A: You did not use enough cement in your mortar.
You used bad cement.
There was dirt in your sand, or you used poor sand. The sand must be sharp and clean.
There was salt in your sand
The moisture was sucked out of your mortar too fast so it dried too quickly and did not have a chance to cure properly. Bonding agent helps to prevent this.
Mortar was used that should have been thrown out. Do not add water to drying out mortar unless you really know what you are doing. Once the chemical reaction has taken place, the mortar could be shot.
Your mortar may have frozen.

    Stone Library

GRANITE
Granite is an important structural and decorative stone. It’s used for immense structural work due to its high compressive strength and durability. It is the hardest structural stone. Granite’s rich colors, stiffness and density, make it an ideal choice for flooring, counter tops, vanities as well as exterior applications. Many stylish patterns and colors make granite the most versatile and trendy of all stones. It is also a very low-maintenance stone. Granite is composed of quartz, feldspars, micas and other minerals. These minerals contribute to the color and texture of the granite stones. Granites get their wonderful variety of colors and patterns from minerals that are melted into the liquid mass as it is formed..

MARBLE
Marble has been valued for thousands of years for its rich palette of beautiful colors and appearance and is perfect, pretty much anywhere in the house. Marble stones consist of limestone that has undergone heat and pressure. A transformation process takes place when the weight of overlying material, pressure from crystal collisions and heat from the earth's core generate temperatures in excess of 1800ºF.Texture of marble depends on the form, size and uniformity of grains. The element components of marble determine the color of the stone. Generally calcite and dolomite marbles are of pure white color. Variations of whiteness of pure marbles are due to the mixture of foreign substances. Such impurities form streaks and clouds.

TRAVERTINE
Travertine is generally used for floors, walls, countertops and for outside as cladding and pavement. Travertine is generally filled with grout before it is honed or polished, which produces a uniform surface more like other marbles. Unfilled travertine is also quite beautiful, and is often seen as exterior surfaces of buildings. Travertine stones result from hot spring water penetrating up through underground limestone. When the water evaporates, it leaves behind layers of dissolved limestone and other minerals, giving it its banded appearance. Travertine stones are generally light-colored beiges and tans.

LIMESTONE
The soft limestone stones are ideal for today's more casual and comfortable environments. Generally these soft beiges and tans are suitable for bathrooms, fireplaces, counters and floors. Limestone is a sedimentary rock that is formed at the bottom of lakes and seas, as organic matters settle to the bottom. As more and more layers build up over thousands and millions of years, adding more and more weight, the heat and pressure cause chemical reactions to take place to harden the sediments into solid stone

SLATE
The lively colors and unique texture make slates appropriate for interior as well as exterior applications. Slate is formed of compressed layers of sediments formed under ocean. Since it is formed in layers, it can easily be split to expose beautifully textured surfaces. The usual colors of slates are earthy browns, beiges, yellows, black, dark-grays, and greenish-gray, pinks, purples and copper are also found. They usually exhibit lot of variations even in the same quarry. Slates are used for flooring, cladding and landscaping.

SANDSTONE
Sandstone consists of sand covered by silica, iron and lime. Cementing material determines the color of sandstone, iron sandstones are reddish, brownish; carbonaceous, black; siliceous, white and argillaceous, earthy to buff. The hardness of sandstones is important in deciding suitability for applications for sandstone.

QUARTZITE
Quartzite stones are the same with slates, but with higher content of quartz in them, which gives them, a bright texture. They are very similar to slates. Like slates, quartzites are used for various applications depending on their hardness.