Week 4 of 12
Module 3: Material Characteristics and as a Building Material
David Odgers, an accredited conservator with a background in chemistry, today gave an introduction to mortars, with a focus on lime mortar. This will be the topic of most of this week's classes. My work at the Scottish Lime Centre Trust is mostly associated with lime mortars and therefore I have been really looking forward to the focus of this week. It is a good chance to consolidate or challenge my understanding of this material. A good understanding of mortars is essential for understanding the performance of stone as a material because they are almost always used in combination.
David briefly introduced us to the main types of binders that have been used in stone buildings and the variations in the types of setting process; clay-rich earth binders set by simply drying out- a loss of water, gypsum plasters set by absorbing water and pure lime, hydraulic lime and cement all set by the occurrence of a chemical reaction.
We were shown a clip from a BBC television series called 'Human Planet' in an episode about rivers (link only available in UK), which showed how mud has been used for centuries in Mali for construction and how it is still used today.
Gypsum was briefly discussed and David listed some of the advantages/disadvantages (some could be either) of using this binder; fast setting, pH neutral, readily available, slightly soluble in water, very susceptible to salt attack, comparatively impermeable and rigid when set.
We briefly touched on the use of Ordinary Portland Cement (OPC) and natural cement but the rest of the day was focussed on an introduction to the use of lime mortars; the lime cycle, natural hydraulic lime, pozzolanic materials, additions (e.g. hair, animal fat, casein, etc.) and aggregates. There is a broad range in the understanding of lime mortars within our group from some of us who deal with lime mortars frequently to those who do not use them in their work (museum collections) or who pass specification work onto others. David asked each of us about the types of mortars used in each of our countries to see how widely the use of the material varies; the result seemed to be that the majority use lime putties, some gauged with hydraulic lime, pozzolan or cement, and/or hydraulic limes although few specified what strength they tend to use. In Goa they currently use OPC because there is no local supply of limestone for burning (although shells have occasionally be burnt instead) and because of a skills shortage. In China and South Korea the majority of their monuments are dry stone and therefore there is not much information on the historic (or current) use of mortars.
Importantly we ran over one of the most important factors in the use of lime mortars; the set. We covered both the carbonation of pure lime (CaO) and the hydraulic set of natural hydraulic lime (NHL), and why protection of new work is essential, because water (H2O) is needed to dissolve atmospheric Carbon Dioxide (CO2) for both of these chemical sets to occur. If the mortar dries out too quickly, then CO2 is not dissolved and the mortar will not gain a chemical set, therefore resulting in a very weak mortar that is likely to fail. In this case it would essentially have the strength of a dried mud, which may survive in some climates, but in most it would fail in a very short period of time.
I am interested to learn more about the types of additives that have been used historically and are in use at present internationally and David briefly touched on this today giving us a short list of some of those that have been used; casein, air entraining agents, animal glue, animal fat, hair, oil, blood, urine... Apparently a paper has recently been written by researchers at Aristotle University of Thessaloniki about 1,000 of the different additives that have been used in mortars (I will try to source the details), which I'm very interested to read.
David ran over the various issues associated with the use of OPC including its brittleness and lack or permeability. I was interested to learn that the addition of OPC in lime putty supposedly reduces the strength of the mortar unless it is used as over 50% of the binder, by which point this would be a cement mortar gauged with lime rather than the other way around.
This afternoon we were divided into six groups of 3 in the lab and have created a range of test mortars (which we will revisit I believe in week 9) based on the following;
1. Investigating the different types of lime putty.
2. Investigating different types of lime; slaked, hydrated and hydraulic.
3. Investigating different types of pozzolan.
4. Comparing hydraulic lime with hydrated lime plus pozzolan, and slaked lime plus pozzolan.
5. Comparing different quantities of aggregate.
6. Investigating gradings of aggregate.