Open Access
Issue
Wuhan Univ. J. Nat. Sci.
Volume 29, Number 4, August 2024
Page(s) 383 - 390
DOI https://doi.org/10.1051/wujns/2024294383
Published online 04 September 2024

© Wuhan University 2024

Licence Creative CommonsThis is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

0 Introduction

Trinity mixture fill[1,2], an ancient Chinese engineering construction adhesive material, is made by mixing lime, clay, sand, and water. The addition of sticky rice pulp, which is rich in branched starch that acts as an inhibitor, controls the formation of calcium sulfate crystals. More importantly, it generates a dense microstructure, resulting in higher compressive strength, as well as enhanced corrosion and moisture resistance for the improved trinity mixture fill, also known as sticky rice lime mortar[3-5].

Mainly aiming at the restoration of ancient buildings, the research on sticky rice lime mortar is quite advanced[6,7]. In terms of its macro performance[2,8,9], all argued that there are compatibility of macroscopically homogeneous materials between trinity mixture fill and sticky rice pulp, and amylopectin in sticky rice would play a role of inhibitor. Moreover, the addition of sticky rice pulp improves the water retention, the water absorption and the curing effect of the trinity mixture fill, and prolongs its setting time and slows down the carbonation of the mortar. Therefore, the sticky rice lime mortar has improved physical and mechanical properties. Yang et al[10] concluded that sticky rice lime mortar subject to wetting-drying exhibited a heterogeneous strain field that correlates well with its heterogeneous microstructure. On the basis of analysis for the micro mechanism of sticky rice lime mortar, Zhao et al[11] found that gelatinized amylopectin would induce carbonation of calcium carbonate, promote the production of Mg2+ and Fe3+ in calcite, and adsorb Pb(II) for an extremely high level. Yang et al[12] considered that the formation of hydration products of cementitious materials was strongly affected by amylopectin as a main ingredient in sticky rice.

Since the emergence of cement in 1824, cement-based materials such as cement mortar and concrete have rapidly developed into some of the most mainstream construction materials. Although these materials exhibit satisfactory compressive strength, their brittleness during compression and weaker tensile and flexural performance indicate a need for improvement. The addition of sticky rice pulp may offer a solution by enhancing these properties, necessitating research into its effects. Currently, there is almost no research in this area. This paper, therefore, conducted experimental research on cement mortar with sticky rice pulp to understand the compressive and flexural strength of this new material and to explore the underlying mechanisms of how sticky rice pulp influences strength.

1 Experimental

1.1 Base Materials

The base materials include slag Portland cement with grade of P.S 32.5, medium sand with fineness modulus of 2.7, tap water, and common water milled sticky rice flour.

The preparation for sticky rice pulp can be divided into three steps:

Step 1: Mixing we calculated the dosage of sticky rice flour and tap water as follows:

s t = m g m g + m w × 100 % (1)

where st denotes the concentration of the thick sticky rice pulp and it is 5% in the process, mg the dosage of sticky rice flour, and mw the dosage of tap water. Accordingly, both sticky rice flour and tap water were weighed for mixing. It should be noted that the actual dosage of water should be obtained by deducting the water content of the natural accumulated sand on the basis of dosage of the tap water calculated in formula (1). After determination, the moisture content of the sand was 5%. In order to avoid agglomeration during heating, we stirred the sticky rice flour in water at normal temperature.

Step 2: Boiling Its concentration should remain constant in the process of keeping boiling. This process usually takes about two hours as least, and hence it needs to be stirred all the time during the process to prevent the concentration from decreasing due to the paste pot; at the same time, it is also necessary to add tap water regularly to the initial volume scale marked in the pot to make up for the water loss caused by vaporization.

Step 3: Dilution with different concentrations The prepared thick pulp is divided into four parts by volume and then diluted into sticky rice pulp with concentration of 1.5%, 1.0%, 0.6%, 0.3% according to the volume ratio of the thick pulp to tap water is 1:2.3, 1:4.0, 1:7.3 and 1:15.8, respectively.

