Generally, the water content of masonry mortar is 260–300 kg/m3, and the cement content of ordinary masonry mortar is 180–300 kg/m3. The main is to ensure that the mortar does not absorb too much water from the block material before it sets and hardens, and that the cement in the mortar does not hydrate without sufficient water due to excessive water loss, so as to avoid reducing the strength of the mortar itself and the bond strength between the mortar and the block material.
As we all know, the moisture required for the complete hydration of cement is 26% of the cement mass. The amount of water used in mortar greatly exceeds the amount of water required for the hydration of cement in mortar. The excess water is mainly used to meet the needs of construction. The strength of cement stone is mainly related to the water-cement ratio. The larger the water-cement ratio, the higher the porosity of cement stone and the lower the strength of cement stone, and the strength of mortar will also decrease accordingly. Therefore, as long as the water retention of the masonry mortar ensures the workability of the mortar and the water required for the hydration of the cement in the mortar, the mortar strength will be fine.
If the water retention of the masonry mortar is too good, then the actual amount of water retained in the mortar is high, the true water-cement ratio of the mortar is high, the actual strength of the mortar is low, and the bond strength with the block material is correspondingly low. In addition, if the water retention of the mortar is too good, the water is not easily absorbed by the block material, which also affects the bond between the mortar and the block material, 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 the masonry mortar should be related to the block material. If the pore structure of the block material is open, the block material is easily soaked through with water, such as sintered bricks, then the water retention rate of the masonry mortar can be lower, as long as it reaches 80% or more. For example, using traditional mortar to build sintered bricks is very effective.
If the pore structure of the block material is closed, the porosity is high, the block material is not easily soaked through with water, or the block material is not allowed to be wetted with water during construction, then the water retention of the masonry mortar should be increased to meet the moisture required for the hydration of the cement in the mortar. For example, when autoclaved sand-lime bricks are being built, a masonry mortar with a water retention rate of 80% is used to build the sand-lime bricks. Due to the low water retention rate of the mortar, the moisture in the mortar is easily absorbed by the sand-lime bricks, resulting in the cement hydration required moisture is seriously insufficient, making the cement hydration can not be carried out normally, reducing the real strength of the mortar and the bond strength of the mortar and sand-lime bricks, which is also one of the reasons for the cracking of masonry caused by traditional mortar masonry sand-lime bricks.
Therefore, the water retention rate of the mortar used for masonry sand-lime bricks should be controlled at 88% or higher. However, if we increase the water retention rate of the mortar used for masonry sand-lime bricks to 95% or higher, 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 bricks will also be reduced. Furthermore, if the water retention of the mortar is too high, the bricks will not bond with the mortar during masonry, and the height of the masonry will be limited. Therefore, the water retention of masonry mortar is not the higher the better, and there should be a corresponding water retention range for different block materials.
