Effects of Different Kinds of Defoamer on Properties of Geopolymer Mortar

Abstract

As a new type of green inorganic material, geopolymers have excellent mechanical properties, durability, and less environmental pollution. It is considered a new building material that can replace cement, but it also has some disadvantages such as high viscosity, poor fluidity, and more apparent pores after hardening. In this study, the uniaxial compressive strength test of geopolymer mortar was carried out, and the effects of alkali equivalent, alkali activator modulus, bone glue ratio, and silica fume content on the mechanical properties of geopolymer mortar were analyzed. The test results show that when the alkali equivalent is 13%, the alkali activator modulus is 1.4, the bone glue ratio is 2.0, the silicon powder content is 8%, and the metakaolin-based geopolymer mortar has higher uniaxial compressive strength. Through the comparative test of adding different kinds of defoamers and dosage, the effect of defoamers on the compressive strength, fluidity, density, and water-absorption of geopolymer mortar was further studied. The fluidity, density, and water-absorption were improved, and the uniaxial compressive strength was reduced. The formation of cementitious material in the mortar was confirmed by scanning electron microscope (SEM) observation. It was found that the pore structure and pore distribution changed with the content of different defoaming agents, and the microstructure of mortar after defoaming agent material treatment was shown. The proportion and distribution of Na, Al, and Si atoms were analyzed by energy dispersive spectroscopy (EDS). This experimental study shows that the defoamer can be an effective additive for geopolymer mortar.

1. Introduction

Construction sites often encounter weak or easily cracked concrete buildings. These concrete buildings in the construction period, and the use of the period due to lack of strength, often find it difficult to support the upper structure of the load. The upper structure is subjected to different degrees of cracking and potholes and other phenomena, resulting in the decline of the function of the building [1]. In order to improve the strength, stiffness, and fluidity of concrete buildings, inorganic cementitious materials such as lime and cement are widely used in engineering for chemical reinforcement. Cement in cement soil makes the reinforced building dense through hydration reaction and pozzolanic reaction. In lime soil, the concrete particles become more compact by flocculation and pozzolanic reaction. However, the main problem with traditional curing agents is that the process of producing cement and lime consumes too much energy and produces a large amount of greenhouse gases. For example, 0.66–0.82 kg of carbon emissions will be produced during the production of 1 kg of ordinary Portland cement, and about one ton of carbon dioxide will be emitted to produce one ton of cement [2,3,4]. The construction industry is already looking for new environmentally sustainable alternatives to cement and lime. Geopolymer is considered to be the most promising new green cementitious material, carbon dioxide emissions of ordinary Portland cement is only 24%, which can greatly reduce the carbon emissions caused by traditional Portland cement [5,6], is a green mate-rial with three-dimensional silicon and aluminum tetrahedral network structure, because of its excellent mechanical properties, fire resistance, high temperature resistance, corro-sion resistance [7,8] and simple process, low energy consumption, low environmental pollution, etc. Therefore, it is considered as a new green building material that can replace cement [7]. Geopolymer (GP) is a low carbon binder that is clinker free [8]. Formed by the activation of solid aluminosilicates, such as fly ash (FA) [9], silica fume (SF) [10] and me-takaolin [11,12], using alkaline sols such as silicates, carbonates, alkali hydroxides and sulfates.

At present, geopolymers have also been widely used in engineering practice, but geopolymers have problems such as high viscosity, poor fluidity, and more pores after hard-ening. At present, the fluidity, density, and mechanical properties of defoamer-modified cementitious materials at home and abroad are concentrated in cement-based materials and have never appeared in the single-doped experiment of geopolymers. Therefore, it is of frontier significance to carry out the experiment of defoamers on the performance of geopolymers. For example, Yu Feng [13] studied the effect of a defoaming agent on the frost resistance of cement-based concrete, and found that a defoaming agent can achieve the effect of synergistically improving the working performance, pore structure, and frost-resistance of concrete. When a 0.15 wt% defoaming agent reduces the air content loss by 64.28%, the expansion loss rate is reduced by 55.04%. Song Putao [14] found that when the content of the polyether defoamer is 0.06%, the fluidity of sea sand UHPC is the largest, the 7 d compressive strength is the highest, and when the yield is 0.12%, the 28 d compressive strength is the highest and the water absorption is the lowest. However, the above tests are the progress of the research on the modification of cement-based mate-rials. H. Süleyman Gökçe [15] found that the addition of the silica fume will lead to an in concrete density of 55%, and the incorporation of the silica fume makes the water absorption of concrete decrease by 67%. Wenjuan Wu [16] found that silicon powder, in reducing the water absorption of coral aggregate concrete and chloride ion permeability and microhardness, also has a positive effect. In addition, in recent years, some progress has been made in changing the law of mechanical properties of geopolymer mortar from the aspects of alkali activator type, the incorporation ratio of geopolymer material, and the liquid-solid ratio [17,18,19,20]. In this study, firstly, through the design of an orthogonal test and a verification test, and uniaxial compressive strength as the index to select the optimal mix ratio under experimental conditions, the orthogonal test results were analyzed to determine the alkali equivalent, water glass modulus, bone glue ratio, and silica fume content on the mechanical properties of geopolymer mortar. Additionally, we set up a polyether and a silicone defoamer as a control group. The uniaxial compressive strength tests of 3 d and 7 d with different dosages and different ages were carried out, and the fluidity, density, and water absorption were tested to evaluate the effect of defoamers on geopolymer mortar. In addition, the microstructure and elemental composition of geopolymer mortar were characterized by scanning electron microscopy (SEM) and (EDS), and the reaction mechanism of geopolymer mortar was described.

