Academic Journal of Science and Technology 
ISSN: 2771-3032 | Vol. 11, No. 2, 2024 

 

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Study on Alkali Reduction Effect of SAP on Cementitious 
Composites 

Wenxing Xu, Shuai He and Yangyi Zhu 

School of Central South Forestry University of Science and Technology, Changsha 410004, China 

 

Abstract: In order to improve the high alkalinity environment inside the cementitious composites, the effect of SAP doping 

on the pH value and compressive strength of cement slurry was investigated by doping Super Absorbent Polymer (SAP). The 

test results show that SAP can reduce the pH value of cementitious composites at 28d to a certain extent; the pH value of 

cementitious composites at 28d is significantly reduced when SAP doping is 0.3%, and the compressive strength is improved 

compared with that of the control group; SAP particles will form cracks with the middle of the cement matrix after the release 

of water to reduce the alkaline material outflow from the cement matrix, which will make the pH value of SAP specimens reduced. 

Keywords: SAP; pH; Mechanical Properties; Cement. 

 

1. Introduction 

With the development and progress of society, people's 

demand for environmental ecology is increasing, in order to 

comply with the concept of building sponge cities, the 

concept of Ecological Concrete (EC) is proposed, that is, a 

new type of porous concrete structural material composed of 

a single particle size coarse aggregate, no sand or a small 

amount of sand, low-plasticity cementitious cementitious 

materials, and to meet the symbiosis of vegetation. The 

concrete is a new type of porous concrete structural material. 

However, during the hydration process of concrete, the pH 

value of pore water environment is as high as 12, which does 

not meet the conditions for plant growth. Therefore, the 

research on alkali-reducing technology for concrete is a key 

technology for the development of eco-concrete.  

In order to reduce the pH value of concrete pores, the alkali 

reduction methods commonly used at home and abroad are: 

single or compound chemical methods, physical methods, 

agronomic methods and so on. Gao Ting[1] controlled the 

alkalinity within the pores of eco-concrete in a suitable range 

by mixing appropriate amounts of mineral admixtures, 

preferring low alkalinity cement varieties, spraying acidic 

modifying materials, and carbonation; Hu[2] prevented alkali 

efflux by mixing silane emulsions into eco-concrete, which 

formed a stable protective film on the surface; Huang[3] 

prepared eco-concrete by using a low alkalinity supersulfate 

cement to concrete, which lowered the pH of eco-concrete 

pore solution; Chen[4] effectively lowered the pH of concrete 

internal void solution by adding appropriate amount of 

phenylpropylene emulsion instead of sulphoaluminate 

cement; Tang[5] lowered the alkali by adding fly ash, and the 

optimum eco-concrete pH could be maintained at 8.7. 

Super Absorbent Polymer(SAP) is a polymer with a three-

dimensional spatial network structure, the complex spatial 

lattice structure so that it has a better water absorption, water 

release properties, often used in high-performance concrete 

internal maintenance materials, the formation of a certain 

number of “reservoir” within the concrete It forms a certain 

number of “reservoirs” inside the concrete, releasing water 

when the humidity inside the concrete decreases, providing 

conditions for the continuation of the hydration reaction of 

cement[6,7]. However, due to the complexity of SAP and 

cementitious composites, a unified understanding of the 

relationship between SAP dosage, optimum water-cement 

ratio, etc. and the performance of cementitious composites 

has not yet been reached. Based on this, this paper 

investigates the effect of different SAP dosage on pH and 

compressive strength of cementitious composites. 

2. Raw Materials and Test Methods 

2.1. Raw Materials 

Cement grout test raw materials: cement for P·O 42.5 

ordinary silica cement, produced by Hunan Pingtang South 

Cement Co., Ltd., its basic characteristics are shown in Table1; 

mixing water for tap water; water reducing agent for high 

performance polycarboxylic acid water reducing agent, the 

water reduction rate of 30%; SAP by the Yixing.  

 

Table 1. Basic characteristics 

Specific surface area(m2·kg-1) 

Setting time(min)  Flexural strength (MPa)  Compressive strength (MPa) 

Initial setting Final setting  3d 28d  3d 28d 

336 178 228  5.1 8.1  25.0 46.2 

2.2. Preparation of Specimens 

In the test, the baseline water-cementitious ratio of 0.30 

was used, and nine different SAP dosages were set, which 

were 0.10%, 0.15%, 0.20%, 0.25%, 0.30%, 0.35%, 0.40%, 

0.45%, 0.50%, respectively, and the specific test programme 

is shown in Table 2. First of all, the cementitious material, 

SAP, water reducing agent fully mixed, according to the 



 

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water-cementitious ratio of 0.30 to join the water solution, at 

a mixing speed of 200 r/min uniform, mixing 120s can be 

loaded into the mould to get prismatic specimens, specimen 

size of 40mm × 40mm × 160mm, to be maintained at room 

temperature for 24h after the removal of the mould, and put 

into a standard maintenance box, respectively, maintenance to 

3d, 7d, 28d. The pH value and compressive strength of the 

specimens were determined. 
 

