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Technology of mortars and their properties

BUILDING MORTARS AND DRY MIXTURES

Building mortars unlike the concrete do not contain the coarse aggregates and can be placed, normally, on porous base without intensive mechanical action. The building mortars according to the row of structural features and properties are similar to the fine-grained concrete. Comparatively low strength allows at the mortars production to use successfully local binder materials and various industrial wastes (ash and slag, lime and gypsum containing products etc).

Lime, gypsum, cement and composite mortars (cement-lime, cement-clay etc) are mainly applied in construction. Cements are used as a binder for mortars which are exploited at high humidity. Heavy-weight mortars have an average density over 1500 and light-weight less than 1500 kg/m3. For light-weight mortars natural or artificial porous sands can be used as aggregates. The building mortars according to the purpose are divided on masonry, intended for arrangement of brick, large panel and other walls structures, finishing and special. Among the special mortars grouting, injection, waterproofed, heat-resistant, chemically proofed and other are used in building.

Modern construction is characterized by widely application of dry building mixtures - carefully batched and mixed industrially mortar and concrete mixtures mixed by water on a building site.

Efficiency of dry mixtures consists in increasing of mechanization level, essential reduction of terms of construction, decreasing of labor and industrial expenses, high quality supply.

Dry building mixtures are classified by:

§ basic destination (a kind of works);

§ type of binders in a mixtures;

§ modification degree by mix admixtures;

§ the most typical property after hardening;

§ application conditions.

By destination dry building mixtures can be divided as masonry, facing, seam, spackling, plastering, gluing, sealing mixtures, etc.; by a type of the main binder - gypsum, anhydrite, lime, magnesia, cement, cement-lime, polymer, etc.; by modification degree - economic, standard, high-quality; by characteristic property after hardening - adhesive, weather-proof, rapid-hardening, water-proof, frost-resistant, high-strength, self leveling, elastic, etc.; by application conditions - manual and machine placing, for porous materials, etc. The same mixture can be often used at production of different types of works.

The dry mixtures containing in the composition one type binder, are called simple mixtures; mixed binders - composite mixtures.

 

Technology of mortars and their properties

 

 
 

Preparation of mortars. Building mortars are prepared in movable and stationary mortar-mixing plants in the form of ready-mixed mortar mixtures with required plasticity or as dry mixtures which are mixed with the water before the use (Fig. 12.1). It is effectively to use the hydrophobic cements which provide the long-term of their storage at the production of dry mixtures.



The technological process of manufacture of the ready-made mortar mixture includes preparation of the initial materials (screening-out of the gravel impurities, warming up in winter etc), dosage and careful mixing.

The automated mortar-mixing machines with the productivity 18-20 m3/hour are widely applied at the centralized manufacture of mortars.

The compositions and modes of mortar mixtures making, graphics of mortars delivery on the building objects can be determined with a computer help on the modern plants.

The forced mixing of mortar components can be realized in mortar-mixers with the cyclic or continuous mode. The productivity of widespread mortar-mixers is 30-1200 l/hour of the prepared batch. The duration of the mortar mixing should provide the obtaining of sufficient homogeneity of the mix. The duration of mixing should be, usually, not less than 1 minute for heavy-weight mortars, for light-weight mortars – 2 minutes.

The mortars are manufactured with antifreezing admixtures in winter.

The truck mortar-mixers are used to transport the mortar mixtures. Transporting of mortars to the building site is carried out with the help of the mortar pumps.

Properties of mortars. The workability and water-retaining ability are the basic properties of mortar mixtures.

The workability is a mortar ability to be placed on the base by a homogeneous layer. It can be measured by Abrams cone test or by penetration test with special cone (Fig. 12.2). It is specified taking into account the type of mortar, method of it application, humidity and porosity of base, temperature of air. For example, the depth of mortar cone penetration is specified at placing at the ordinary brick – 9-13 cm.

It is possible to attain required workability of mortar mixtures without segregation by application of plasticizers and fillers. The plasticizers of mortar mixtures of two kinds are used: inorganic (lime and clay paste) and organic - surface active substances. The mechanism of inorganic plasticizers action consists in creation of an adsorption hydrate layers on the particles surface that provides grains sliding of the aggregates, and organic – in their dispersive action. Unlike the inorganic organic plasticizers are entered to mortar mixtures in considerably less amount (0.03-0.3% by weight of binder).

Water-retaining capacity prevents segregation of the mortar mixtures. For this purpose can be diminished water – binder ratio (due to correct proportioning of composition) and can be added fine mineral fillers and plasticizing agents.

Mortars are placed at the porous base in the most cases. The process of the normal cement hardening is impeded at the superfluously intensive sucking of water by base. At the same time fly ash can be used in mortars as active admixture, microfilling agent and plasticizer. Its presence improves plasticity, water-retaining ability and other properties of mortars, substantially allows reducing the cement and lime content. Fine-grain fly ash that is taken away from the last fields of electrostatic collectors is the most effective.

The compressive strength of the mortars is in the range of 0.4-30 MPa. Mortars with ultimate strength 0.4-1 MPa are made mainly of lime. Strength of mortars can be determined on specimens - cubes at 28 days hardening. At making of specimens is important to take into consideration the real conditions of mortar hardening.

Strength of mortars, as well as concretes, depends, mainly, from a binder strength and water-binder ratio. At the placing on a dense base, the strength of mortars ( ) can be calculated by a formula:

, (12.1)

where - cement strength; C/W - cement-water ratio.

At the water suction by the porous basis; strength of mortars can be calculated depending on binder content (B):

, (12.2)

where Ê – sand quality coefficient.

The frost resistance of building mortars is determined by the number of cycles of freezing and thawing, which are specimens withstand without destruction. The mortar grade of the frost resistance depends on operating conditions. For wall laying and external plaster grade is determined, usually, in the range of F10-F50, and at the humidity conditions in the range of F100-F300. The frost resistance of mortars, as well as frost resistance of concrete, is determined by a type of binder and admixtures, water-binder ratio, quality of aggregates and conditions of hardening. It goes down at a presence of leakage, cavities and large voids, conditioned by insufficient workability.

The structure of mortars is modified by admixtures of organic surface-active substances. Thus water absorption and capillary suction go down, a freeze-resistance and watertightness grow.

Compositions of building mortars are proportioned according to tables or calculation and in both cases are corrected by experimental way on the given initial materials. The some compositions of the composite masonry mortars (measured in volume parts - Cement: Lime paste: Sand) are resulted in Table 12.1.

Òable 12.1

Compositions of the composite masonry mortars

Ultimate binder strength, MPa Compressive strength, MPa
2.5 5.0 7.5 10.0
1:0.9:8 1:0.5:5.5 1:0.4:4.5
1:1.4:10.5 1:0, 6:6 1:0.3:4 1:0.2:3.5
1:0.8:7 1:0.3:4 1:0, 1:2.5

 


Date: 2015-12-18; view: 1059


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