Water Retention Mechanism of HPMC in Cement Mortar

3.1. Water Retention and Plastic Viscosity

The water holding capacity of mortar reflects the ability of fresh mortar to hold water under an external force. As shown in Figure 4 , when the content of HPMC was 0.03–0.10%, the water retention rate increased greatly with the HPMC content and reached 92.1% at 0.10% HPMC. When the HPMC content was 0.30–0.50%, the water retention rate increased less than 0.10% HPMC, and 0.10% HPMC was the inflection point of water retention. As the water-holding capacity of cement mortar increased, its viscosity increased, and when the HPMC content was less than 0.10%, the mortar was sticky and fluid. When the content of HPMC was 0.10–0.50%, the mortar mainly showed stickiness. In addition, 0.10% HPMC was also the inflection point of the change in the mortar state, which indicated a relationship between the viscosity and the water retention of the mortar [ 13 ].

h

represents the viscosity of the mortar mixture as a homogeneous continuous medium and can be used to measure the plastic state of cement mortar. Therefore, it is necessary to study the relationship between plastic viscosity

h

and water retention. As shown in

h

of mortar and the water retention of mortar were positively correlated, and HPMC enhanced the water retention of cement mortar by increasing its plastic viscosity.

If cement mortar is treated as a Bingham fluid, the plastic viscosityrepresents the viscosity of the mortar mixture as a homogeneous continuous medium and can be used to measure the plastic state of cement mortar. Therefore, it is necessary to study the relationship between plastic viscosityand water retention. As shown in Figure 4 , when the HPMC content was 0.01%, the plastic viscosity was only 0.43 (N·mm·min), and the water-holding capacity of the mortar was constant. When the content of HPMC was 0.03%, the plastic viscosity increased to 0.90 (N·mm·min), and the water retention of mortar increased to 47.6%. Thus, as the HPMC content increased, the water-holding capacity of the mortar increased due to the increased plastic viscosity. The plastic viscosity of the mortar increased upon increasing the HPMC content, which quantitatively reflected the effect of HPMC on the cement mortar viscosity. The plastic viscosityof mortar and the water retention of mortar were positively correlated, and HPMC enhanced the water retention of cement mortar by increasing its plastic viscosity.

2 = 0.95. The physical meaning of the slope of these two straight lines is the contribution of the plastic viscosity of mortar to its water holding capacity. The HPMC content in samples was in the range of 0–0.10%, for each 1 (N·mm·min) increase in the plastic viscosity of the mortar, the water retention increased by 18.91%. The results show that the water-holding capacity of cement mortar can be improved when the plastic viscosity is low, and HPMC can be used to improve its plastic viscosity and water retention performance.

As shown in Figure 5 , the plastic viscosity and water retention of cement mortar showed a linear relationship with different slopes in the range of 0–0.1%. When the HPMC content was ≤0.1%, y = 27.88 + 18.91x, r= 0.95. The physical meaning of the slope of these two straight lines is the contribution of the plastic viscosity of mortar to its water holding capacity. The HPMC content in samples was in the range of 0–0.10%, for each 1 (N·mm·min) increase in the plastic viscosity of the mortar, the water retention increased by 18.91%. The results show that the water-holding capacity of cement mortar can be improved when the plastic viscosity is low, and HPMC can be used to improve its plastic viscosity and water retention performance.

n is the contribution of various components of cement mortar to its plastic viscosity, and HPMC has an important influence on the plastic viscosity h of cement mortar:

W R = A h + B ,

(2)

h = ∑ 3 n h n , n ≥ 3 .

(3)

Plastic viscosity occurs due to interactions between cement, aggregate, additive, and water in cement mortar, and it can be increased by adjusting their proportion. The test methods for plastic viscosity and water retention were kept constant, and the plastic viscosity h and water retention rate WR of several groups of cement mortars were tested by adjusting the viscosity and HPMC content. The distribution between WR and h is shown in Figure 6 , which is consistent with Figure 5 . Based on the above research, the relationship between plastic viscosity h and water retention rate WR of cement mortar is presented as shown in Formulas (2) and (3). A and B are parameters related to the test method and apparatus, his the contribution of various components of cement mortar to its plastic viscosity, and HPMC has an important influence on the plastic viscosity h of cement mortar: