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Hydroment Ditra-Set:  Technical Discussion

There are various aspects of Hydroment Ditra-Set that make it a perfect match for the Schluter^®-DITRA uncoupling membrane.  Below, we’ll look at the more technical aspects of the product and how they relate to ensure excellent working characteristics and high early strengths.

1.  Cement Content

All other parameters being equal, the higher the cement factor the higher the strength.  Ditra-Set contains higher concentrations of portland cement than most, if not all, competitive thin-set mortars.

The higher cement content may raise concerns about shrinkage, but these concerns are easily addressed.  The shrinkage of cementitious materials is due to two factors:

First, chemical shrinkage occurs as water and portland cement combine in the hydration reaction, because the volume of the products is less than the volume of the reactants.  Second, drying shrinkage occurs as water leaves the system through evaporation or absorption into adjacent materials, which also reduces the volume of the material. 

Typically, the amount of drying shrinkage far exceeds the amount of chemical shrinkage.  In the case of Ditra-Set, the higher cement content will actually reduce the potential for drying shrinkage by binding more water through the hydration reaction.  Further, the DITRA provides an ideal environment for controlling drying shrinkage.  DITRA is an entirely impervious substrate that eliminates wicking into the substrate.  Large-format porcelain tile installed with Ditra-Set reduces evaporation of water of convenience to a minimum.  The absorption of porcelain tile is < 0.5%, which means virtually no wicking, or loss of water into the tile.  This environment essentially provides a wet cure of the cement system, which is the absolute best method for promoting proper hydration of cement.  Proper hydration of the cement promotes strength.

The higher cement content in Ditra-Set results in higher strengths without any adverse affects on shrinkage within the DITRA system.

2.  Cement Fineness

In general, the finer the cement, the faster the hydration reaction.  This is because finer cement has a greater surface area per volume, which enables the water to contact and hydrate the total volume of cement faster.  Ditra-Set contains white cement, which is significantly finer than typical grey cements.

There are a number of methods for evaluating the fineness of cements.  The two measures that Bostik typically looks at are Blaine fineness and % passing a 45-micron sieve.  The higher the Blaine numbers the finer the cement.  Typical Blaine values for Type I/II grey cements are 350 to 400 m²/kg.  Typical Blaine values for the white portland cements used in Ditra-Set are 490 to 510 m²/kg.    This is a significant difference in fineness.

The % passing a 45-micron sieve is as important, maybe more important than simply reporting a Blaine fineness value.  This is because any cement particle that is larger than a 45-micron size will not hydrate completely, so that most of it simply becomes inert filler.  The % passing a 45-micron sieve for white portland cements Bostik uses is > 95%.  Typical values for grey cements are ~ 90% passing a 45-micron.

The finer cement used in Ditra-Set speeds the cement hydration, resulting in a higher rate of early strength gain.

3.  Chemical Composition of Cement

The use of white portland cement is also important to the performance of Ditra-Set in that it has a different chemical composition compared to grey portland cement, which results in improved early strength gain. 

There are four major phases in the composition of portland cement, as shown below:

            C₃S     Tricalcium Silicate

            C₂S     Dicalcium Silicate

            C₃A      Tricalcium Aluminate

            C₄AF   Tetracalcium Aluminoferrite

Early strength gain is directly proportional to C₃A and C₃S content with the C₃S content being the most important.  The C₃S phase hydrates to form CSH, calcium silicate gel.  Calcium silicate gel is the main component that defines the strength and durability of hydrated portland cement.

Grey cements are grey because they have a fairly substantial iron content, in the form of C₄AF.  Typical values range from 6 to 8%.  White cement is white because it has very low iron content, less than 1% C₄AF.  To maintain mass balance, the C₃A and typically the C₃S contents are increased when the C₄AF content is lowered.

The white cement used in Ditra-Set features a chemical composition that improves early strength gain compared to grey cement.

4.  Blend of Additives

All thin-set mortars contain additives that retain water and help control the hydration reaction to improve open time, adjustability, pot life, and set times of the product.  The use of proprietary non-latex additives in Ditra-Set has been optimized, in terms of additive types and concentrations, to provide excellent working characteristics without retarding the cement hydration.

The optimization of additives provides excellent working characteristics without compromising early strength gain.

Each of the above aspects of the Ditra-Set mortar (cement content, fineness, and chemical composition, along with the blend of additives) has been related to early strength gain of the product through cement hydration.  Hydration of cements is easily studied by the technique of isothermal conduction calorimetry.  One can follow the heat generated from the reaction of cement and water or a cement-based product and water. 

Heat of hydration is the heat generated when cement and water react.  The amount of heat generated is dependent chiefly upon the chemical composition of the cement, with C₃A and C₃S being the compounds primarily responsible for high heat evolution.  The hydration of C₃A and C₃S also control and determine the early properties such as initial set, final set, and early strength gain.  The rate of heat generation is also dependent upon the fineness of the cement.  As discussed previously, finer cements react quicker with water, which results in more rapid heat production.  Within reason, the heat of hydration is proportional to strength and strength gain.  Over any given time frame studied, the higher a system’s heat of hydration the greater the strength should be.

Comparative hydration curves clearly show that Ditra-Set achieves a significantly higher heat of hydration in the first 24 hours than competitive products.  The implication is that Ditra-Set gains strength through cement hydration quicker than competitive products.  This high early strength is the reason tile setters can grout after only 12 hours when using the Ditra-Set mortar, compared to 24 hours for most competitive products.

In the end, the balance of excellent working characteristics and early strength gain makes Ditra-Set an ideal mortar for use with Schluter^®-DITRA.

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