1.2 Grouping and Preparation of Test Blocks

Test blocks of cement mortar with design concentrations of 0.3%, 0.6%, 1% and 1.5% of sticky rice pulp are set as the four test groups, and one group of test blocks of common cement mortar is used as the control group. Firstly, we designed the mortar in the control group according to the mix proportion of the common cement mortar with strength grade of Chinese M20, which represents a characteristic cube mortar compressive strength of 20 MPa with 95% guarantee rate. The mortar in the test groups were designed with the same proportion, except replacing water with equivalent sticky rice pulp completely and directly. The dosage of each base material is listed in Table 1. There were 24 blocks in each group, which included 12 characteristic cube blocks of 70.7 mm × 70.7 mm × 70.7 mm for compressive strength test and twelve characteristic prism blocks of 40 mm × 40 mm × 160 mm for flexural strength test. The blocks were cured to the ages under standard conditions required by Chinese Code [13]. Let it be added here that the cement mortar with sticky rice pulp in a concentration of 1.5% had poor fluidity, and the consistency was only about 30 mm.

Table 1

The dosage of base materials

2 Test Results

2.1 Compressive Strength and Flexural Strength

The compressive strength test and the flexural strength test were completed in accordance with the provisions of the Chinese Codes [14,15]. After the test, the mean value of the measured cube compressive strength and the measured prism flexural strength were calculated respectively first, and then the characteristic values of the strength with a 95% guarantee rate were calculated according to the following formulas:

f c u , k 3 d = f c u , m 3 d - 1.645 σ c 3 d (2)

f c u , k = f c u , m - 1.645 σ c (3)

f f , k 3 d = f f , m 3 d - 1.645 σ f 3 d (4)

f f , k = f f , m - 1.645 σ f (5)

where fcu,k3d and fcu,k are the characteristic cube compressive strengths; ff,k3d and ff,k are the characteristic prism flexural strengths; fcu,m3d and fcu,m are the mean cube compressive strengths; ff,m3d and ff,m are the mean prism flexural strengths; σc3d,σc,σf3d and σf are the standard deviations corresponding to the strength test results, respectively. The above parameters with a superscript of 3 d denote they are related to 3 d age, as well as the ones without superscripts denote they are related to 28 d age. The test results are as listed in Table 2.

Table 2

The test results

2.2 Influence Functions of the Strength

The influence functions of concentration of sticky rice pulp on strength of cement mortar are defined as follows:

η c 3 d ( s ) = f c u , k 3 d / f c u , k 3 d ,   0 (6)

η c ( s ) = f c u , k / f c u , k 0 (7)

η f 3 d ( s ) = f f , k 3 d / f f , k 3 d ,   0 (8)

η f ( s ) = f f , k / f f , k 0 (9)

where ηc3d(s), ηc(s), ηf3d(s) and ηf(s) are the influence functions of 3 d compressive strength, 28 d compressive strength, 3 d flexural strength and 28 d flexural strength, respectively; s is the design concentration of sticky rice pulp and all the functions take it as the variable; fcu,k3d,0, fcu,k0, ff,k3d,0 and ff,k0 are the 3 d characteristic cube compressive strength, 28 d characteristic cube compressive strength, 3 d characteristic prism flexural strength, 28 d characteristic prism flexural strength of common cement mortar (i.e. its design concentration of sticky rice pulp is 0) in group I. The fitting curves are as shown in Fig. 1.

thumbnail Fig. 1 Fitting curves for influence function of strength

2.3 Strength Characteristic Analysis

The following strength characteristics can be seen from Table 2 and Fig. 1:

1) Early strength The early strength of all cement mortar in the test groups was lower than that in the control group, including both compressive and flexural strengths. Among them, the mortar in group II (i.e., with a pulp concentration of 0.3%) had the relatively lowest early strength, being about 40% lower than the control group (i.e., common cement mortar). The early strength of the mortar in the test groups showed an increasing trend with the increase in pulp concentration from 0.3% to 1.5%. The mortar in groups III, IV, and V (i.e., with pulp concentrations of 0.6%, 1.0%, and 1.5%, respectively) had relatively similar early strengths. Their early compressive strength was 10% to 16% higher than that in group II, while their early flexural strength was even 40% to 50% higher than that in group II.

