Which of the following solutions is not allowed for cleaning masonry walls?

Water solutions are generally the safest cleaning method for masonry walls due to their non-reactive nature. However, cleaning masonry can be sensitive to the types of solutions used because certain chemicals can damage the material. Acid solutions, often used to remove efflorescence or mortar stains, can etch or otherwise harm the masonry if used improperly. Caustic solutions, which are strongly alkaline, can also lead to damage and discoloration of masonry. Alkaline solutions, while sometimes used, can still pose risks similar to those of acid solutions, particularly if they are too concentrated.

Using water solutions minimizes these risks as they do not involve harsh chemicals that could react with the masonry. Therefore, water solutions are preferred and widely accepted for general cleaning purposes. Understanding the appropriate methods for cleaning masonry helps ensure the durability and aesthetic appeal of the materials, promoting proper maintenance practices.