The performance of mortar before hardening is the operability of mortar, specifically manifested in the consistency, layering degree, water retention, density and setting time.
The consistency index reflects the softness of mortar. If the consistency is great, mortar will be soft and easy for spreading; if the consistency is small, mortar will be relatively hard and the plastic strength will be great.
The volume stability, cohesiveness and ability to retain moisture of mortar are generally indicated by water retention. In the past, it is expressed in the layering degree in China; while at present, ready-mixed mortar has gradually adopted water retention instead of layering degree, and water retention is also adopted abroad. The focus of two indicators varies. The layering degree focuses on the water retention, overall volume stability and degree of separation of each component of mortar; while the water retention focuses on the capacity of mortar to maintain moisture.
If the overall volume stability is poor, such as the layering degree greater than 20mm, then the cement paste is likely to produce segregation and internal layering phenomena with sand particles, resulting in poor bonding between the mortar layer and base course, and dusting on the surface. If mortar has poor water retention, then the water loss of mortar will be very fast, leading to neither sufficient hydration of cement in the mortar nor normal development of strength, also leading to peeling, cracking and dusting of mortar layer. The water retention of mortar should be closely related to the service conditions. If the mortar used is thin, then the requirements of its water retention is high; if the mortar used is thick, then the water retention of mortar should be controlled within limits. For the thin-layer mortar with a thickness no more than 5mm used, whether the adhesive mortar, or surface crack-resistant mortar or masonry mortar, the indicator of its water retention should not be less than 98%. While for the ordinary plastering mortar with a thickness of about 10mm, if its water retention is too good, such as the layering degree less than 10mm or water retention more than 92%, then the moisture inside the mortar layer will not be easily adsorbed by the base course or evaporated to the atmosphere. So the actual water content of mortar is much higher than that required for cement hydration, therefore prolonging the setting time of mortar, leading to dry surface and humid inside of mortar, as well as slowing down the construction speed. And it is not easy for leveling of mortar layer. More seriously, the mortar contraction will be increased and the mortar will be easy for cracks. Therefore, the layering degree and water retention of ordinary plastering mortar should be controlled within limits. The general requirements for ordinary plastering mortar are the layering degree ranging from 10mm to 20mm and water retention no less than 88%.
The density of mortar reflects the compaction degree of mortar. Generally, the longer the mortar is in the damp environment, the greater the density of mortar will be; the stronger its ability to resist the aqueous solution corrosion and the freeze-thaw damage will be.
The setting time of mortar represents the time interval from the moment mortar is stirred with water to the moment mortar itself achieves certain strength and can preliminarily resist the external force. In general, mortar is required to have a certain setting time. If the setting time is very short, then the operable time will be very short, thus affecting the construction operation process; while if the setting time is very long, then it will affect the construction speed, such as its water shrinking and press polishing time. The setting time of dry-mixed plastering mortar is required to be 3-8H.
The properties of mortar after hardening include the strength and durability indicators. The mortar strength reflects the ability of mortar to resist the damage from external force. If the strength is high, then the ability to resist the external force will be strong; and vice versa. According to the types of external forces, the mortar strength can be divided into the compressive strength and bond strength, of which the latter is represented by the tensile bond strength. In general, if the compressive strength is high, then the bond strength will also be great, but the two are not exactly proportional. When the compressive strength is too high, mortar appears to be hard and brittle, doing harm to its bond strength instead. Therefore, in a certain strength range, the increase of compressive strength should be avoided as possible to achieve the increase of mortar bond strength.
The durability indicators of mortar include water, frost, corrosion and contraction resistance. If mortar is not water-resistant, then its scope of application will be restricted. Therefore, Chinese construction specifications state that cement-lime mixed mortar should not be used in the humid environment and long-term water-saturated state, but that cement mortar should be used instead. Another durability indicator of mortar is frost resistance, one of the important parameters to measure the durability of materials. For the wall materials exposed to the outdoor environment, the indicators of frost resistance are F50 in severe cold regions, F35 in cold regions, F25 in hot-summer and cold-winter zone, and F15 in hot-summer and warm-winter zone. If the masonry mortar and plastering mortar with cementing effects are used in exterior masonry and plastering work, the frost-resistant indicator should be equal to that of wall materials. If mortar is used in the foundation works below ±0. 0m, then it will be in the groundwater environment for a long period. Some of the ground water in China has a PH value less than 7, while some greater than 7. Therefore, mortar should also have resistance to corrosion of various weak acid, salt and base solutions, which we call corrosion resistance.
The stress generated by the capillary water loss of cement hydrates has led to the contraction deformation of cement paste. If Portland cement is used for the cementitious materials of mortar, the contraction is inevitable. The higher amount of cement is used, the greater the contraction will be; the higher amount of lime is used, the greater the contraction will also be; the finer the sand is, the greater the contraction will also be. The great contraction means promoting the cracking tendency. Experience has shown that the cracks of cement plastering mortar are less than those of cement-lime mortar. The mortar with a high amount of cement used is hard and brittle; while the mortar prepared with fine sand is easy for peeling and cracking.
In designing the mix proportion of ordinary plastering mortar, we should also consider its operability, including the water retention, cohesiveness, thixotropy, initial crack resistance, bond strength, compressive strength and later crack resistance and other multiple indicators of mortar. Emphasis should not be laid on one certain indicator to affect other properties of mortar. The technical indicators of dry-mixed plastering mortar include the water retention, compressive strength, tensile bond strength, contraction, etc., as well as requirements for frost resistance if used for exterior walls.
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