Improving Drying Shrinkage Performance of Metakaolin-Based Geopolymers by Adding Cement

Abstract

Geopolymers, as sustainable alternatives to conventional cement, face application limitations due to pronounced drying shrinkage. This study systematically investigates the effects of cement incorporation (0–40%) on the drying shrinkage mitigation and performance evolution of metakaolin-based geopolymers (MKBGs) through multi-scale characterization of mechanical properties, reaction kinetics, and pore structure refinement. Key findings reveal that 10% cement addition optimally reduces drying shrinkage through pore structure densification and elastic modulus enhancement. The cement–geopolymer hybrid system exhibited a distinctive dual-reaction mechanism: cement hydration produced C-S-H gels that refined the pore structure while simultaneously competing with and delaying the geopolymerization kinetics, as demonstrated by the extended duration of the reaction exotherm. However, cement contents exceeding 20% induce detrimental self-desiccation shrinkage, resulting in net shrinkage amplification. Microstructural analysis confirms that the optimal 10% cement dosage achieves synergistic phase evolution, with N-A-S-H and C-S-H gels co-operatively improving mechanical strength and dimensional stability. This work provides quantitative guidelines for designing shrinkage-resistant geopolymer composites through controlled cement hybridization.

1. Introduction

Geopolymer is a novel green binding material with a zeolite-like three-dimensional reticular structure. Compared with traditional Portland cement, geopolymers exhibit superior mechanical properties [1], durability [2,3], and heavy metal immobilization capabilities [4]. Moreover, geopolymers are typically prepared from solid waste materials (such as fly ash and slag) or nonmetallic minerals (such as metakaolin) that are calcined at much lower temperatures than cement. This not only enables the recycling of industrial waste and reduces its subsequent environmental pollution but also decreases the energy consumption during the raw material preparation process of geopolymers and mitigates the increase in greenhouse gases. However, geopolymers tend to experience significant shrinkage under dry conditions, which limits their large-scale application in engineering projects. During the service life of engineering structures, this shrinkage can easily lead to cracking. In mild cases, it results in a decrease in material strength and permeability resistance, while, in severe cases, it can directly cause structural instability.

Improving the shrinkage behavior of geopolymers can be achieved through various means. The surface tension of pore solution can be altered by adding air-entraining agents and water-reducing agents, thereby reducing the shrinkage of alkali-activated slag concrete [5]. The addition of shrinkage-reducing admixtures can reduce the surface tension of pore solution by half, thereby mitigating the shrinkage caused by capillary water loss [6,7,8]. By adjusting the composition and ratio of precursor raw materials and activators, as well as curing conditions, the gel structure and pore structure of the geopolymer can be modified. A large number of studies have shown that various types of shrinkage in geopolymers are closely related to their composition parameters (such as NaOH concentration, liquid–solid ratio, Na₂SiO₃-to-NaOH ratio, calcium content, etc.) and curing conditions [9,10,11]. It is also possible to slow down the contraction by adding an expansive substance that expands during the reaction. For example, the addition of a small amount of gypsum to alkali-activated slag concrete can reduce shrinkage because the formation of a large number of silicates, AFt, and AFm phases generates expansion, which has a certain compensatory effect on shrinkage [12,13]. In addition, the strength of gels and the density of geopolymers can be increased by adding powder materials, thereby improving the anti-shrinkage performance of geopolymers.

The reaction process of geopolymer can be summarized as four stages: dissolution, diffusion, polymerization, and solidification [14,15]. During the polymerization stage, water is removed and expelled into paste [16,17]. The water released during these polymerization reactions can be utilized by mixing with cement. The reaction products formed through continuous hydration between cement and water fill the original pore structure of the geopolymer, further refining the pore structure and densifying the material. Meanwhile, the hydration reaction products of cement, particularly C-A-S-H gels, exhibit superior anti-deformation performance compared to N-A-S-H gels. The addition of cement can further enhance the anti-shrinkage performance of geopolymers. All these mechanisms are beneficial for inhibiting the dry shrinkage of geopolymers. The idea of incorporating cement into geopolymers to improve their drying shrinkage behavior is worth exploring.

Metakaolin-based geopolymers (MKBGs) are selected as the research object in this study due to their stable composition and reaction products, as well as their relatively uniform pore size distribution. The primary objective of this study is to investigate the influence of cement incorporation on the drying shrinkage behavior of MKBGs. The mechanical properties, early shrinkage, and drying shrinkage of MKBGs were evaluated under various cement content ratios. Additionally, the underlying mechanisms by which cement addition affects the drying shrinkage of MKBGs were explored based on the test results of phase transformation, reaction heat release, and pore structure characteristics.

