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Development of sustainable, high performance mortars that are robust and easy-to-use is an important target for the construction industry. Since the 1940 s, use of lime mortars for construction has almost vanished as they are slow to cure and more difficult to use compared to cement products. Natural hydraulic limes remain as alternative binders to cement however their slow rate of set and unpredictable, low strengths prove problematic for specifiers and construction companies. Despite its popularity, cement mortars fail to meet most criteria for sustainability due to CO2 emissions from cement manufacture and inability to recycle the material for reuse as binder. Further negative characteristics of cement mortars include excessive strength, inherent brittleness under load and lack of vapour permeability. Our research has explored the development of synthetic or pozzolanic hydraulic lime (PHL) technology to exploit its potentially more attractive sustainable credentials and physical characteristics. Work has focused on potentially highly active and novel additives to improve both the rate of set and overall final strength of mortars. We present preliminary data regarding the use of amorphous synthetic aluminosilicate materials (Al:Si > 1:5) to prepare synthetic hydraulic limes. In contrast to binder reactions which contain either only fly ash or metakaolins, the presence of high ratio synthetic aluminosilicate uniquely increases the rate of set, giving mortars with economically useful compressive and flexural strengths. Their reactive efficiency also retains high levels of free calcium hydroxide within a mortar which adsorb significant amounts of CO2 on carbonation giving binders with class leading low CO2 footprints in-use. It is postulated that the presence of dilute sialate species within the alkaline aqueous calcium hydroxide solution may be responsible for the increased reaction rate, producing discrete oligo-sialite species that give rise to long range 3D calcium disilicate/orthosialate species, similar to geopolymer structures. We speculate that such structures are responsible for rate of set and increased strength observed.
Gibson, Robin R.
Doctor, Limeco Limited,