throughout the volume. They minimize the formation of cracks, improve frost resistance, manufacturability.

Dispersants (plasticizers) are additives that are absorbed on the surface of the particles of the mortar mixture, reducing the formation of lumps during its preparation.

Cellulose fibers are incorporated into gypsum mixtures to reduce cracking and shrinkage.

Dry mixes for self-leveling floors have a more complex composition. In addition to these components, these gypsum mixtures include such functional additives as superplasticizers, hardening accelerators, antifoam agents.

  • Superplasticizers - additives that reduce the water-cement ratio with increasing fluidity.
  • Hardening accelerators are additives that shorten the time it takes to walk in anhydrite floors.
  • Defoamers - additives that are hydrocarbons and polyglycols on an inert carrier (amorphous silica), they reduce the formation of macropores during the preparation of the mortar mixture, and in the future, after solidification, the risk of craters on the surface.
Portland cement - injected into a- floors to reduce cracking and increase water resistance.

In the composition of gypsum plaster mixes as a binder, mainly building gypsum grades from G3 to G5, G6 are used, a mixture with anhydrite can be used in light stucco mixtures.

Hydrated lime is added to the gypsum plaster mix to improve plastic properties, reduce volumetric deformations during hardening and to slow down setting.

Limestone sand plays the role of aggregate, the dispersion is less than 1 mm, the main requirements are a clean, dry, rational particle size distribution (that is, approximately the same ratio of fractions), as they say, there should be a continuous particle size curve.

Perlite is used as a lightweight aggregate to reduce the density of the solution, the particle size is 0-1 mm (sometimes expanded vermiculite is used).

Limestone flour is used in some plaster compositions to create the necessary packing density of particles, to create a rational particle size distribution of the aggregate. After hardening, the risk of shrinkage is reduced.

A water-retaining additive is an important component of plaster mixes, the content ranges from 0.16 to 0.3.

Rheological additive - gives the plaster mixes the necessary plastic and viscous properties, is introduced in small amounts from 0.01 - 0.03%.

Air-entraining additive - promotes the formation of a microporous structure. It is introduced into all plaster compositions in an amount of 0.01 - 0.03%.

Setting retarders - are introduced into all plaster compositions in an amount depending on the type of retarder. Basically, it is citric acid and its salts (sodium citrates) and tartaric acid and its salts (sodium tartrates).

Aqualon offers Silipon RV series retarders specially developed for gypsum mixtures: RV 1730 and RV 1731. 1730 is a mixture of calcium complex compounds, 1731 is a sodium salt of alkylphosphinic acid. The recommended concentration in the dry mixture is 0.1 - 0.4%.

Dispersant - improves the preparation of the mortar mixture, eliminates the formation of lumps. Can recommend Genapol PF80.

Fillings, grouts and gypsum adhesives differ from stucco compounds in both component composition and dispersion. A feature of these compounds is that building gypsum with a fineness of less than 0.1 mm is used as a binder, limestone, dolomite flour is a filler, and chalk with a fineness of also less than 0.1 mm. In this regard, the amount of water-retaining additives increases to 0.5 - 0.8%.

The composition of gypsum putties, grouts and adhesives includes RPP, which improves bending strength, toughness, resistance to deformation, adhesion, processability of processing. Depending on the type of mixture is introduced in an amount of 1 - 4%.

In order to reduce the water absorption of gypsum solutions, Wacker proposes to use Vinnapas RI551Z with a hydrophobic effect. For the full manifestation of the hydrophobicity effect of RI551Z, alkaline activation is required. In addition, it is recommended for the same purpose in gypsum systems to observe the ratio of gypsum and filler 30/70. A tangible decrease in water absorption can be achieved with the introduction of 2 - 6% RI551Z. However, it must be borne in mind that the hydrophobic properties of materials using RI551Z strongly depend on the presence of additives: defoamers, blowing agents, thickeners, plasticizers, setting retarders, etc.

The list of tests (according to DIN 1168) of the properties of dry gypsum mixtures, depending on the type, is as follows:

  • Screen analysis (sieving on sieves) - complies with GOST 125 for testing gypsum.
  • Determination of the water-gypsum ratio - by mobility (they shut 2.5 dm3 of the gypsum mixture, put it in a cone, then put it on a glass plate, shake it 15 times, the cone spread should be 165 mm).
  • The start of setting is on the Vic device.
  • The test features, in contrast to the setting time tests according to GOST 125: instead of a needle with a diameter of 1.1 mm, a cone is used with the dimensions: lower diameter - 1 mm, upper - 8 mm, height 50 mm. The beginning of the setting of the gypsum mortar is taken as the time when the cone does not reach the glass plate of 18 mm.
  • Determination of compressive and bending strength on beams of 40x40x160 mm. Feature - tests are carried out after 7 days of hardening under normal conditions, then the samples are placed in a heat cabinet, where they are kept at a temperature of 40 ° C to constant weight.
  • Adhesive strength (adhesion). It is carried out by measuring the force of separation of the hardened solution from the substrate (asbestos-cement slabs or drywall sheets).
Dry gypsum mixtures for self-leveling floors and screeds have the following composition:

  • Astringent - in self-leveling floors and screeds use either building gypsum in the form of a-hemihydrate, or anhydrite, or a mixture of both.
  • Aggregate - quartz sand with a particle size of up to 0.8 mm is used in screeds, quartz sand with a dispersion of up to 0.4 mm is used in bulk floors, as well as a fine filler - limestone flour, ash, chalk, dolomite (less than 0.1 mm) in order to seal the structure of the hardened solution, reduce the risk of shrinkage deformation.
  • Hydrated lime - is introduced in small quantities into screeds and self-leveling floors in order to reduce shrinkage deformations.
  • Superplasticizer - is introduced into the composition of gypsum floors in order to increase mobility and reduce the water-gypsum ratio, in the amount of 0.3 - 0.7%.
  • Water-holding additive - in this case it is used to stabilize high-water gypsum mortar mixtures. In screeds, a water-retaining additive can be omitted. In bulk floors on a-gypsum, low-viscosity and ultra-low-viscosity cellulose ethers are used in an amount of 0.02 - 0.2%.
  • RPP - specially designed redispersible polymer powders for dry mixes for bulk floors are used for self-leveling floors.
  • Antifoam - helps reduce air entrainment during the preparation of the mortar mixture and prevents the appearance of craters on the surface of the hardened solution. For gypsum systems, the use of Agitan P803 is recommended.

Setting retarders - are not always used in gypsum floors, since the setting time of a-gypsum can provide the necessary survivability, and anhydrite requires the introduction of hardening catalysts. It is recommended to introduce sodium citrate or tartrate if necessary.

Hardening accelerators - introduced into the composition of anhydrite floors in order to reduce walking time, potassium sulfate can be used.

Rheological additive - sometimes introduced into bulk floors as a thickener.

The quality control of gypsum mixtures for the installation of bulk floors is recommended to be performed according to the following indicators:

  • Flowability - according to the cone diffusion (should be 210 - 230 mm).

  • Vitality - must be at least 30 minutes according to DIN 18560.

  • Walking time (achievement of compressive strength of at least 1.5 MPa).

  • Strength in compression and bending depending on the strength class according to DIN 18560.

  • Shrinkage strain.

  • Adhesion to substrate.

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