Bricks, Masonry, Asphalt and Concrete - Mortar Failure

I mixed up mortar to grout a granite walkway failed to set up properly. After drying, you can stick a screwdriver right into the mortar and it just crumbles to dust. What was wrong with the mix? To wet? What is the proper consistency for Type N Mortar? Is there any special trick to mixing it?

What ingredients did you use to mix it with? What was it laid onto (not the granite, the sub-base). How long has it cured and what is the temp range during that time?

I used Quikcrete Type S (in the green and white bag) and the bag said just to add water which is what I did. The temperature Saturday was 65 degrees. The mortar was placed around the flagstones with a small trowel. About 12 hrs later, I could tell that something was wrong. The best way I can describe it is, there is a thin top "crust" that is hard and beneath that the mortar is a powdery consistency. (If a lady wearing high heels stepped directly on the mortar, they would probably go right through the mortar.)

About the porch-- it is approx 8 x 12 ft. with one front step and a curved walkway leading to the street. I believe the porch has a concrete slab sub-base, topped with a layer of sand, topped with the granite/or limestone flagstones set in an irregular pattern. The stones are various sizes/shapes but all are approx 2 inches thick and are all smooth on the top surface. The porch/walkway are very level, especially considering the age of the house (77 years) and it appears to be original to the house.

It sounds like the sand sucked all of the moisture from the grout. Other than that, I have seen cases where the pre-packaged mortars contain too much sand, not as a part of the normal formula, but from a clogged feed line on the bagging machine. When you mixed it did it have a creamy consistancy or a sandy one?

it seemed to have a lot of grit in it, so I guess that was the sand.

It is possible you used the wrong material or had an error in the method of application.

You mentioned an "irregular" pattern. What is the size (width) of the areas you tried to fill with mortar. If the joints are wide, you could be better off with a flowable concrete mix instead of a zero slump type concrete like mortar.

Masonry mortar is intended to be compacted to fill the joint. With a vertical wall, this is easy because of the weight of the wall above and the size of the joint (3/8"). Mortar relies on compaction to acheive strength and bond. If you have wide joints you may have the wrong material. It has a low moisture content because it must support the wall during construction.

Concrete and mortar require moisture to cure and gain strength. For your application, you should keep the mortar moist to help it cure. By moistening the mortar now, it will begin to cure where it left off and get the most out of what you have.

I question the advisability of trying to put mortar or grout between stones that are sitting on a sand bed. The sand is not strong or rigid enough to permit you to create a rigid surface over it. Even if the mortar (or grout) is perfect, you will still get cracks no matter how strong the mortar is.

Sand setting beds are meant for use with pavers like interlocking concrete pavers where you can tolerate very small movement with no adverse effects. In this method, no mortar is ever used.

The proper way would be to set the stone on a grout bed directly on the concrete. Then you would fill the joints.

The chances on getting an over-sanded bagged mixes are very, very remote because the masonry sand used in pre-proportioned mixes is clean is oven dried. In many operations the sand and cement are mixed together in large batches before bagging.

Dick

I am not going to disagree with what ConcreteMasonry said should be done.

In the past, that is not the way it was done and I have seen 10-20 even 50+ year old pavements made of sand bed grouted flag that are in fine shape. That only means that those particular pavements held, not how good a method it was or is.

That said, to use mortar over a sand bed, you would have to keep everything damp as you grouted, including your mortar. Joints should be 1" or less and minimum rock size should be 6x6. I would not rely on a prebagged mix, personally, but then I would only lay concrete pavers in a sand bed...

Cement was used the last time the porch was tuckpointed (the neighbor told me and he's lived next door 50+ years), so that is what I am going with. Found out it is granite stones that form the porch/walkway and laid on a cement slab base. The sand is just a thin layer spread on top of the slab base to level the granite flagstones. Hopefully cement will work better than the mortar. Nope, sorry I am going with premix cement in a bag.

About keeping the cement moist after application--I am assuming you are talking about just a light mist with the garden hose? How often do you do that and for how long? Every hour or so for the first day? Also should you moisten the edges of the flagstones before you start applying the cement? (Just wondered if the granite would absorb the water in the cement, thus drying it out too quickly).

What should be the consistency of the mixed cement? I've been told everything from toothpaste to peanut butter. I've also been told it should be thick enough to stick to the trowel if you turn the trowel sidways. I will be mixing small batches in a 5 gallon plastic bucket--how long should I mix the cement? Just until I can see that the cement mix and water have combined fully? Or do you continue mixing for a few minutes after?

Thanks for all your helpful comments.

"cement" has no meaning. What type of cement and in what porportions? The granite will absorb little to no moisture, it is the sand that will suck the moisture out and is what should be dampened. Wet curing will help but isn't needed unless it is extremely dry and or windy.

It is almost impossible to mix cementious compunds in a five gallon bucket without a proper mixing paddle and 1/2" drill; it is better to do it in a wheelbarrow or on a sheet of plywood.

I plan to buy Quikrete brand - Type N Masonry Cement in the gold and white bag. I believe it comes in 60 lb bags. I will probably use about 10-15 lbs at a time, but that is just a guess. Temps will be around 65, so I will not do the moistening. I will check on getting the mixing paddle.

My main question inow s about the consistency that the cement should be.

Type N masonry cement has to be mixed with 3 parts sand by volume. The sand needs to be moistened before you mud the joints. Mix it to a workable consistancy, place it and then come back and tool the joints after inital set. Perform your primary cleaning of the granite at this point as well.

Note that I don't think this is the best way to do the job, but it is the best way with what you state you are going to do.