2. Experimental Program

2.1. Materials

(1) Solid powder raw materials

In this study, a highly active metakaolin named MK KAOPOZZ, sourced from Inner Mongolia, China, was used to prepare the geopolymer. The cement used was P·O 42.5 ordinary Portland cement, produced by South Cement Factory in Changsha, China. The main chemical components of the materials are shown in

Table 1. Main chemical composition of powder material (%).

Oxide/%	SiO2	Al2O3	CaO	MgO	SO3	TiO2	K2O	Na2O	Other
Metakaolin	52.53	45.42	0.26	-	0.04	0.97	0.18	-	0.6
Cement	31.59	7.6	42.43	2.33	-	-	0.46	2.38	13.21

and the particle size distribution is illustrated in Figure 1.

(2) Alkali activator

The alkaline activator was prepared by mixing water glass solution (sodium silicate solution; produced in Shijiazhuang, Hebei, China), flake sodium hydroxide solid (purity ≥ 99.9%, produced in Changsha, Hunan, China), with a purity of 99.9%), and deionized water. In the water glass solution, Na₂O accounted for 8.35%, SiO₂ accounted for 26.54%, and water accounted for 65.11%. After mixing the alkaline activator, it was necessary to seal the entire container to prevent water evaporation and loss, which could occur due to the heat released by sodium hydroxide dissolving in water. Additionally, the mixture was stirred and left to stand for at least 6 h to ensure complete mixing of all components and cooling to room temperature before use.

2.2. Design of Mix Proportion

The ratio of MKBG-S0 (blank control group without cement addition) is as follows: activator modulus 1.2, liquid–solid ratio 1.2, activator concentration 35%, and curing temperature 20 °C. Activator modulus refers to the molar ratio of SiO₂ to Na₂O in the activator. Liquid–solid ratio refers to the mass ratio of activator to solid raw materials. Activator concentration is defined as the ratio of the combined masses of Na₂SiO₃ and NaOH to the total mass of the activator. The cement powder is mixed by the internal mixing method, and the mass content is increased from 10% and then to 40%, respectively, recorded as 10%CE, 20%CE, 30%CE, and 40%CE.

Following the specified mix ratio, metakaolin and cement powder were evenly blended. Subsequently, the alkaline activator was added, and the mixture was stirred at a low speed of 140 ± 5 r/min for 1 min, followed by high-speed stirring at 285 ± 5 r/min for an additional 2 min to achieve a uniform paste. The paste was then injected into the mold and vibrated for 3 min to eliminate bubbles. Afterward, the specimens were placed in a curing box (20 °C, ≥95% RH) and, after curing for 1 day, the mold was removed. The specimens were then further cured until the desired age.

2.3. Test Methods

(1) Compressive strength and elastic modulus test

The strength test method adopted was in accordance with JGJ/T 70-2009 “Standard for Test Methods of Basic Properties of Building Mortar” [18]. The compressive strength test mold had dimensions of 70.7 mm × 70.7 mm × 70.7 mm, and three samples were prepared for each set of mix proportions. The average value of the three samples was taken as the compressive strength of that set of specimens.

The elastic modulus test method followed GB/T 50081-2002 “Standard for Test Methods of Mechanical Properties of Ordinary Concrete” [19]. The elastic modulus test mold was a 40 mm × 40 mm × 160 mm triple PVC mold. For each mix proportion, six prismatic specimens were prepared. Three of these six prisms were subjected to compressive tests to determine their failure loads. The average value of one third of the failure load was used as the maximum load for the elastic modulus test. The remaining three specimens were tested for elastic modulus, and the average value of these three specimens was taken as the final result.

(2) Early shrinkage test

Early shrinkage tests can not only examine the chemical and self-shrinkage of geopolymers but also establish the commencement point for dry shrinkage measurement. The point at which chemical shrinkage is largely concluded and self-shrinkage begins to decrease is designated as the starting time for dry shrinkage measurement, followed by conducting the dry shrinkage test through alterations in the humidity environment. The early shrinkage test method used ASTM C1698-09 “Standard Test Method for Autogenous Strain of Cement Paste and Mortar” [20]. PVC bellows with a length of 420 mm and a diameter of 29 mm were utilized to measure the early shrinkage. After the paste was stirred, it was poured into the bellows, with the entire process being completed on a shaking table. Simultaneously, a metal rod was used to provide auxiliary vibration inside the bellows to minimize the presence of large cavities caused by trapped air, and then the bellows were sealed. The filled bellows were placed in a curing chamber maintained at a temperature of 20 (±2) °C and a relative humidity of not less than 95% RH for curing, with the length being measured every 24 h. Three specimens were prepared for each mix proportion, and the average value was taken as the final result.