Table 2. The specific test programme 

Numble SAP(%) Cement(kg/m3) Water reducing agent (%) Total water consumption(kg/m3) 

1 0.00% 1953 0.02% 586 

2 0.10% 1953 0.02% 586 

3 0.15% 1953 0.02% 586 

4 0.20% 1953 0.02% 586 

5 0.25% 1953 0.02% 586 

6 0.30% 1953 0.02% 586 

7 0.35% 1953 0.02% 586 

8 0.40% 1953 0.02% 586 

9 0.45% 1953 0.02% 586 

10 0.50% 1953 0.02% 586 

2.3. Test Methods  

The pH value test of the specimen adopts the method of 

soaking the net pulp, which reflects the acidity and alkalinity 

between the voids of the net pulp by determining the pH value 

of distilled water after soaking. Specimen maintenance to a 

certain age after placing the plastic box, injected into the mass 

ratio of 1:1 distilled water, distilled water should not be more 

than the height of the test block 1 ~ 2cm, sealed for 24h after 

the use of Sigma instrumentation PH818 type PH pH pH 

meter test pH value, the test will be immersed in cement 

mortar specimens stirred in the aqueous solution, to be read 

after reading the stability of the readings, the test 3 times to 

take the average value.  

Cubic compressive strength test with reference to ‘GB/T 

17671-2021 Cementitious Sand Strength Test Method (ISO 

Method)’, using DYE-300B microcomputer servo cement 

flexural and compressive testing machine. 

3. Test Results and Analyses 

The pH and compressive strength of the specimens with a 

water-cement ratio of 0.3 were determined for 3d, 7d and 28d, 

and the test data are shown in Table 3.  

 

Table 3. The test data 

Numble SAP 
pH  compressive strength(MPa) 

3d 7d 28d  3d 7d 28d 

1 0.00% 11.21 10.01 9.31  54.28 57.48 61.03 

2 0.10% 11.65 10.71 9.16  38.93 55.88 66.08 

3 0.15% 11.66 10.36 9.23  42.28 50.25 63.45 

4 0.20% 11.60 10.99 8.91  39.50 51.95 67.23 

5 0.25% 11.72 10.28 8.83  37.83 53.95 66.40 

6 0.30% 10.80 9.62 8.78  46.93 59.17 70.10 

7 0.35% 11.02 9.71 8.89  44.55 47.40 66.70 

8 0.40% 11.20 10.21 8.77  40.93 50.53 65.63 

9 0.45% 11.35 10.51 8.95  43.05 51.43 64.95 

10 0.50% 11.57 11.12 9.45  42.60 52.08 64.38 

3.1. Effect of SAP on the Alkalinity of 

Cementitious Slurries  

P-O 42.5 cement and SAP particles were used to prepare 

cement net paste with SAP dosage of 0.10%, 0.15%, 0.20%, 

0.25%, 0.30%, 0.35%, 0.40%, 0.45%, 0.50% and cement 

paste without SAP addition was the control group with a 

water-cement ratio of 0.30, and the results of the tests are 

shown in Fig. 1. 

As can be seen from Fig. 1, the pH value of cement paste 

with the increase of SAP dosage shows a trend of first 

increase, then decrease and then increase. 3d age, when the 

SAP dosage is 0.25%, the highest pH value of the specimen, 

compared with the control group increased by 4.5%; when the 

SAP dosage is 0.30%, the specimen reduces the alkali effect 

is obvious, and the pH value of the specimen is reduced by 

3.7% compared with the control group and the pH value is 

reduced by 7% compared with the specimen with 0.25% SAP 

dosage. The pH value of the specimen was reduced by 3.7% 

compared with the control group, and by 7.8% compared with 

the 0.25% SAP specimen. This is due to the fact that SAP 

absorbed part of the free water, which increased the 

concentration of alkaline ions (Ca2+, K2+, etc.) in the cement 

paste, resulting in an increase in the pH value of the SAP 

specimens. 

