Keywords

1 Introduction

With cement being the world's second-largest consumable material in industrial production, its high energy consumption and significant industrial CO2 emissions pose challenges in meeting the needs of green ecological engineering construction. Geopolymer cement, an eco-friendly and low-carbon cementitious material, has been increasingly emphasized to promote carbon dual-carbon strategies and facilitate low-carbon development [1]. Widely applied in construction, subways, bridges, tunnels, and water conservancy, geopolymer [2].

Geopolymer cement is produced from amorphous aluminosilicate solid waste materials with alkaline activators. It has been proven to possess mechanical properties and durability comparable to ordinary Portland cement (OPC) [3]. However, geopolymer cement still suffers from the drawback of short setting time, hindering its extensive application in engineering [4]. Therefore, the incorporation of a certain amount of retarder is necessary to control the setting time. Previous research has indicated significant variations in the effects of traditional cementitious retarders on the setting time of alkali-activated cementitious materials [5]. Moreover, the influence of retarder performance on concrete specimens remains understudied, necessitating further research on whether the addition of retarders affects the compressive strength of geopolymer concrete [6]. Therefore, there is a lack of exploration into the optimal retarder for a specific fixed mix proportion of geopolymer cement with alkali-activated materials.

This study primarily focuses on slag-fly ash concrete and cement, through experiments, aims to explore a suitable retarder that effectively prolongs the setting time of geopolymer cement while minimally affecting the workability and compressive strength of the concrete.

2 Experiment

2.1 Materials

The materials used in this study are detailed as follows: (1) Slag, provided by Longze Purified Water Materials Co., Ltd., Gongyi City, China, with a density of 3100 kg/m3; (2) Fly ash, provided by Henan Borun Foundry Materials Co., Ltd., China, with a density of 2550 kg/m3; (3) Solid activator with a purity of over 95% NaOH; (4) Liquid activator, sodium carbonate, prepared by blending sodium hydroxide and water glass, produced by Qingdao Yousuo Chemical Technology Co., Ltd., China. The specific composition of water glass is shown in Table 1.

Table 1 Composition of water glass
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2.2 Experimental Design

Experimental Equipment

The experimental equipment used in this study includes a ring knife, 100 cm*100 cm mold, cement slurry mixer, cement mortar mixer, humidity curing box, penetration tester, electronic scale, electro-hydraulic servo universal testing machine, vibration table, etc.

Experimental Procedure

The experimental design procedure of this study is as follows, ensuring the prolongation of the setting time of geopolymer concrete while maintaining its strength. The specific process flowchart is shown in Fig. 1.

Fig. 1
A process flow diagram. It begins with determining the matrix proportion of geopolymer cement, followed by preparing geopolymer cement test blocks, determining the effect of retarders on geopolymer cement, preparing geopolymer concrete test blocks, testing, and obtaining the most sustainable retarder.

Experimental design flowchart

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Step 1: Determine the mix proportion of geopolymer cement for this experiment.

Step 2: Prepare geopolymer cement test blocks. Prepare the control group geopolymer cement test blocks and prepare experimental group geopolymer cement test blocks with 1% sucrose, 1% borax, 1% barium chloride, 3% sucrose, 3% borax, and 3% barium chloride as retarders, then place them in a humidity curing box for setting.

Step 3: Determine the initial and final setting times of each test block to understand the impact of different types and proportions of retarders on the setting effect of geopolymer cement.

Step 4: Prepare geopolymer concrete test blocks using geopolymer cement as a binder material. The control experiment method is adopted. Prepare geopolymer cement according to the mix proportion in Step 1, then add aggregates in a certain proportion, mix the aggregates with the binder material thoroughly, and pour the mixture into the mold to form the control group geopolymer concrete test blocks and geopolymer concrete test blocks made with 1% sucrose, 1% borax, 1% barium chloride, 3% sucrose, 3% borax, and 3% barium chloride as retarders.

Step 5: Demold the test blocks after 12 h of setting, then perform high-temperature curing, and conduct compressive strength tests on the geopolymer concrete test blocks to investigate the impact of retarders on the compressive strength.

