Why put fairy liquid in cement?

Adding dish soap, such as Fairy liquid, into a mortar or cement mix is an old-school mason trick used to act as an improvised plasticizer and air-entraining agent. The surfactant properties of the soap introduce millions of microscopic air bubbles into the mixture, which reduces surface tension, lubricates the aggregate particles, and makes the mortar dramatically smoother, creamier, and much easier to trowel. This increased workability prevents the mortar from drying out too quickly and helps bricklayers spread it evenly across masonry units. However, professional builders generally discourage using household dish soap in modern structural work because excessive or uncontrolled air entrainment can severely compromise the ultimate compressive strength and durability of the cured concrete.