With the increase of specimen age, the pH value of cement 

paste at 7d age was reduced compared with that of 3d, and the 

pH value of specimen 7d was the lowest when SAP doping 

was 0.30%, which was 3.9% lower compared with that of the 

control group. This is due to the fact that SAP promotes the 

hydration of cement to generate C-S-H gel, which fills the 

small capillary pores and small cracks inside the cement 

matrix, reduces the connected pores, prevents the alkaline 

material from flowing out, and reduces the pH value. 

At the age of 28d, the hydration of cement paste was 

basically completed, and the pH value of the test group doped 

with SAP was generally lower than that of the control group, 

in which the pH value of the specimen was the lowest when 



 

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the dosage of SAP was 0.30%, which was reduced by 5.7% 

compared with that of the control group. Due to the large 

specific surface area and pore structure of SAP particles after 

the release of water, it can adsorb alkaline cations in the 

cement, so that the alkaline cations are fixed on the surface of 

SAP and in the pores, and at the same time, after the release 

of water, SAP volume shrinkage occurs to form a shell-like 

morphology, which forms a crack in the middle of the cement 

matrix, slowing down the rate of alkaline ions flowing out of 

the cement matrix, and making the pH value of the SAP 

specimen significantly lower. 

 

Fig 1. The results of the tests 

 

3.2. Effect of SAP on the Compressive Strength 

of Cementitious Mortar 

The effects of different dosages of SAP on the 3d, 7d and 

28d cubic compressive strength of cementitious composites 

are shown in Fig. 2. 

As can be seen in Figure 2, the compressive strength of the 

cement paste increased with the SAP dosage and showed a 

general trend of first increase and then decrease. 3d age, the 

compressive strength of SAP specimens compared with that 

of the control group have different degrees of reduction, of 

which, when the SAP dosage of 0.30%, the specimen's 

compressive strength was the highest, and its compressive 

strength compared with that of the control group decreased by 

13.5%. This is due to the SAP water absorption makes the 

early hydration of cement incomplete, the compressive 

strength decreased, and SAP water absorption and expansion 

in the cement paste to produce large pores, is not conducive 

to the development of early strength of cement.  

At the age of 7d, the compressive strength of the specimens 

increased, of which, when the SAP doping of 0.30%, the 

compressive strength of the specimens was the largest value, 

and compared with that of the control group increased by 

2.9%, and when the SAP doping of 0.10% and 0.25%, the 

specimens had the largest increase in the compressive 

strength of the specimens compared with their 3d age, with an 

increase of 43.5% and 42.6%, respectively. This is due to the 

fact that as SAP exerts the ‘internal conservation’ effect, it 

continuously releases water to promote cement hydration and 

enhance the strength of the cement paste [8]. 

 

Fig 2. The effects of different dosages of SAP on the 3d, 7d and 28d cubic compressive strength of cementitious composites 



 

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With the increase of the age of the cement matrix, the 

compressive strength of the specimens doped with SAP was 

significantly improved at the age of 28d, and their 

compressive strength values were higher than those of the 

control group; when the SAP doping rate was 0.35%, the 

compressive strength value of the specimens was the largest, 

and it was increased by 14.8% compared with that of the 

control group. This is due to the continuous and slow release 

of water in the late stage of SAP, which promotes the 

hydration of cement to generate C-S-H and Aft, filling the 

pores in the cement paste, improving its densification and 

increasing its strength. 

3.3. Relationship between Alkalinity and 

Compressive Strength of Cement Screeds 

Figure 3 shows the relationship between alkalinity and 

compressive strength of specimens at 3d, 7d and 28d for SAP 

doping. 

As can be seen from Fig. 3, with the increase of SAP dosage, 

the pH value of the cement paste shows a trend of first rise, 

then fall and rise again, and the compressive strength of the 

cement matrix shows a trend of first rise and then fall. At the 

age of 3d, the pH value of the specimen is reduced by 3.7% 

compared with that of the control group when the dosage of 

0.30% of SAP is used, and the compressive strength of the 

specimen is reduced by 13.5% compared with that of the 

control group. This is due to the better water absorption of 

SAP particles can make it absorb part of the water, so that the 

concentration of alkaline ions in the cement paste increased, 

which led to an increase in the pH value of SAP specimens, 

and SAP absorbed water and swelled to produce large pores 

in the cement matrix [9], resulting in a significant reduction 

in its compressive strength. 

