Screed admixtures combining plasticiser technology

AptiCem has harnessed new technology to develop new screed admixtures, which give rapid drying times and high early strengths. Dan Devine discusses how the company developed these screed admixtures in response to customer and industry demand.

Screed admixtures are used to improve strengths and reduce the levels of moisture within a screed, therefore allowing floor coverings to be laid over the screed earlier than they could be using traditional additives.

A recent customer required a screed admixture that combined plasticising and water reduction properties. Crucially the customer required – and was unable to source – an admixture that could help enable the screed to be pumped over long distances under high pressure, without changing consistency or consistence. Drying times can also be an issue with conventional screed admixtures. According to industry guidelines, a 75mm traditional screed would require up to 110 days to dry at 20°C and 50% relative humidity (RH) at the rate of 1mm per day for the first 40mm and 0.5mm thereafter. However, it is much shorter for modified screeds which use water reducing and additives and admixtures to reduce the water/cement ratio in the screed mix.

In response to the demand, the AptiScreed Max screed additive was formulated, which gives rapid drying times, high early strengths and pumpability of a screed as requested by our customer. Development of this additive has addressed the following: 1) products that do not require constant mixing and have a lower viscosity than currently marketed products; 2) products that provide a seven-day flexural and compressive strength in excess of those on the market; and 3) a product that reduces separation during pumping.

Formulation

AptiScreed Max was specifically developed to provide rapid drying times, high early and ultimate compressive and flexural strengths. In addition, some screed admixtures tend to separate in storage and therefore need to be remixed before use. All raw materials were miscible to avoid phase separation within the admixture, thus making it easier to handle and more user friendly.

Additionally, standard rapid drying screed admixtures that face high internal pressures during pumping can cause issues with separation of the sand and aggregates, which results in decreased strength. The pumping aid in AptiScreed Max acts as a binder to resist the high pressures in the pump, which can cause the sand–cement and water to separate or cause over-plasticisation of the mix.

Performance

Designed for use in a variety of applications including underfloor heating, interior and exterior applications, AptiScreed Max achieves compressive strengths of up to 60MPa and enhanced drying times permit foot traffic after 24 hours. Its properties consist of a blend of plasticisers and specialist additives that help improve the finishing of the screed and aid with rapid evaporation of the free water to help with the reduction in drying times. The plasticising action allows the screed to be produced with moisture contents as low as 6% without affecting the workability as generally low moisture content would be too dry to lay. The low moisture content along with the evaporation aids help promote quicker drying of the screed.

AptiScreed Max has a drying time of 48 hours for tiles and 72 hours for any other coating or covering to be applied, providing the area is well ventilated.

In-house testing, using gravimetric moisture content analysis, was undertaken in which screeds were cast into large metal trays and placed in a curing chamber at 21°C and 65% relative humidity. These tests showed that screed with the admixtures has 18% less moisture when compared with a traditional screed over a 36-hour period. To allow impermeable floor coverings to be placed, the screed must achieve 75% relative humidity or below; the screed with AptiScreed Max is already down to 82% moisture content after 36 hours.

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