2) Long-term strength Among the four test groups and the control group, the mortar in group II exhibited the highest long-term strength; its long-term flexural strength was about 14% higher than that in the control group, and its long-term compressive strength was just slightly higher than that in the control group. The long-term strength of the mortar in the test groups showed a downward trend with the increase in pulp concentration from 0.3% to 1.5%. Their long-term compressive strength declined slowly, such that the strength of the mortar in group V was only 11% lower than that in group II. With the increase in pulp concentration, their long-term flexural strength initially dropped slowly and then more rapidly. The strength of mortar in groups III and IV was both slightly higher than that in the control group and slightly lower than that in group II, while the long-term flexural strength of the mortar in group V was only 57% of that in group II.

According to the above strength characteristic analysis, it could be preliminary inferred:

1) In Fig. 1, it is evident that the addition of sticky rice pulp at concentrations between 0.3% and 1.0% results in excellent performance at both 3rd and 28th days flexural strength of cement mortar. Notably, the 0.3% concentration exhibits the lowest flexural strength at 3rd day but the highest at 28th day. However, when the concentration of sticky rice pulp exceeds 1.0% up to 1.5%, there is a significant decrease in flexural strength, particularly at 28th day.

2) In the tests at 3rd day, the compressive strength of the cement mortar with the addition of 0.3% sticky rice pulp concentration is 40% lower than that of common mortar. As the concentration increases to 0.6%- 1.5%, the compressive strength improves by 10% to 16%. By the 28th day, the compressive strength at 0.3% concentration is slightly higher than that of common mortar, while it is 11% lower at a 1.5% concentration. This indicates that the addition of an appropriate sticky rice pulp concentration can significantly affect the performance of cement mortar, making the enhancement of its compressive strength crucial.

3) It can be understood that the reduction in early strength with the addition of sticky rice pulp may be attributed to its retarding effect on the setting of cement mortar.

3 Microstructure Analysis

After strength tests, cement paste block samples with sizes of 10 mm to 20 mm were taken from crushed test blocks of each age of each group. They were retained in absolute ethanol to terminate the hydration reaction. After all the samples were collected, they were taken out for drying, sample preparation, gold spraying, and then the microstructures with magnification of 2 000 times, as shown in Fig. 2, were obtained by the Scanning Electronic Microscope (SEM) of FEI Q45-type.

thumbnail Fig. 2 SEM images for microstructures in magnification of 2 000 times

Circular shapes represent calcium silicate hydrates (C-S-H), while rectangular shapes denote pores

According to Fig. 2, the microstructure analysis is carried out as follows:

1) The common cement mortar It can be seen from a comparison between Fig. 2(a) and Fig. 2(b) that the flocculent gel substance of C-S-H was constantly produced in the mortar of the control group, and the holes and some other defects at the early stage were gradually filled and patched by these hydration products as the hydration reaction progressed. Consequently, the compactness of the microstructure increased, and meanwhile, the separation between the grains decreased.

2) The cement mortar with sticky rice pulp in a concentration of 0.3% As can be seen from Fig. 2(c), the mortar in group II initially had few hydration products, indicating that the hydration reaction was somewhat hindered at this stage. This likely explains why both its compressive and flexural strengths were lower than those in the control group initially. Over time, however, the hydration products in group II became well-distributed, as evidenced by Fig. 2(c), indicating that the hydration reaction eventually occurred fully. As a result, the grains were well aggregated, and almost no obvious hole defects remained in the microstructure, resulting in higher compactness at this stage compared to the control group. This likely contributes to its long-term compressive strength being significantly higher and its flexural strength slightly higher than those in the control group.