At the age of 7d, the pH value of the specimen was reduced 

compared to 3d, and the compressive strength was increased 

to different degrees compared to 3d. When the dosage of SAP 

was 0.30%, the pH value of the specimen at 7d was the lowest, 

which was 3.9% lower than that of the control group; and the 

compressive strength of the specimen at 7d was the largest, 

which was 2.9% higher than that of the control group. This is 

due to the fact that the internal maintenance effect of SAP can 

promote the generation of hydration products to fill the small 

capillary pores, reduce the small cracks inside the cement 

matrix, reduce the connecting pores, reduce the alkaline 

material outflow inside the cement matrix, reduce the pH 

value, and make the structure denser, so the 7d compressive 

strength of SAP specimens was increased significantly. 

With the growth of age, at the age of 28 d, the pH value of 

the test group specimens doped with SAP was generally lower 

than that of the control group specimens, and the compressive 

strength was higher than that of the control group, in which 

the pH value of the specimens was the smallest when the 

dosage of SAP was 0.30%, which was reduced by 5.7% 

compared to that of the control group, and the compressive 

strength was as high as 70.10 MPa. This was due to the fact 

that SAP particles form a layer of organic film after the release 

of water [10] and forms cracks with the middle of the cement 

matrix, reducing the outflow of alkaline ions from the cement 

matrix, resulting in a significant decrease in the pH of the 

specimens. The gradual release of internal conservation water 

in the SAP gel at a later stage improves the hydration of the 

cement, induces the transformation of low-strength C-S-H to 

high-strength C-S-H in the matrix [11], and improves the 

compressive strength of the cement matrix. 

 

 

Fig 3. The relationship between alkalinity and compressive strength of specimens at 3d, 7d and 28d for SAP doping 

 

4. Micro-morphological Analysis 

Scanning electron microscope tests were carried out on the 

SAP specimens with 0.30% dosing at the age of 28d, and Fig. 

4 shows the microscopic morphology of the SAP specimens. 

As shown in Fig. 4, at the age of 28d, irregular slumping 

cracks were formed between SAP and cement matrix after 

water release, and there were more flocculent and fibrous C-

S-H gel in the cracks between SAP particles and cement 

matrix, which indicated that SAP was slowly releasing water 

and slumping phenomenon, and the slumping cracks were 

able to slow down the rate of alkaline ions out of the cement 

matrix and reduce the rate of alkaline ions in the cement 

matrix, and the slumping cracks were able to slow down the 



 

34 

rate of alkaline ions in the cement matrix. The slumping 

cracks can slow down the flow rate of alkaline ions out of the 

cement matrix and reduce the activity of alkaline ions in the 

cement matrix. At the same time, the porous structure and 

large specific surface area of SAP can adsorb alkaline cations 

in the cement, reduce the alkaline ions in the cement, and 

lower the pH value of the cementitious materials. Due to the 

‘internal conservation’ effect of SAP, it slowly releases water 

and promotes the unhydrated cement clinker in the vicinity to 

continue hydration, generating a large number of C-S-H gels 

in the cracks, and the cluster-like C-S-H gels grow in a 

staggered manner, and at the same time, continuously refining 

the pore structure, enhancing the densification of the cement 

structure, and increasing the strength of the cement matrix. 
 

 

Fig 4. The microscopic morphology of the SAP specimens 

 

5. Summary 

(1) The incorporation of appropriate amount of SAP 

reduced the alkalinity of the cement matrix to a certain extent, 

in which the SAP specimens dosed with 0.30% had the best 

overall effect, the pH at 28d was reduced by 5.7% compared 

to that of the control group, and the pH at 28d was reduced to 

8.78. 

(2) SAP doping significantly enhanced the compressive 

strength of the cement matrix, with the increase of SAP 

doping, the compressive strength of the specimens showed a 

trend of increasing and then decreasing, when the SAP doping 

of 0.30%, the compressive strength of the specimens at 28d 

was increased by 14.8% compared with that of the control 

group, and the value of the compressive strength of the 

specimens at 28d reached 70.10 MPa. 

(3) After the release of water from SAP particles, cracks 

will be formed in the middle of the cement matrix, reducing 

the alkaline ions from the cement matrix, making the pH 

value of SAP specimens significantly reduced, and the pH 

value of 28d decreased to 8.78; the gradual release of water 

from the internal conservation in the SAP gel improves the 

hydration of the cement, and promotes the transformation of 

low-strength C-S-H to high-strength C-S-H in the matrix, 

improving the 28d compressive strength of SAP specimens at 

28d. 

Acknowledgments 

The authors would like to express their sincere thanks to 

the Graduate Student Science and Technology Innovation 

Fund Project of Central South Forestry University of Science 

and Technology (2023CX02096) for financial support. 

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