bricklaying mortar

[sm_jamieson]

I have a tendency to use too much cement in the mix generally, and I want to get this right before tackling the extension walls etc. I think I do this because with a weaker mix, when the mortar has cured in a couple of days, it seems quite weak if you scrape with a finger nail etc. (no it's not drying out before curing) Also, hydrated lime does not seem to be generally available at builders merchants, so is it correct that most brickies would use cement, sand and additive instead these days ? If so, what is the best additive and typical proportions for brick laying mortar ? Cheers, Simon.

[TMC]

When I did the mixing for the brickies for my last extension they insisted on 16 sand to 3 cement with about the same Fairy liquid per 2 gallon bucket of water as I would use for a sink of water when washing up

Tony

[dg]

Internal blockwork should be between 6:1 and 10:1 - a weaker mix is important to allow flexibility of the mortar and especially if using aerated concrete blocks - as too strong a mix will cause the blocks to crack as the wall moves.

With external brickwork, use a 3:1 mix up to DPC and a 5:1 for facing bricks. The weathering protection of the mortar is done by pointing and compacting the surface of the joint, and not by having a strong mix.

Flexibility is important for domestic builds, and only high-rise and engineering projects require more rigidity.

Whilst the 'official' line is to use proper air-entraining additive in the mortar, detergent such as fairy liquid can be used with no detriment above ground level. With normal building sand and a typical 'belle' size mixer, just a tiny squirt will be enough in the mixer (about 1 teaspoon)

[Andrew Gabriel]

Mortar takes weeks to reach final cure. You are being impatient.

You can buy plasticiser at most places which sell cement. Many cements have it included anyway (it will say on the bag).

[tom.harrigan]

3:1 with any proprietary plasticiser (following the manufacturer's instructions) seems popular below dpc. 6:1:1 (the extra "1" being lime) is the most commonly used mortar by builders near me. I have never come across a builders merchant that doesn't stock hydrated lime. In fact it sounds a bit ridiculous to me. I prefer to use lime as it's all too easy to make large percentage errors with small quantities of plasticiser.

Sand is another issue. "Building" sand aka soft sand seems not to be used much in my area. The preferred sand is known as "plastering" sand and looks to me to be a mix of sharp:soft in about 60:40. I have asked brickies and stonemasons why they use it, and a typical answer goes something like "soft sand's too soft" or "sharp sand's too sharp". When I mention that I have seen brickies using soft sand they are usually disgusted.

The most important thing however is matching the mortar. To achieve the colour I need, I have to buy "yellow" sand from a particular builders merchant (which is a sharp/soft mix with a fair bit of grit) and their brand of cement. The hydrated lime I can get from anywhere.

T

[sm_jamieson]

Thanks, now I'm in a quandry about using lime or additive. Lime is a pain - one more bag of stuff to buy. And anyone use lime under DPC ? If I can really use detergent, and lots of brickies do, thats fine. I can also mix in a bit of sharp sand for good measure. Simon.

[The Natural Philosopher]

Should be pretty much rock hard if you mixed it 'strong'

Is at all mne.

Yes, cos lime is expensive and takes longer to set, and indeed never does set quite in the way that cement does. If you want to lay ten course of bricks in a day and come back the next morning and work without fear of knocking it all down, you need cement, not lime.

If so, what is the best

I don't use additives except in winter. It would cvary fro 2:1 sand:cement for below ground work with non porous engineering bricks, to about 4:1 for general sheltered above ground stuff,.or evem sandier if its a scratch job and I don't care.

[tom.harrigan]

to use too much cement in the mix generally, and I

I believe the practice of adding fairy liquid is somewhat frowned upon these days.

T

[tom.harrigan]

Is your extension going to match your house? Lime is not used under dpc.

T

[mail]

Dear Simon Up until approximately 1900, most buildings were made with a lime mortar. That is why London survived the Blitz as it is very flexible. It operates by virtue of changing the hydroxide to carobonate using CO2 in the air over a long period of time. Lime putty is obtained by taking say chalk CaCO3 (calcium carbonate) which has been heated to

1000 degrees C to make it the oxide (quick iime) which was then slaked to make the hydroxide (lime putty) which, after a period of "gestation", in a lime pit was taken out as a gelatinous mess and "knocked" up (by the "boy") to a plastic state (hence the expression) before mixing with sand. The ratio is defined by the natural voids in between the particles of the sand. It is usually 3 parts sand to one part binder (be it lime or Portland cement). Hydrated lime is (as far as I know) simply ground up calcium hydroxide. See, for example, you will see suggested ratios to use both below and above dpc. I use lime putty for plasters where they were orginally lime and cement for renders/plasters when it is orginally cement. You can find advice on new build ratios and merits from an unbiased source by looking up the BRE guidelines which have different ratios for different areas of houses eg chimneys and below ground differ from above the dpc and also according to the likely weather. It all depends on what you are wanting it to do. Personally, I ensure the total ratio of binder to sand is 3 to 1 for building bricks above dpc and that is the classic 1:1:6 - The 1 of lime and 1 of cement when added together forming a total that makes it 1:3. Chris

[sm_jamieson]

Well, conclusion so far :- Different brickies use different mixes. It must be sufficiently flexible and more so than the blocks, so aerated blocks need a very flexible mix (which makes the aircrete walls seem even more feeble to me !). And using additive instead of lime is very widespread. If additive has the same effect as the lime, I may get a proper additive and use that instead.

