Improvement Effects of Hydroxypropyl Methylcellulose on Mortar and Concrete
Release Time:
2021-05-21
Source:
Improvement Effects of Hydroxypropyl Methylcellulose on Mortar and Concrete
In recent years, with the continuous development of external thermal insulation technology for building envelopes, the ongoing advancement of cellulose production technologies, and the inherent superior properties of HP cellulose, HP cellulose has found widespread application in the construction industry. To gain a deeper understanding of the interaction mechanisms between HP cellulose and cement-based materials, this paper introduces HP fiber. The improvement effect of the additive on the cohesive performance of cement-based materials.
Setting time
The setting time of concrete is primarily determined by the setting time of cement, with only a minor influence from the aggregate; therefore, the setting time of mortar can be used as a surrogate to investigate the effect of HP cellulose on the setting time of underwater non-dispersible concrete mixes. Since the setting time of mortar is affected by the water-to-cement ratio and the paste-to-aggregate ratio, in order to evaluate The effect of HP cellulose on the setting time of mortar requires that the water-to-cement ratio and the paste-to-aggregate ratio be kept constant.
Experimental results show that the addition of HP cellulose has a significant retarding effect on the mortar mix, with the setting time progressively increasing as the dosage of HP cellulose rises. Under the same HP cellulose dosage, mortar molded underwater exhibits a longer setting time than mortar molded in air. During testing in water, the addition of Compared with the control specimen, the setting time of mortar containing HP cellulose is delayed by 6 to 18 hours for initial set and by 6 to 22 hours for final set. Therefore, HP cellulose should be used in combination with an early-strength admixture.
HP cellulose is a polymeric macromolecule with a linear molecular structure, bearing hydroxyl groups on its functional groups. These hydroxyl groups can form hydrogen bonds with water molecules in the mix, thereby increasing the viscosity of the mixing water. Moreover, the long molecular chains of HP cellulose attract one another, leading to intermolecular entanglement and the formation of a network structure that encapsulates the cement and mixing water. Come. Due to the formation of a film-like network structure by HP cellulose and its encapsulating effect on cement, it effectively prevents the evaporation of moisture from the mortar, thereby inhibiting or slowing down the rate of cement hydration.
Bleeding tendency
The bleeding phenomenon in mortar is similar to that in concrete: both lead to significant aggregate settlement, which increases the water-to-cement ratio of the top-layer paste, resulting in substantial plastic shrinkage—and even cracking—in the early stages, while also weakening the strength of the paste’s surface layer.
When the dosage exceeds 0.5%, bleeding virtually ceases. This is because, upon incorporation into mortar, HP cellulose forms a film-like, network structure and its hydroxyl groups on the long polymer chains exert an adsorptive effect, causing the cement in the mortar to aggregate with the mixing water into a flocculent state, thereby ensuring the stable structural integrity of the mortar. Furthermore, When HP cellulose is added to mortar, numerous discrete microbubbles are formed. These bubbles are uniformly distributed throughout the mortar, thereby inhibiting the settling of aggregates. This technical property of HP cellulose has a significant impact on cement-based materials and is frequently employed in the formulation of novel cement-based composites, such as dry-mix mortars and polymer-modified grouts.
This imparts excellent water retention and plasticity.
Water demand of mortar
Even at very low dosages, HP cellulose has a significant impact on the water demand of mortar. When the slump of fresh mortar is kept essentially constant, the relationship between the dosage of HP cellulose and the water demand of the mortar exhibits a linear trend over a certain period: initially, the water demand decreases, followed by a marked increase. The dosage of HP cellulose
When the dosage is less than 0.025%, increasing the dosage results in a reduction in the water demand of the mortar under the same slump condition, indicating that a low dosage of HP cellulose has a water-reducing effect on the mortar. The air-entraining property of HP cellulose introduces a large number of fine, isolated air bubbles into the mortar; these bubbles act as lubricants, enhancing the flowability of the mortar. The workability is improved. When the dosage exceeds 0.025%, the water demand of the mortar increases with increasing dosage, because the network structure of HP cellulose becomes more complete, the gaps between flocculent aggregates on the long molecular chains are reduced, and the resulting attractive and cohesive effects diminish the flowability of the mortar. Therefore, when the spread is essentially the same,
Under these conditions, the slurry exhibits an increased water demand.