Step 6: Draw conclusions and identify the most effective retarder that effectively prolongs the setting time of geopolymer cement and minimally affects the workability and compressive strength of the concrete.

2.3 Experimental Mix Design

In this experiment, the precursor materials of geopolymer cement are slag and fly ash, with a mass ratio of 2:3. Sodium hydroxide and liquid sodium silicate are used as activators. The specific mix ratios of each material in geopolymer cement are shown in Table 2. Considering the feasibility of the candidate materials as retarders, this experiment employs the materials and proportions of retarders at 1% and 3%, namely, barium chloride [7], sucrose [8], and borax [9].

Table 2 Geopolymer cement mix designs
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2.4 Sample Preparation

Preparation of Geopolymer Cement

Before preparing the sample, prepare a sodium silicate solution according to the design concentration, and then cool it to room temperature for later use. Subsequently, the retarder is added to the prepared solution to obtain a mixed solution of the retarder and activator. Then, slag, fly ash, and solution are mixed using a cement sand mixer. These test samples are named Z1, Z2, Z3, Z4, Z5, Z6, and Z7, respectively. The types and proportions of retarders in different geopolymer cement test blocks are shown in Table 3. After mixing, inject the slurry into the circular test mold, use the vibration method to remove bubbles, and place it in a moisture curing box for curing. It should be noted that the entire process was carried out in a room with a room temperature of 23 °C [10].

Table 3 Types and proportions of retarders in different geopolymer cement test blocks
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Preparation of Geopolymer Concrete

The aggregates for geopolymer concrete in this study are 5–10 mm crushed stone, 10–20 mm crushed stone, and river sand. The specific proportions of cementitious materials and aggregates are shown in Table 4. The specific preparation process is as follows: after the production of geopolymer cement is completed, a retarder and aggregate are quickly added, and then mixed to prepare geopolymer concrete. Next, quickly place the concrete into a 100 * 100 * 100 mm cube mold, named L1, L2, L3, L4, L5, L6, L7 (Table 5). To reduce test errors and ensure accuracy, each group of concrete test blocks has the same three blocks (Fig. 2), and the average compressive strength of each group of three test blocks is taken [11]. The materials and dosage of retarder for each test block are shown in Table 5. Subsequently, the mold is placed on a vibration table to temporarily damage the flocculent structure of the concrete, reducing its viscosity and increasing the fluidity of the cement slurry in the concrete, thereby reducing the pores in the concrete test block [12] (Fig. 3).

Table 4 Test plan for mix proportion of geopolymer concrete (kg/m3)
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Table 5 Types and proportions of retarders in different geopolymer concrete test blocks
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Fig. 2
A photograph of 4 geopolymer concrete test blocks before demolding.

Geopolymer concrete test block before demolding

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Fig. 3
A set of two photographs of geopolymer concrete blocks.

Excluding bubbles

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2.5 Determination of Setting Time

To investigate the effect of retarders on the initial setting time, the initial setting time of the test block was measured. As the initial setting approaches, place the test block under the test needle and lower it until it comes into contact with the surface of the cement paste. After tightening the screw, it suddenly relaxes and the test needle sinks vertically and freely into the cement slurry. Observe the reading of the pointer when the test needle stops sinking. When the test needle sinks to a distance of 4 ± 1 mm from the bottom plate, it indicates that the cement has reached its initial setting state [13], as shown in Fig. 4. After measuring the initial setting time, immediately remove the test mold and slurry from the glass plate in a translational manner, flip it 180°, and then place it in a moisture curing box to continue curing. Insert a circular attachment at the bottom of the test needle, and the measurement method for the final setting time is consistent with the measurement method for the initial setting time. The difference is that when the test needle sinks 0.5 mm into the specimen, that is, when the circular attachment cannot leave any marks on the specimen, the cement reaches the final setting state, as shown in Fig. 5 [14].

Fig. 4
A photograph of a measuring scale.

Measure the initial setting time

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Fig. 5
A photograph of a solid test block.