(3) Dry shrinkage test under gradually decreasing humidity

To investigate the effect of cement incorporation on the drying shrinkage behavior of MKBGs, drying shrinkage specimens were tested in variable humidity conditions. The drying shrinkage test was conducted in accordance with JC/T 603-2004 “Test Method for Drying Shrinkage of Cement Mortar” [21]. Specimens with dimensions of 25 mm × 25 mm × 280 mm were prepared, with three specimens for each mix proportion. The paste was poured into a three-part cast-iron mold, which was then placed in a programmable curing chamber at 20 (±2) °C and a relative humidity of no less than 95% RH for one day to allow initial strength development. After demolding, the specimens were wrapped with PVC film and cured until the initial measurement time, at which point the initial length was measured. Subsequently, the PVC film was removed from the specimens, and the humidity in the curing chamber was adjusted to the preset relative humidity for the drying shrinkage test. The variable humidity drying shrinkage test employed a gradient humidity approach, with the relative humidity being adjusted from 90% RH down to 30% RH. The length change of the specimens was measured every five days, and the shrinkage and mass changes were recorded. When the difference between two consecutive linear shrinkage changes was less than 100 με, the humidity was reduced by 20% RH to proceed with the drying shrinkage measurement under the next humidity gradient.

(4) Reaction heat test

The measurement of reaction heat was conducted using the TAM Air isothermal calorimeter produced by TA Instruments. A 1 g powder of the raw material was mixed with a certain amount of activator liquid (the mass of the activator was calculated based on the liquid-to-solid ratio of the mixture). The mixture was then placed in a sealed ampoule, which was inserted into the calorimeter. The heat flow during the reaction was automatically collected by a computer using the TAM IV Lab Assistant software. The heat release of the geopolymer within 24 h was measured, providing insights into the reaction kinetics and thermal behavior of the material.

(5) Pore structure characterization

Pore structure analysis was conducted using mercury intrusion porosimetry (MIP) with a Poremaster 33GT automatic analyzer (Quantachrome Instruments, Boynton Beach, FL, USA). The system operated at a maximum pressure of 33,000 psi, enabling quantification of pores spanning 0.0064–950 μm in diameter. Specimens crushed during compressive strength testing were trimmed into 4–5 mm particles, immersed in absolute ethanol for 24 h to arrest hydration, and subsequently oven-dried at 60 °C for 24 h prior to MIP measurements.

3. Results and Discussion

3.1. XRD Result Analysis

Figure 2 shows the XRD results of metakaolin-based geopolymers (MKBGs) with different cement contents. Figure 2 clearly reveals that, in MKBG-S0, the most prominent phases are silica and kaolinite. The kaolinite phase is mainly derived from unreacted metakaolin. Cement incorporation resulted in the emergence of new substances in MKBGs, including calcium silicate (Hatrurite phase), dicalcium silicate (Lamite phase), and hydrated calcium silicate gel (C-S-H). With the increase in cement mixing proportion, the kaolinite phase in MKBGs gradually disappeared, while cement-related phases increased, such as dicalcium silicate and tricalcium silicate. The appearance of hydrated calcium silicate gel confirmed the hydration of cement.

The hydration of tricalcium silicate (3CaO·SiO₂) and dicalcium silicate (2CaO·SiO₂) in cement both produce calcium hydroxide (Ca(OH)₂). However, in MKBGs with added cement, no significant calcium hydroxide crystalline phase was detected. This indicated that the calcium hydroxide generated by the hydration reaction was consumed by the geopolymerization reaction. It is worth noting that, in high-calcium alkaline-activated material systems, calcium aluminosilicate hydrate (C-A-S-H) gel often forms. However, this gel was not observed in MKBGs with 10% to 30% cement addition, suggesting that calcium ions primarily participated in the hydration reaction of cement at this stage. When the cement content reached 40%, calcium aluminosilicate phases (such as Parthéite) were detected in the MKBGs, indicating that sufficient calcium was available in the mixed paste to participate in the geopolymerization reaction.

