Refurbishing 1 - Lime & Cement Mortars

I recently moved into an old London house (built in 1864). It has not been refurbished, but had some work done on it in the 1970's when the outside toilet was modified to lead off the kitchen and become a bathroom. Central heating was installed in the 1980's

The old steel gas pipes are still in place and connected although the fittings have long gone and ends capped off.

Electricity has been added in stages. There is no "consumer unit" just a series of isolation switches and large bakelite fuses. I have some lead sheathed cable with cotton and rubber insulation still in use with 15Amp and 5Amp round pin sockets.

The mains water supply is lead pipes.

Windows are single glazed wooden sash, apart from two at the back which are draft louvre.

Much of the external concrete mouldings, window sills and brickwork is in poor condition and been badly repaired in the past.

Internally there is some damp that has rotted some joists and damaged plaster work.

The house is in a conservation area which limits what I can do. I'm retired and decided to undertake the refurbishment myself (slowly) This is a 1st of a series on my progress as I seek to find out what and how to do it.

The first job is to stop the damp. One area is under the front door step that has rotted the hall joists. This has happened before as I can see where the joists have been replaced, however little to nothing was done to prevent the water coming in and they have rotted again.

I need to take out the door step and investigate and prevent for the future.

Knocking out the doorstep is one thing but I need to replace it after and this lead me to investigate mortars. Here is the summary of my finding on mortars...  


Why Lime Mortars?
A1) You mustn’t have a mortar stronger then the masonry (because it damages the masonry).
A2) It’s better than cement only mortars 

Background
Cement was invented in 1824 and not perfected until 1860 dominating construction in the UK in the 1930’s. Lime mortars are the principle mortar used in construction until then. 
Pure lime mortars behave as if they are flexible and lime cement mortars are slower hardening and remain more flexible than cement sand mortars. Lime, therefore, enhances the ability of the brickwork to accommodate stresses caused by building movement and cyclical changes without excessive cracking.  A water saturated brick can be as much as 10% bigger then when full dry. These natural movements in a building cause stress. In cement only mortars the mortar can be stronger than the brick and can cause the brick to crack and/or spall (flakes off).

Mortar is an expendable part or the construction and is expected to last around 100-years before repair (e.g. re-pointing). It will turn to dust over time as part of its job of holding the brickwork together and allowing movement.

Benefits of Lime

Workability - Lime improves the plasticity and workability of mortar, while providing a high degree of cohesiveness it also spreads easily under the trowel.

Water retention - Lime mortars have higher water retention, creating an improved bond as there is more contact between masonry and mortar. Retention of water in mortar improves carbonation in pure lime mortars and results in best conditions for early hydration of cement lime mortars – thus reducing cracking and water penetration into hardened mortar joints. The closer bond also reduces water penetration after hardening and has a positive effect on thermal insulation.

Mortar strength - The use of lime in mortar reduces the compressive (load carrying) and flexural (bending) strength of the hardened mortar.

Vapour permeability and Freeze-thaw - Vapour permeability of mortar improves with increasing lime content. A high lime mortar can act as a 'wick', to allow water vapour to pass out from the building enabling the structure to effectively 'breathe'. Vapour permeability allows any moisture to evaporate, thus reducing the risk of freeze thaw deterioration.

Autogenous (self) healing - Should hairline cracks develop in the mortar, the combination of lime, moisture and carbon dioxide from the air can help to seal the crack by the formation of calcium carbonate (limestone). The crystals that are subsequently formed by this process help to plug the cracks.  Repeating the process turns the lime based mortar to dust over a period of around 100 years. Cement based mortars crack and need replacing earlier.  Most commonly affects the exposed surface (pointing).

Types of Lime

Air Lime – non-Hydraulic/Quicklime/Putty Lime - Uses atmospheric carbon dioxide to form calcium carbonate.

Hydrated Lime – Will not set while in contact with water and is added to cement mixes to give the benefits of lime.

Hydraulic Lime  - Fat lime -  Uses the process of Hydrolysis to set, so will set while in contact with water. Used in Lime+Sand only mixes. Setting time for lime only mixes is much slower. 

Motars
Usually consist of 3 or 4 parts, but other additives may be used.

1. Aggregate – e.g. sand, provides the bulk of the mortar
2. Binder – e.g. Lime and/or cement, binds the aggregate together and the mortar to the masonry
3. Water – Catalyst for binder and bonds the mortar parts together as a cohesive substance.
4. Filler – materials which are added to a mortar to bulk up the mix and fill voids

Mix

Hydrated Lime Mix – by volume
                   Use                                 Sand Lime       Cement
General Building above ground       5   1           1
General Building below ground             6   1           1
Internal Walls                                       9   2           1

1. Dry mix until very even mix
2. Add water slowly and stop when the mix looks slightly too dry
3. Mix the wet mortar for a good 10 minutes and the mix will eventually become more workable