Drymix Mortar is the most widely used and one of the essential materials in modern construction engineering. It is composed of cement, sand and admixtures. Cement is the main cementing material. Today let’s learn more about the basic properties of drymix mortar.
Construction mortar: It is a construction material prepared by cementing material, fine aggregate, admixture and water in proper proportions.
Masonry mortar: The mortar that binds bricks, stones, blocks, etc. into masonry is called masonry mortar. Masonry mortar plays the role of cementing blocks and transmitting load, and is an important part of masonry.
1. Composition materials of masonry mortar
(1) Cementing material and admixture
The cementing materials commonly used in masonry mortar include cement, lime paste, and building gypsum.
The strength grade of cement used for masonry mortar should be selected according to the design requirements. The strength grade of cement used in cement mortar should not be greater than 32.5; the strength grade of cement used in cement mixed mortar should not be greater than 42.5.
In order to improve the workability of mortar and reduce the amount of cement, some lime paste, clay paste or fly ash are often mixed into cement mortar, and the mortar prepared in this way is called cement mixed mortar. These materials must not contain harmful substances that affect the performance of the mortar, and when they contain particles or agglomerates, they should be filtered with a 3 mm square hole sieve. Slaked lime powder shall not be directly used in masonry mortar.
(2) Fine aggregate
The sand used for masonry mortar should be medium sand, and the rubble masonry should be coarse sand. The mud content of the sand should not exceed 5%. For cement-mixed mortar with a strength grade of M2.5, the mud content of the sand should not exceed 10%.
(3) Requirements for additives
Like the addition of admixtures in concrete, in order to improve certain properties of mortar, admixtures such as plasticizing, early strength,cellulose ether, antifreeze, and retarding can also be added. Generally, inorganic admixtures should be used, and their types and dosages should be determined through experiments.
(4) The requirements for mortar water are the same as those for concrete.
2. Technical properties of masonry mortar mixture
(1) Fluidity of mortar
The performance of mortar flowing under its own weight or external force is called the fluidity of mortar, also called consistency. The index indicating the fluidity of the mortar is the sinking degree, which is measured by a mortar consistency meter, and its unit is mm. The selection of mortar consistency in the project is based on the type of masonry and construction climate conditions, which can be selected by referring to Table 5-1 (“Code for Construction and Acceptance of Masonry Engineering” (GB51203-1998)).
The factors that affect the fluidity of mortar are: the water consumption of mortar, the type and amount of cementitious material, the particle shape and gradation of aggregate, the nature and dosage of admixture, the uniformity of mixing, etc.
(2) Water retention of mortar
During the transportation, parking and use of the mixed mortar, it prevents the separation between water and solid materials, between fine slurry and aggregate, and the ability to keep water is the water retention of mortar. Adding an appropriate amount of microfoam or plasticizer can significantly improve the water retention and fluidity of the mortar. The water retention of mortar is measured by a mortar delamination meter, and is expressed by delamination (. If the delamination is too large, it means that the mortar is prone to delamination and segregation, which is not conducive to construction and cement hardening. The delamination degree of masonry mortar should not be greater than 3 0mm. If the delamination is too small, drying shrinkage cracks are prone to occur, so the delamination of mortar should not be less than 1 0mm.
(3) Setting time
The setting time of building mortar shall be evaluated based on the penetration resistance reaching 0.5MPa. The cement mortar should not exceed 8 hours, and the cement mixed mortar should not exceed 10 hours. After adding the admixture, it should meet the design and construction requirements.
3. Technical properties of masonry mortar after hardening
The compressive strength of the mortar is used as its strength index. The standard specimen size is 70.7 mm cubic specimens, a group of 6 specimens, and the standard culture is up to 28 days, and the average compressive strength (MPa) is measured. Masonry mortar is divided into six strength grades according to the compressive strength: M20, M15, M7.5, M5.0, and M2.5. The strength of mortar is not only affected by the composition and proportion of the mortar itself, but also related to the water absorption performance of the base.
For cement mortar, the following strength formula can be used to estimate:
(1) Non-absorbent base (such as dense stone)
The non-absorbent base is the main factor affecting the strength of mortar, which is basically the same as that of concrete, that is, it is mainly determined by cement strength and water-cement ratio.
(2) Water-absorbing base (such as clay bricks and other porous materials)
This is because the base layer can absorb water. When it absorbs water, the amount of water retained in the mortar depends on its own water retention, and has little to do with the water-cement ratio. Therefore, the strength of mortar at this time is mainly determined by the strength of cement and the amount of cement.
Bond strength of masonry mortar
The masonry mortar must have sufficient cohesive force to bond masonry into a solid whole. The size of the cohesive force of the mortar will affect the shear strength, durability, stability and vibration resistance of the masonry. Generally, the cohesive force increases with the increase of the compressive strength of the mortar. The cohesion of mortar is also related to the surface state, degree of wetness and curing conditions of masonry materials.
Post time: Dec-07-2022