3.2. Compressive Strength and Elastic Modulus

Figure 3 shows the compressive strength and elastic modulus of metakaolin-based geopolymers (MKBGs) at the age of 28 days with different cement contents. The results indicated that, as the cement content increased from 0% to 30%, the compressive strength of MKBGs rose from 43.2 MPa to 47.86 MPa, an increase of 10.8%; the elastic modulus increased from 8.55 GPa to 8.70 GPa, a rise of only 2%. When the cement content was increased to 40%, compared with the group without cement addition (MKBG-S0), the compressive strength of MKBGs increased by 19.6%, reaching 51.67 MPa; the elastic modulus increased by 10.5%, reaching 9.45 GPa. These trends suggested that the addition of cement was beneficial to improving the mechanical properties of MKBGs. The reason was that, on the one hand, the C-S-H gel generated by the hydration reaction of cement had a denser microstructure and smaller crystal size compared with the N-A-S-H gel, which helped to improve the mechanical properties of the hardened paste. On the other hand, a large amount of cement addition promoted the formation of C-A-S-H gel, and the amorphous N-A-S-H system exhibited characteristics of crystalline transformation, which positively enhanced the mechanical properties of geopolymers [22]. It is worth noting that, within the range of 0% to 30% cement content, the influence of cement on the properties of geopolymers was relatively small. The reason might be that there was a competitive relationship between the hydration reaction of cement and the geopolymerization reaction of metakaolin. A small amount of cement addition could affect the final degree of geopolymerization reaction of metakaolin.

3.3. Reaction Heat Test Results

Figure 4 presents the reaction heat test results of metakaolin-based geopolymers (MKBGs) over 24 h with varying cement contents. The results showed that the position of the first peak in the reaction heat curve remained unchanged, indicating that the geopolymerization reaction remained the dominant process. However, the incorporation of cement significantly reduced the early-stage heat release rate of the MKBGs.

Compared to the control group without cement, the cement-modified MKBGs exhibited not only a lower early reaction heat release, as evidenced by the reduced initial exothermic peak, but also a decreased cumulative reaction heat within 24 h. These effects were attributed to the competitive dissolution between calcium silicate minerals in cement and aluminates in metakaolin within the alkaline activator, where the dissolution and precipitation of Ca²⁺ interfered with the subsequent polycondensation reaction [23]. Additionally, the lack of synchronization between the heat release processes of cement hydration and geopolymerization further contributed to the overall reduction in total reaction heat.

In addition, the initial heat release peak intensified as the cement content increased, and the final total heat release decreased first and then increased within 24 h. The reason is that a small amount of cement incorporation weakened the heat release of geopolymerization of metakaolin but, when a sufficient amount of cement was incorporated, the heat release of the hydration reaction of cement began to reflect, prompting the initial heat release peak to increase again. At the same time, it can also be noted that, compared with MKBG-S0, the MKBGs with cement addition exhibited stable heat release in the range of 4–24 h. The total heat release within 24 h was higher than that of MKBG-S0, and a continuous heat release process persisted even after 24 h. This phenomenon is consistent with the ongoing exothermic process of cement hydration, thereby confirming that the cement incorporated into MKBGs has undergone hydration.

3.4. Pore Structure Characteristics

Figure 5 and Figure 6 illustrate the cumulative porosity and pore size distribution of metakaolin-based geopolymers (MKBGs) with different cement contents, respectively. As seen in Figure 5, the porosity of MKBGs decreased significantly with increasing cement content, from 39.09% to 25.27%. This densification of the material led to improved mechanical properties, which is consistent with the previously discussed mechanical results [24]. The incorporation of cement reduced the porosity, shifted the pore size distribution toward smaller dimensions, increased the material density, and enhanced both the compressive strength and elastic modulus of MKBGs. The cumulative porosity curves of MKBGs with cement addition were similar to those without cement (MKBG-S0), both starting to rise near 100 nm, which is distinctly different from the porosity growth pattern of cement. This can be attributed to two main factors: first, metakaolin has a smaller particle size than cement (as shown in Figure 1); second, the geopolymerization reaction of metakaolin has a higher priority than the hydration reaction of cement. The resulting gel structure remains primarily a three-dimensional network, with smaller pore sizes.

It can be seen from Figure 6 that, with the increase in cement content, the most probable pore size of MKBGs became smaller, indicating that the pore size distribution of MKBGs developed towards a smaller size. The peak value of the most probable pore size decreased, indicating a decrease in the number of micropores, which is consistent with the cumulative porosity results. It can also be found that the number of pore structures below 10 nm in the MKBG-S0 was relatively low. However, as the cement content increased, the number of pore structures below 10 nm showed a slight increase. When the cement content reached 40%, the number of pore structures below 10 nm increased significantly. This pore size distribution characteristic is similar to that of cement paste. In cement, it is generally believed that pore structures below 10 nm are related to the formation of C-S-H gel, as the gel pores are primarily distributed in the size range of 10 nm or even smaller. The marked increase in the number of pore structures below 10 nm in MKBGs with a 40% cement content further suggests the significant presence of C-S-H gel in these geopolymers. Additionally, the increase in pore structures below 10 nm may also be related to the formation of C-A-S-H gel. In alkali-activated slag materials where C-A-S-H gel is the main reaction product, a large number of pore structures below 10 nm are also observed [25].

