The amount of water used in masonry mortar is 260-300 kg/m3. The main role of controlling the water retention of masonry mortar is to ensure that not too much water of mortar will be absorbed by the block material before setting and hardening; otherwise, the cement in mortar will have no enough water for hydration caused by too fast water loss, thus reducing the strength of mortar itself and the bond strength of mortar and block material.
As we all know, the theoretical water amount of water required for complete hydration of cement is 26% that of cement mass, while the actual water amount of mortar has greatly exceeded the amount of water required for hydration of cement in mortar, and the exceeding water amount is mainly to meet the needs of the construction. The strength of cement is mainly related to the water-cement ratio. The larger the water-cement ratio is, the greater the porosity of cement stone and the lower the strength of cement stone, so the strength of mortar will be reduced accordingly. Therefore, it can be OK as long as the water retention of masonry mortar can guarantee the operability of mortar and the amount of water required for hydration of cement in mortar.
If the water retention of masonry mortar is too good, the actual water retained in the mortar will be much and the real water-cement ratio of mortar will be large, so the actual strength of mortar will be low and the bond strength with the block material will be correspondingly low. In addition, when the water retention of mortar is too good, moisture will not be easy to be absorbed by the block material, which will also affect the bonding of cement slurry and block material and extend the setting time of mortar, thus affecting the masonry speed and increasing the construction difficulty. So, the water retention indicators of masonry mortar should correspond to those of bulk materials.
If the pore structure of bulk material is open and the bulk material is susceptible to be drenched by water, such as sintered brick, then the water retention of masonry brick can be lower, just over 80%. For example, using traditional mortar for masonry sintered common brick, the effect is very good.
If the pore structure of block material is closed, with high porosity, and the block material is not susceptible to be drenched by water, or the block material is not allowed to be wetted by watering during construction, then the water retention of masonry mortar should be improved in order to meet the need of water for the hydration of cement in mortar. For example, in autoclaved sand-lime brick building, if the masonry mortar with water retention of 80% is used in plastering sand-lime brick, due to the low water retention, the moisture of mortar will be easily absorbed by sand-lime brick, causing a serious shortage of water required for cement hydration in the mortar joint, making cement hydration not work properly, and reducing the actual strength of mortar as well as the bond strength between mortar and sand-lime brick, which is also one of the reasons why it is easy for the masonry to crack by using traditional mortar in plastering sand-lime brick. Therefore, the water retention of mortar used in plastering sand-lime brick should be controlled up to 88%. However, if the water retention of mortar for plastering sand-lime brick is increased up to 95%, it will make the moisture in the mortar joint difficult to be absorbed, the actual strength of mortar reduce, and the bond strength between mortar and sand-lime brick also reduce. Furthermore, if the water retention of mortar is too good, it will not be easy for stable bonding between brick and mortar in masonry construction, and the masonry height will be restricted. Therefore, the water retention of mortar is not the higher the better and there should be a range of water retention adapted to different bulk materials.
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