3) The cement mortar with sticky rice pulp in a concentration of 1.0% As shown in Fig. 2(e), at the early stage, the hydration reaction of the mortar in group IV was slightly more extensive than that in group II but was much slower than that in the control group. Consequently, its compressive strength was slightly higher, and its flexural strength was much lower compared to those in mortars with a lower concentration of the pulp and the standard one at this stage. Additionally, as shown in Fig. 2(f), although its hydration products seemed to increase significantly at the later stage, they were excessively aggregated in some areas, leading to uneven distribution. Thus, its compressive strength became lower than that in mortars with a lower concentration of the pulp at this stage. However, the microstructure generally appears dense with few defects, so its long-term flexural strength is not lower than that in the control group.

4) The cement mortar with sticky rice pulp in a concentration of 1.5% As shown in Fig. 2(g), the hydration reaction of the mortar in group V had progressed to a certain extent, and therefore its compressive strength and flexural strength were close to those in the control group at the early stage. As shown in Fig. 2(h), at the later stage, the hydration products seriously agglomerated, resulting in a very loose microstructure with a large number of hole defects. Thus, both its long-term compressive and flexural strengths were far lower than those in the control group. It was also evident that the test blocks were prone to crumbling, lacked integrity, and were brittle when crushed or broken. Additionally, with such a concentration, the mortar had very poor fluidity and consistency, which would also make it difficult to ensure construction quality. Therefore, no further comparative tests on cement mortar with sticky rice pulp concentrations higher than 1.5% were conducted.

4 Strength Mechanism Analysis

4.1 "Water Reduction" Effect of Sticky Rice Starch

Sticky rice starch is mainly composed of amylose (sugar starch) and amylopectin (gum starch), whose chemical structures[16,17] are shown in Fig.3. It can be seen from Fig.3 that the molecular composition of amylose includes hydrogen (—H), amino (—NH2) and hydroxyl (—OH), while the molecular composition of amylopectin is mainly hydroxyl (—OH)[17]. Note that hydroxyl is a kind of hydrophilic group.

thumbnail Fig. 3 Chemical structures of sticky rice starch

When boiling (thick) sticky rice pulp, some free water combines with amylopectin, becoming bound water, thus reducing the free water available for the hydration reaction. This creates a "water reduction" effect[16]. If the concentration is low enough that the remaining free water in the pulp is sufficient for the hydration reaction throughout the process, this effect is equivalent to reducing the actual water-cement ratio, and accordingly, the long-term strength of the mortar must be higher than that of the common one. Conversely, if the concentration increases to a point where there is a shortage of free water for the hydration reaction at a later stage, the long-term strength will be lower than that of the common cement mortar.

4.2 Polymerization Effect of Sticky Rice Pulp

Sticky rice pulp contains amylopectin and other bonding substance, thus the added pulp can enhance the polymerization of molecules of hydration products, that is, polymerization effect. According to Ref. [18], the hydration reaction in common cement mortar is mainly going on as the following chemical equation:

C a 3 S i O 5 ( s ) + H 2 O C - S - H ( s ) + C a ( O H ) 2 ( s ) (10)

C a 2 S i O 4 ( s ) + H 2 O C - S - H ( s ) + C a ( O H ) 2 ( s ) (11)

where C-S-H is an amorphous colloidal substance with an indeterminate chemical composition and accounts for about 70% of hydration products of cement. Calcium hydroxide (Ca(OH)2) accounts for only about 20%, existing in a crystalline state. The rest of the products are solid particles, such as calcium aluminate hydrates (C-A-H), among others. The microstructure diagrams are shown in Fig. 4. If the concentration is at a relatively low level of about 0.3%, moderate polymerization will lead to reinforced cohesiveness of the mortar and improved compactness of the microstructure, as shown in Fig. 4(b). In this state, the mortar with the pulp significantly enhances long-term compressive strength and slightly improves long-term flexural strength compared to the standard mortar. However, if the concentration is 1.0% or stronger, excessive polymerization results in serious agglomeration and tremendous heterogeneity among the hydration product molecules, resulting in a loose microstructure, as shown in Fig. 4(c). In this case, the long-term strengths, especially the flexural strength, are lower than those of mortars with lower pulp concentrations and the standard mortar. Moreover, according to the SEM images of microstructures shown in Fig.2(d), 2(f), and 2(h), granularity becomes more pronounced with the increase in pulp concentration. This is also caused by the polymerization effect of sticky rice pulp.