3.5. Early Shrinkage Results

Figure 7 shows the early shrinkage of metakaolin-based geopolymers (MKBGs) under different cement contents. As can be seen from Figure 7, the early shrinkage of MKBGs with cement addition was significantly different from that of the MKBG-S0. The early shrinkage of MKBG-S0 began to slow down on the third day. Compared with the second day, the early shrinkage of MKBG-S0 on the eighth day increased by 38.66%. The MKBGs mixed with cement continued to shrink within 8 days. As the cement content increased from 10% to 40%, the early shrinkage on day 8 increased by 393.01%, 296.86%, 152.78%, and 124.24% compared to the results on the second day, respectively. This phenomenon indicates that the addition of cement leads to a very significant early shrinkage. On the one hand, the addition of cement slowed down the geopolymerization reaction process, prolonging the chemical shrinkage time of MKBGs. Therefore, the shrinkage on the second day was significantly lower than that of MKBG-S0. On the other hand, the cement continuously underwent hydration in the paste, and the ongoing hydration reaction was accompanied by persistent early shrinkage (both chemical shrinkage and autogenous shrinkage), which increased with the increase in cement content.

3.6. Dry Shrinkage Results Under Varying Humidity Conditions

The early shrinkage of the MKBG-S0 tended to stabilize after 7 days. To compare it with the MKBGs with cement addition, the 8th day was chosen as the starting point for the drying shrinkage test. Figure 8 shows the drying shrinkage of MKBGs with different cement contents in the range of 90–50% RH. Previous studies have found that metakaolin-based geopolymers exhibit two abrupt increases in drying shrinkage in the range of 90–30% RH, located at 70% RH and 50% RH, respectively [26]. As can be seen from Figure 8, the addition of cement did not alter the drying shrinkage pattern of the MKBGs, with significant shrinkage still occurring at 70% RH and 50% RH. Based on the capillary stress drying shrinkage mechanism, the reason for the unchanged drying shrinkage pattern is that the pore structure distribution of the MKBGs did not undergo significant changes.

According to Figure 8, it can also be observed that, with the increase in cement content, the drying shrinkage of MKBGs first decreased and then increased under gradually decreasing humidity conditions. From the drying shrinkage results on the 40th day, it can be seen that, when the cement content was 10%, the drying shrinkage of the MKBGs decreased; however, when the cement content exceeded 20%, MKBGs exhibited even greater shrinkage. According to the previous test results, as the cement content increased, the pore structure size decreased, the porosity decreased, and the mechanical properties improved. This led to a decrease in capillary shrinkage stress, an improvement in the anti-shrinkage performance of the MKBGs, and a subsequent decrease in drying shrinkage. When the cement content was 10%, the drying shrinkage of the MKBGs decreased, which was consistent with the above trend. However, when the cement content was increased to over 30%, the drying shrinkage of MKBGs actually increased significantly, which was inconsistent with the above trend. The reason was that large amounts of water were displaced during the geopolymerization process, providing favorable conditions for the continuous hydration of a large number of unreacted cement particles. The refined pore structure of the MKBGs with cement addition also increased capillary shrinkage stress during the self-drying process. All of these factors contributed to long-term and significant self-drying shrinkage in the MKBGs, resulting in a significant increase in the final drying shrinkage.

4. Conclusions

(1) The addition of cement refines the pore structure of metakaolin-based geopolymers (MKBGs), reduces their porosity, and enhances their compressive strength and elastic modulus, thereby improving the anti-shrinkage performance of MKBGs.

(2) The hydration reaction of cement and the geopolymerization of metakaolin interfere with each other. The incorporation of cement slows down the early geopolymerization reaction rate of MKBGs. Additionally, the addition of cement prolongs the exothermic process of the entire reaction, which is related to the continuous hydration reaction of cement.

(3) This study verified the possibility of adding cement to improve the drying shrinkage of geopolymers, laying a practical foundation for subsequent related research. When the cement content is 10%, the dry shrinkage of MKBGs is significantly reduced. However, when the cement content exceeds 20%, it leads to a significant increase in self-drying shrinkage, making the overall shrinkage of MKBGs more severe.