By the way, what is the actual important effect of the lime in a cement/sand/lime mix ? If it just makes it easier to work, I don't consider that important. But if, for example, it makes a strong yet flexible mix, it may be very important for aircrete walls.

If there is no general consensus on these matters, it makes it difficult for the DIYer who doesn't want to mess things up!

Cheers, Simon.

[Stuart Noble]

That is the question. Does a 50-50 mix give you the best, or the worst, of both worlds? Nobody talks about pozzolans much, but these were used extensively in Victorian building to make lime mortar set. Small amounts of cement work in the same way, but this is said to cancel out the benefits of lime. If you can make any sense of it all, either here or in the other place to which you will soon be re-directed, let us know.

[meow2222]

You seem to be confusing cement mortar containing lime with lime mortar. The 2 are quite different, and while lime mortar sets slowly, cement mortar with added lime sets fast and hard. When used in cement mortar the lime only adds workability, it does not bring the other properties of lime mortar.

3:1 is the strongest mix, as stiffer mixes microcrack during cure. So there is no mileage in using more cement than that.

Cement takes a month to cure fully, all mixes are weak a day or 2 later, but come back a week later and they've toughened up pretty good.

Detergents weaken final mortar strength, but for domestic brickwork this isnt too significant.

1:1:6 is a good gen purp brickwork mix, but plenty of other mixes also work.

NT

[sm_jamieson]

Well, I guess, for a typical single storey extension, a few courses of blue bricks, DPC, cavity wall of brick/aircrete, what are the most common mixes used ? I guess a survey could be done. The main thing that concerned me is the aircrete blocks cracking on the inner skin, since these are so feeble anyway. I can't see a brickie building 2 skins at the same time, using two piles of mortar ... Simon.

[Andrew Gabriel]

The 1:1 ratio lime to cement is important. Most other ratios of lime to cement in either direction don't work and give a poorer bond than was expected.

[sm_jamieson]

So, are we saying all lime adds to cement/sand is workability ? What does this really mean ? Why bother with the lime if it adds so little ? I'm sure its more expensive than sand.

Simon.

[dg]

Back in the good old days, when the Clerk of Works ruled the site, you could only build a maximum of 1m of wall per day, and the walls had to go up evenly around the building to avoid differential loading of the foundations. Also we could only build cavity walls in sections between the tie wires (two block courses/six brick courses) and then the ties had to be laid across the two skins - horizontal and at 90 degrees, before proceeding with the next courses. And two mixers were needed for the two different mortars

None of this one leaf up to lintel height and hanging the ties out malarky

You are wrong with your impression of aerated blocks being feeble. They are not.

Whatever the walling material, mortar should be weaker than the bricks/ blocks. So even concrete blocks can crack if a mortar is used which is too strong.

And the question you should be asking yourself is ... should the mortar keep the blocks together, or keep them apart? Answers on a postcard please

dg

[meow2222]

The workability's worth it for many jobs. Cement and sand alone just doesnt behave well, extending work times in many cases.

NT

[meow2222]

Neither, its just cement mortar.

Cement is pozzolanic, but has very different results on lime mortar to other pozzolans.

au contraire, theres not good concensus because its hard to muck it up. Whether you use 5:1, 7:1, 1:1:6 or even lime 3:1, with fairy liquid, feb or nothing, your walls will get built and will stay there.

NT

[mail]

Dear Simon The purpose of a PROPER functional lime in a mortar is to act as a binder. it is an alternative to cement. As, quite correctly in a later post, it is explained that the method of setting is chemically different. Lime is slow (approx 0.4% air volume of CO2?) carbonation of the hydroxide and cement sets quite quicky hence the later post explaining how brick walls used to be built. Bagged lime is NOT an effective binder. For good physical chemical reasons (platelet formation) calcium hydroxide from bagged lime in a set mortar cannot act as a binder. It acts as a plasticer and makes the mixture more fluid. It has practically no setting strength. It effectively weakens the mortar STRENGTH whilst maintaining filling the voids. IF you want to use bagged lime as a mortar binder then you need to put it in a plastic bin for several months or a year to form a "putty" which will then have SOME binding strength. However, the best way of using lime is with a proper lime putty NOT bagged lime. In your case, you are only interested in the mortar. You can dispense with lime if you like but I would not myself as it will be a better mortar with it.Penny pinching at this stage is not worth it. Plasticisers "aerate" the mortar with small bubbles of air to make it easier to lay and mix. I would substitute that with lime myself! Chris