The test block reaches the final setting state

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2.6 Testing of Compressive Strength

To investigate the effect of retarders on the compressive performance of geopolymer concrete specimens, the compressive strength of the specimens was tested. Leave the prepared concrete test block groups L1, L2, L3, L4, L5, L6, and L7 standing for 12 h before demolding, and wrap them with cling film before quickly placing them in a high-temperature curing box for 24 h, as shown in Fig. 6. The cured geopolymer concrete test blocks were placed into an electro-hydraulic servo universal pressure testing machine for pressure testing, as shown in Fig. 7. After completing the test, directly use the screen display pressure testing machine to read the data [15].

Fig. 6
A photograph displays geopolymer concrete specimens arranged in a high-temperature curing box in 2 rows.

High temperature curing

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Fig. 7
A photograph of a pressure testing machine.

Pressure test

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3 Results

3.1 Results and Analysis of the Influence of Initial Setting Time

Table 6 shows the initial setting time of each test block group, and it can be seen that the barium chloride −3% group has the longest initial setting time of 228 min, which is 128 min longer than the control group and has a significant positive impact. Compared to the control group, the saccharose -1% group only extended the initial setting time by 9 min, which had a smaller impact. Figure 8 shows the percentage increase in initial setting time of each effective group compared to the control group, with a significant retardation effect of 3% concentration of barium chloride, an increase of 341.7%.

Table 6 Initial setting time of each group of test blocks group
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Fig. 8
A bar graph depicts the percentage increase in initial setting time for the following. Sucrose 1%, 15. Sucrose 3%, 140. Borax 1%, 180. Borax 3%, 243. Barium Chloride 1%, 280. Barium Chloride 3%, 341.7.

Percentage increase in initial setting time of each effective group

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3.2 Results and Analysis of the Effect of Final Setting Time

Table 7 shows the final setting time of each test block group, and it can be seen that the barium chloride -3% group has the longest final setting time of 420 min, which is 290 min longer than the control group. Figure 9 shows the percentage increase in initial setting time of each effective group compared to the control group, with a significant retardation effect of 3% concentration of barium chloride, an increase of 341.7%.

Table 7 Final setting time of each group of test blocks
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Fig. 9
A bar graph depicts the percentage increase in the final setting time for the following. Sucrose 1%, 13.8. Sucrose 3%, 115.4. Borax 1%, 128.4. Borax 3%, 165.4. Barium Chloride 1%, 192.3. Barium Chloride 3%, 223.07.

Percentage increase in final coagulation time for each effective group

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3.3 Compressive Strength Results

Table 8 shows the compressive strength of the concrete test block group. Overall, compared with the control group, adding 1–3% sucrose, borax, and barium chloride retarders does not have a significant impact on the compressive strength of geopolymer cement. Among them, the average compressive strength of the 3% borax experimental group L5 decreased by 11.2% compared to the control group L1. However, the average compressive strength of the 3% barium chloride experimental group L7 was 29.85 N/mm2, Compared to the control group, only decreased by 4.3%.

Table 8 Average compressive strength of each group of concrete test blocks
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4 Conclusions

In response to the impact of retarders on the initial setting time and compressive strength of geopolymer cement, this study selected three types of retarders: sucrose, borax, and barium chloride, and set corresponding proportions. Through indoor experiments, the influence of these retarders on the setting time of geopolymer cement was explored. The conclusion drawn is as follows:

  1. (1)

    Compared to sucrose and borax, the geopolymer cement with barium chloride as a coagulant has the longest setting time. When the dosage is 1%, the initial setting time and final setting time of geopolymer cement can reach 228 and 380 min, respectively. When the dosage is 3%, the initial setting time and final setting time of geopolymer cement can reach 265 min and 420 min, respectively.

  2. (2)

    Compared to barium chloride −1%, barium chloride −3% has a better retarding effect. The percentage increase in initial and final retarding time is 341.7 and 223.7, respectively, which are 61.7 and 30.77% higher than barium chloride −1%.

  3. (3)

    In the six groups of test blocks, the effects of 1% and 3% concentrations of retarders on setting time were not the same, and the retarding effects of sucrose, borax, and barium chloride all became stronger with the increase of proportion.

  4. (4)

    There are differences in the impact of different types of retarders and their dosage on the compressive strength of geopolymer concrete. When the dosage is 1% −3%, retarders such as sucrose, borax, and barium chloride do not have a significant negative impact on their compressive strength.