Why isn’t the water retention of masonry mortar the higher the better?

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Why isn’t the water retention of masonry mortar the higher the better?

Masonry mortar should have good workability, including fluidity, cohesion and thixotropy. The mortar with good workability is easy to spread into a uniform thin layer on the rough block surface, and can be tightly bonded to the bottom surface. The use of mortar with good operability not only facilitates construction operations, improves labor productivity, but also ensures project quality. At the same time, the masonry mortar should have better water retention, so as to prevent the moisture in the mortar from being absorbed by the blocks prematurely and too much, which will affect the further hydration of the cement. Secondly, it is required that the hardened mortar should have a certain compressive strength, bonding strength, etc. to ensure the strength and integrity of the masonry.

The water consumption of masonry mortar is 260~300kg/m3, and the cement consumption of traditional masonry mortar is 180~300kg/m3. The main function of controlling the water retention rate of masonry mortar is to ensure that the mortar will not absorb too much water by the blocks before setting and hardening. The bond strength of mortar and block.

As we all know, the water required for complete hydration of cement in theory is 26% of the cement mass. The water consumption of mortar greatly exceeds the water required for hydration of cement in mortar, and the excess water is mainly to meet the needs of construction. The strength of the cement stone is mainly related to the water-cement ratio. The larger the water-cement ratio, the greater the porosity of the cement stone, the lower the strength of the cement stone and the corresponding reduction in the strength of the mortar. Therefore, as long as the water retention performance of the masonry mortar ensures the operability of the mortar and the water required for the hydration of the cement in the mortar.

If the water retention of the masonry mortar is too good, then the

The more actual moisture is left, the larger the real water-cement ratio of the mortar, the lower the actual strength of the mortar, and the lower the bond strength with the block. In addition, the water retention of the mortar is too good, and the water is not easily absorbed by the blocks, which will also affect the bonding between the cement slurry and the blocks, and will prolong the setting time of the mortar, thereby affecting the speed of masonry and increasing the difficulty of construction. Therefore, the water retention index of masonry mortar should be related to the block material.

If the pore structure of the block material is open, and the block material is easily penetrated by water, such as sintered bricks, the water retention rate of the masonry mortar can be lower, and it only needs to reach more than 80%. The effect of building sintered ordinary bricks is very good. If the pore structure of the block material is closed, the porosity is high, the block material is not easily penetrated by water, or the block material is not allowed to be wetted by water during construction, then the water retention of the masonry mortar should be increased to meet the requirements of the mortar. Moisture required for hydration of medium cement.

When building autoclaved lime-sand bricks, masonry mortar with a water retention rate of 80% is used to build lime-sand bricks. Due to the low water retention rate of the mortar, the moisture of the mortar is easily absorbed by the lime-sand bricks, resulting in the cement hydration in the mortar joints. The serious lack of water makes the cement hydration unable to proceed normally, which reduces the real strength of the mortar and the bond strength between the mortar and the lime-sand bricks. Therefore, the water retention rate of the mortar used for building lime-sand bricks should be controlled at more than 88%. By adding a small amount of hydroxypropyl methylcellulose, the water retention of the mortar can be controlled and the strength can be improved.

However, if we increase the water retention rate of the mortar of the lime-sand bricks to more than 95%, the water in the mortar joints will be difficult to absorb, the actual strength of the mortar will be reduced, and the bond strength between the mortar and the brick will also be reduced, and The water retention of the mortar is too good, the bricks are not easy to bond with the mortar stably during masonry, and the masonry height is limited. Therefore, the water retention rate of masonry mortar is not as high as possible, and there should be a corresponding range of water retention rate for different block materials.

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