Technical Properties of Mortar
For the sake of projects, the newly mixed mortar should have good workability, and the hardened mortar should have the required strength, the bending power to the bottom face, little deformation and durability.
1. Workability of Fresh Mortar
The workability of fresh mortar refers to the comprehensive properties of mortar easy for construction and good for quality, including mobility and water retention. The mortar with good mobility is easy to be paved thinly and evenly on bricks and bonded with floors well.
a. Mobility (Consistency)
The mobility of mortar is the property that mortar can flow under the role of dead weight and exterior force. Fluidity is expressed by “sinking degree”, usually determined by the consistency of mortar. The bigger the sinking degree is, the better the mobility will be.
The selection of mortar mobility should be determined by masonry types, construction conditions, and weather. The consistency of masonry mortar should be selected according to project requirement.
b. Water Retention
The water retention of mortar refers to the property of mortar to maintain moisture. When the mortar with good water retention is used in transportation, standing and pavement, water will not escape from mortar soon and the necessary consistency can be maintained. To keep a certain amount of water in mortar is easy to manipulate and also guarantees the normal hydration of cement to maintain the strength of masonry.
The water retention of mortar is expressed by layering degree, measured by mortar layering degree instrument. The mortar with good water retention has the layering degree of 10-30mm, and if it is more than 30mm, the water retention will be bad and easy to segregate; the mortar whose layering degree is less than 10 mm is not good for construction. Based on many experiments, the layering degree of cement mortar should be no more than 30mm, and the layering degree of cement mix mortar should be no more than 20mm.
2. Strength of Hardened Mortar
The hardened mortar should have big strength which is expressed by strength grade. Compressive strength is the main basis for mortar strength. The strength grade of mortar, expressed by fm, is determined by compressive strength average (MPa) measured through curing a group of six cube specimens with side length of 70.7mm for 28d, the standard test method.
The strength grades of mortar include: M20, M15, M10, M7.5, M5 and M2.5, the six grades.
The strength of mortar is connected with its surface material. For the ordinary cement mortar, the following equation can be used to calculate its compressive strength.
3. Adhesion Stress of Mortar
Brick and stone masonry is a solid entity composed of many blocks that are bonded by mortar as a whole. Thus, it is required that mortar must have a certain adhesion stress over bricks and stones. Generally, the more the compressive strength of mortar is, the bigger its adhesion stress will be. In addition, its adhesion stress is related to the surface, cleanness, and humidity of bricks and stones, as well as the construction and curing conditions. For example, bricklaying needs watering, and the surface without clay will improve the adhesion stress and ensure the quality of masonry.
4. Deformability of Mortar
It is easy for mortar to get deformed when it bears loads or the temperature changes. If it deforms greatly or unevenly, the quality of masonry and surface will decrease and cause shrinkage and crack. When fine aggregates are used to mix the mortar, its deformation is bigger than the ordinary mortar. In order to prevent the cracks caused by uneven shrinkage deformation, hemp cut, paper strip and others fabric materials can be mixed in the surface mortar.
5. Durability of Hardened Mortar
The durability of mortar refers to the property to withstand wear and tear in the long-term use. The hydraulic masonry that usually contacts with water should be impermeable and frost-resistant, so the impermeability and frost resistance of hydraulic masonry should be considered.
a. Frost Resistance
The frost resistance of mortar refers to the property to resist freeze-thaw cycle. Mortar is frozen and damaged because the water in its pores expands due to freeze and breaks the pores. Thus, dense mortar and the mortar with closed pores have good frost resistance. In addition, the factors influencing the frost resistance of mortar also include cement types, strength grades, and water-cement ratio.
The frost resistance of mortar is the property to resist the infiltration of pressure water. It is mainly related to density and size and structure of the inner pores. The connecting pores inside mortar and the cellular structures and, pores formed when it is moulded, all of which can lead to water seepage of mortar.