Why is mortar added to the first truck when pouring concrete?

[McHeidi]

To ensure a tight bond between the new and old concrete surfaces. The mortar used in the construction process is typically mixed on-site for ease of construction: The cement mortar mix ratio of 1:3 refers to the weight ratio, which is then converted to a volume ratio based on the capacity of the on-site mixer or material container. In structural construction, pre-mixed mortar is often used, usually indicated by mortar strength grades such as M5, M7.5, and M10. Cement mortar is suitable for masonry in dry environments: Walls in multi-story buildings generally use cement-lime mortar with a strength grade of M5; brick columns, brick arches, and reinforced brick lintels typically use cement mortar with a strength grade ranging from M5 to M10; brick foundations generally use cement mortar with a strength grade no less than M5; low-rise buildings or single-story houses may use lime mortar; simple structures may use lime-clay mortar. The mortar used in masonry must have a strength no less than that of the masonry blocks. Construction drawings often specify the mortar grade, emphasizing its strength.