Admixtures that create a higher water demand and reduced internal water absorption within semi-dry concretes for brick, paviours and block manufacturers can be hugely beneficial. These admixtures allow a higher water content in the concrete without affecting the semi-dry consistence which also improves the cement hydration.
Dan Devine, Commercial Director at Devine Chemicals, discusses how we developed DeCAL 9060 and 9061 in response to industry demands.
In semi-dry concrete the water demand is dictated by the consistence required to cast and demould instantly. However, through working closely with a number of industry associates it was identified that the standard products in the marketplace, usually traditional compaction aids, were routinely failing to hydrate a significant volume of cement.
The theory behind enhanced water demand in semi-dry concrete is based on in-house research and development which indicated that at low water: cement ratios a significant portion of cement can remain un-hydrated due to the low levels of moisture required to achieve the desired consistence. If the moisture content is too high, it will provide a consistency which is too wet and unusable on plant. This also means that the cement can be little more than an expensive filler.
Although there are a range of admixtures available, we approached finding a solution differently. As a leading speciality chemicals manufacturer, we designed admixtures that increases water demand and furthermore in the case of DeCAL 9061, added a reactive water repellent that demonstrates water beading on block surfaces for more than 12 hours.
These admixtures also present a commercial benefit to the manufacturer by reducing production downtime due to wet aggregates. Aggregates intended for precast products are frequently stored in bays exposed to the elements, resulting in there being an excess of moisture in the materials, making the resultant concrete too wet to cast. DeCAL 9060 and 9061 effectively ‘mop up’ the excess water and allow the semi-dry consistency to be retained, even with overly wet aggregates. The admixture effectively acts as an integral curing aid by absorbing excess water and then making this water available for hydrating previously un-hydrated cement.
We set a challenge to formulate an ‘all inclusive’ concrete admixture that functioned as a water inducer/water repellent when added to concrete brick mixes to provide higher strengths and lower water absorption. Through conducting further research and development, we incorporated a blend of materials within the admixture which resulted in an additional 24% water demand above current levels achieved with incumbent admixtures. Higher water content can increase water absorption of the product, which can cause excessive and rapid suction from the mortar during laying, effecting the mortar’s hydration, strength, and durability in the joint. This issue was addressed into formulation of the admixture.
Results from trials have shown that these new admixtures provide a 24% increase in water demand when compared with other admixtures. The additional water content gives the semi-dry product more efficient cement hydration with a 20% increase in compressive strength from 7 days. The admixtures also provide a 51% reduction in water absorption, which in turn reduces the suction of the brick and allows the mortar to function normally within the joints. DeCAL 9061 also demonstrates improved water repellence via reaction of the active components with the calcium ions within the concrete, which in turn reduces the occurrence of efflorescence.
Products incorporating the admixtures exhibit a significantly improved water repellent, meaning the blocks dry far more quickly, generally reaching a constant air-dry weight after 24 hours. This result is remarkable when taken in comparison with conventional concrete, which can take between three and eight days to dry to a constant weight.