Due to mourning over the destruction of Jerusalem, the Sages decreed that it is prohibited for one to plaster his entire house with quality plaster. When plastering, one must leave a spot the size of a square cubit unplastered to commemorate the destruction of the Temple. In the course of discussing the volume of sand that would make someone liable for carrying on, the Sages dispute which lime may not be used.
The Gemara teaches:
The measure that determines liability for carrying out coarse sand is equivalent to that which is used to place on a full spoon of plaster. We learned in the Mishna: The measure that determines liability for carrying out coarse sand is equivalent to that which is used to place on a full spoon of plaster.
A taught in a Tosefta: An amount equivalent to that which is placed on the opening of a plasterer’s trowel, and not on a spoon used for eating. The Gemara asks: Who is the tanna who holds that sand is beneficial for plaster and is, therefore, mixed with it? Rav Ḥisda said: It is Rabbi Yehuda, as it was taught in a: In mourning the destruction of the Temple, one may not plaster his house with plaster, which is white, unless he mixed straw or sand in it, which will make the color off -white and less attractive. Rabbi Yehuda says: Straw is permitted, but sand is prohibited because when mixed with plaster it forms white cement [teraksid]. Apparently, Rabbi Yehuda holds that sand is typically mixed with plaster. Rava said: Even if you say that our Mishna is in accordance with the opinion of the Rabbis who disagree with Rabbi Yehuda, we can say that its ruination is its improvement.
Even though the Rabbis hold that mixing sand with plaster is not beneficial, since following the destruction of the Temple only partially ruined plaster may be used, adding sand to plaster enables its use.