Hydrated lime - a fundamental partner Hydrated lime - a fundamental - PDF document

Hydrated lime - a fundamental partner Hydrated lime - a fundamental partner for for high performance, durable & sustainable mortar high performance, durable & sustainable mortar 1 1 Mai 2008 1

Introduction on the use of lime in mortars • Masonry mortar is a mix of binders, aggregates, water and sometimes addition and/or admixtures • Lime used as a binder for centuries • The use declined during the nineteenth century largely due to the introduction of portland cement (OPC) • Used in site made mortar, less in factory made mortar. Factory made mortars are gaining market share • The presentation is giving the result of a bibliographic study of more than 220 articles dealing with the use of lime in mortars 2 Masonry mortar is composed of binders such as air lime, hydraulic lime, masonry cement and cement, aggregates such as sand, water and sometimes addition and/or admixtures for use in masonry. Lime has been used as a binder for centuries. It is one of the oldest known types of mortar, dating back to the 4 th century BC. With the introduction of portland cement (OPC) during the nineteenth century the use of lime mortar in new constructions has gradually declined, largely due to portland cements ease of use, quick setting and compressive strength . Lime in mortars are mostly used in site made mortar and less in factory made mortar. Factory made mortars are recovering market share. Danish Technological Institute (DTI) has made a bibliographic study on behalf of EuLA. DTI has studied more than 200 articles, of which 4/5 of them have been delivered by EuLA, gathered amongst its members. The remaining articles have been selected by DTI. The aim of this study was to provide an exhaustive overview of the internationally available scientific publications relating to the functionality which are in general devoted to lime containing mortars The background for the initiative is the competitive situation for the use of air lime in mortars. As the initiative is market related and mainly involving the masons and the building designer in the choice of mortars to be used, the functionalities have been grouped according to the interest of these two parties. 2

Functionalities in relation to the interest of the Masons ‐ General The interests of the masons are that the mortar is easy: • to handle, • to mix, • to use, • to control and • cheap. 3 The economy plays a great roll when the masons are choosing a mortar. In that respect the logistics are quit important. The interests of the masons are that the mortar is: easy to handle, easy to mix, easy to use, easy to control and cheap. The tendency is that the mason would like to have mortars delivered to the building site premixed or ready to use. The mixing of mortars on site requires less and less man power and less and less mixing time. It is only in relation to renovation works that individual mixes make it more appropriate to operate with flexible systems. The examined articles are only covering functionalities in relation to the interest “easy to use”. As the background for the initiative is the competitive situation for the use of air lime in mortars the functionalities in relation to the other interests of the mason need to be taken into account too when considering the market situation. 3

Functionalities in relation to the interest of the Masons ‐ Easy to use The functionalities of interest in relation to “easy to use” are: • Workability • Water retention • Air content 4 The functionalities dealt with in relation to “easy to use” are: Workability, Water retention and Air content , where the workability is the most important functionality followed by water retention. The air content is a functionality linked to workability as the use of air entraining agents mainly in cement based mortars is to improve the workability where it is not developed by other means e.g. by lime. To make a virtue of necessity it is then often emphasized that use of air entraining agents is to improve the durability. The necessity of that is often discussed. The picture is a lime paint from a Danish church of a brick layer from 14th century. 4

Functionalities in relation to the interest of the Masons ‐ Workability • Lime ‐ based mortars have superior workability compared with cement mortars where no admixtures are added • Workability is affected positively by ageing of lime putty • High air content in cement mortars increase workability, but will affect negatively the bond strength • Development of new test methods. Non of them are directly related to practice • Sand grading and filler play key role 5 Description: Workability is the sum of the application properties of a mortar which give its suitability The general conclusion is that lime ‐ based mortars have superior workability compared with cement mortars where no admixtures are used. Many articles demonstrate that the use of lime putty in mortars leads to a good workability of the mortar. The same is the case if the mixing time is extended (15 ‐ 20 minutes) when using dry hydrated lime in the mortar. The improved workability of lime ‐ based mortars ensures a higher quality of the finished masonry, resulting in better bonding between mortar and bricks and thus better resistance against water penetration. By adding air entraining agents to cement mortars the same workability can be obtained as for lime ‐ based mortars, but the increased air content will have a negative effect on the bond between mortar and bricks. The same effect can also be the case when using plasticizer to improve the workability. Very often a combination of different admixtures are used. Some articles are dealing with development of new test methods for workability. No articles have related the test method and test result to experience from application in practice e.g. the amount of energy needed for placing the units into the right position in the mortar, the time before mixing water is released from the mortar in the bowl (the need for remixing the mortar in the bowl) and how clean the mason can lay the masonry units. The sand grading and the amount of filler also play a role as well as the use of binders with a high specific surface. With well graduated sand (a) the amount of voids among the sand grains is smaller compared to sand where a great part of the grains have equal size (c). 5

Functionalities in relation to the interest of the Masons ‐ Water retention • Water retentivity important when laying the units • Good water retentivity is established in the same way as good workability • Lime in mortars ensures good workability and adequate water for hydration of cement 6 Description: Water retention is the ability of a fresh hydraulic mortar to retain its mixing water when exposed to substrate suction The water retentivity is also an important functionality in relation to laying the masonry units. Especially when using high suction units, it is necessary that the mortar has a good water retentivity because otherwise the time, where correction of the unit is possible, is too short. The use of mortars with low retentivity in combination with high suction bricks easily leads to masonry with a high risk of water penetration as the time for bringing the bricks into the right position. The results demonstrate the strong water retaining characteristics of lime, which enhances the workability and ensures that adequate water is available to hydrate cementitious components of the mortar. Investigations also indicate that the use of aged lime putty is ideal because the material has higher plasticity and water retention capacity which results in mortars of higher strength that carbonate faster. A good water retentivity of mortars is established in the same way as a good workability. 6