R.L., North Andover

A. That cement is mortar, and to remove it, treat it with muriatic acid, which attacks the lime in the mortar. Mix equal parts acid and water (always pour the acid into the water), and paint this solution on the mortar. You will see it fizz up; when it stops fizzing, scrub with a wire brush or a bristle brush, and rinse. Wear protective clothing and eye protection when working with acid. You may have to do this more than once, but eventually all the mortar will be gone, except in the joints, where it is supposed to be.

Q. You wrote recently about cement dirt, a concrete of sorts that can be made out of soil and cement. I forget the formula, but I think it would be a good thing to line a trench leading to a culvert to prevent erosion, and to keep water from getting under the culvert. What is that formula? Are there alternatives to prevent erosion and keep the water where it belongs?

HARRY LEVIN, Sugar Hill, N.H.

A. It's called soil cement. Make it by mixing earth with Portland cement to the desired depth, add water and mix again. Tamp, and cover with plastic to let it cure properly. Use 6 to 16 percent cement by volume according to the density of the soil. The denser the soil (clay, for instance), the higher percentage of cement to use. Six percent translates to 1 part cement to 15 parts soil; 16 percent translates to 1 part cement to 6 parts soil.

An alternative is to pour 3 or 4 inches of regular concrete, or install sod or seed the area. You can buy a concrete mix called Sakrete at building stores; it is expensive if you are usinig a reasonable amount of concrete. You probably won't use enough to order it ready-mixed. You could mix your own with this formula: 1:2:3 (1 part Portland Cement, 2 parts sand and 3 parts crushed stone. Incidentally, I suggest you wait until next spring to do any of this work; mixing and curing concrete in freezing weather is not going to work.

Q. Last fall I put down a flagstone patio with mortar between the stones. It looked good with dark gray mortar. Last spring some of the mortar cracked and I had to chip it out and put in new mortar, but it is much lighter than the original. How can I get the new mortar to look like the old?

FRED MAFRICI, Braintree

A. Wait a while; it will turn dark before long. If you want to speed up the darkening, paint the mortar with a dark gray semitransparent stain, the kind you would use on shingles or clapboards. That sounds weird, but if you have ever spilled semitransparent stain on concrete, you know it seems to last forever. The same will happen with the stain on the mortar.

If you laid the flagstones on sand or stone dust, the mortar will continue to break up as the stones move up and down with frost heaving. It will be a constant battle. To avoid this continuous chipping out of broken mortar and filling in with new, chip it all out and fill the joints with stone dust. It will pack down very nicely and will drain water well, but it won't break up. And it will be a dark gray color.