thumbnail Fig. 4 Microstructure diagrams

Besides, cement mortar and sticky rice pulp are compatible, meaning they easily accommodate each other and form a macroscopically uniform material. According to Refs. [19,20], amylopectin in sticky rice pulp, a bonding material with a multi-branch structure, can fill the hole defects inside the microstructure of cement mortar to increase its compactness, as shown in Fig.2(d) and 2(f). Thus, the tensile capacity of the mortar is improved, which is ultimately beneficial to its flexural strength.

4.3 Retarding Effect of Polysaccharide Molecules

Polysaccharide molecules in sticky rice pulp have a retarding effect on cement hydration[21]. Two major changes occur when sticky rice pulp, rich in starch—a polysaccharide molecule—participates in the reaction. First, the dissolved calcium ion (Ca2+) and silicate ion (SiO44-) agglomerate into a dense but thin layer of C-S-H at stage I . This layer acts as a barrier, delaying hydration for a certain period at stage II, thereby maintaining fluidity for construction before hardening. Additionally, the added polysaccharide molecules can increase the solubility of cement[21]. Second, the polysaccharide molecules likely detrimentally affect the surface of hydration products[22]. They are always adsorbed on Ca(OH)2 and C-S-H, inhibiting the nucleation and growth of crystals at stage III and thus further slowing down the hydration reaction[23]. Consequently, mortar with sticky rice pulp generally has substantially lower compressive and flexural strength compared to common mortar at an early stage; however, this difference may not persist at later stages[24].

5 Conclusion

1) The addition of sticky rice pulp at a concentration of about 0.3% significantly benefits the long-term flexural strength of the cement mortar. However, concentrations higher than 1.0% can severely harm this strength, while those lower than 1.5% only slightly affect its long-term compressive strength.

2) The results of SEM analysis of microstructures indicated that the added sticky rice pulp reduced the hydration products of cement at the early stage. If the pulp is added at a relatively low concentration, the hydration reaction of cement can fully proceed at the later stage and the microstructure of the mortar tends to be dense. However, if the concentration is high, the hydration products seriously agglomerate at the later stage, resulting in a microstructure that is loose and grainy with many holes and other defects.

3) The strength of cement mortar with sticky rice pulp is affected by three effects: the "water reduction" of sticky rice starch; the polymerization of sticky rice pulp; and the retarding of polysaccharide molecules. Among them, at low concentrations, the effects of "water reduction" and polymerization benefit the strength by reducing the actual water-to-cement ratio and improving the compactness of the microstructure, respectively. At higher concentrations, however, these two effects harm the strength due to the shortage of free water for the cement hydration reaction and the molecular agglomeration of hydration products. The retarding effect mainly makes the strength of cement mortar with sticky rice pulp significantly lower than that of common cement mortar at the early stage.

In addition, the fluidity and consistency of the mortar will decrease as the increase of concentration of the added sticky rice pulp. This problem related to its construction performance is also an aspect that needs further consideration in future research.

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All Tables

Table 1

The dosage of base materials

Table 2

The test results

All Figures

thumbnail Fig. 1 Fitting curves for influence function of strength
In the text
thumbnail Fig. 2 SEM images for microstructures in magnification of 2 000 times

Circular shapes represent calcium silicate hydrates (C-S-H), while rectangular shapes denote pores

In the text
thumbnail Fig. 3 Chemical structures of sticky rice starch
In the text
thumbnail Fig. 4 Microstructure diagrams
In the text

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