2. Materials and Methods

2.1. Materials

The metakaolin used in this experiment is a silicon–aluminum inorganic polymer material calcined at 800–850 °C from Inner Mongolia Chapadi Metakaolin Company (inner mongolia, China). The particle size is 10 microns. The main components are active Al₂O₃ and SiO₂, and their mass fractions are 43.4% and 49.1%. Therefore, it has high reactivity in the alkali activation process. The chemical composition of metakaolin is shown in

Table 1. Chemical composition of metakaolin. (%).

Materials	SiO2	Al2O3	SiO2	Fe2O3	P2O5	K2O	CaO	Others
MK	49.1	43.4	0.05	0.09	0.69	0.13	0.24	0.53

. The particle size distribution measured by the laser particle size distribution analyzer is shown in Figure 1, and the metakaolin particle size distribution (PSD) is shown in

Table 2. Particle size distribution (PSD) of metakaolinite.

PSD	Particle Size (μm)
D10	0.554
D50	1.39
D90	5.56

.

The silica fume used in this test is from Henan Dingnuo Purification Materials Co., Ltd. (Zhengzhou, China). The main component is active SiO₂, with a mass fraction of 91.6% and a particle size of 13 microns. The chemical composition of silicon powder is shown in

Table 3. Chemical composition of silicon powder.

Material	SiO2	Al2O3	Fe2O3	CaO	MgO	Na2O
SF	91.6	0.36	0.08	1.13	0.6	0.9

.

The polymerization reaction of geopolymers occurs in an alkaline environment, and the flake sodium hydroxide and sodium silicate powder are used to make an alkaline activator. The flake sodium hydroxide is selected from Inner Mongolia Junzheng Chemical Co., Ltd. (Wuhai, China), with a mass fraction greater than 98%. The sodium silicate powder is selected from Xi’an Huachang Water Glass Co., Ltd. (Xi’an, China). The content of Na₂O is 21.53%, the content of SiO₂ is 60.1%, and the sodium silicate modulus is 2.86. The modulus of alkali activator was changed by a different NaOH content to meet the design value of test requirements. In this experiment, flaky sodium hydroxide and sodium silicate powder were fully mixed and dissolved in 100 °C water and stood at room temperature for 24 h. The water in the experiment is distilled water.

As shown in Figure 2, the aggregate used in this experiment was taken from quartz sand of Guangxi Shiling Building Materials Co., Ltd. (Guigang, China). Three continuous gradations of 0.2–0.5 mm, 0.5–1 mm, and 1–2 mm were taken, and a quartz sand SiO₂ content of 99.75% is shown in

Table 4. Chemical composition of quartz sand.

Material	SiO2	Al2O3	Fe2O3	CaO	MgO	K2O	Na2O
Sand	99.7	0.13	0.0037	0.008	0.001	0.054	0.012

.

This test selected polyether and silicone, two kinds of concrete mortar defoamer, from Guangdong Nanhui New Material Co., Ltd. (Dongguan, China). The polyether defoamer was a pale-yellow liquid, and the silicone defoamer was a transparent liquid. Both defoamers have small running speed blocks, high temperature resistance, strong alkali resistance, good dispersion in the system, do not affect the firmness of the product, and have strong stability, especially in alkaline environment. Defoaming and inhibiting bubbles have dispersion properties in concrete and mortar.

2.2. Test Method and Mix Design

The main purpose of this test is to study the effect of different defoamers and dosages on the performance of geopolymer mortar and to find the optimal dosage of defoamers. The microstructure of geopolymer mortar was observed by scanning electron microscope (SEM) and energy dispersive spectrometer (EDS), and the mechanism of cracks and pores formed in the microstructure of mortar was analyzed. Therefore, the test is divided into three parts.

• Research on the optimal mix ratio design of geopolymer mortar.

The main purpose of the test is to find the ideal mix ratio design of cementitious materials and alkali activators in geopolymer mortar. Four factors and three levels L (3⁴) orthogonal test was established, and four main factors were determined, namely alkali equivalent [21,22], water glass modulus [23,24], bone glue ratio [25,26] and silicon powder content [27,28], as shown in

Table 5. Orthogonal experimental table of four factors and three levels.

Level	Factor
	A:Alkali Content	B:Modulus	C:Bone Glue Ratio	D:Silica Fume
MKSF-1	13%	1.2	1.5	16%
MKSF-2	13%	1.4	1.0	8%
MKSF-3	13%	1.6	2.0	12%
MKSF-4	16%	1.4	2.0	8%
MKSF-5	16%	1.2	1.0	12%
MKSF-6	16%	1.6	1.5	16%
MKSF-7	19%	1.6	1.0	12%
MKSF-8	19%	1.2	1.5	8%
MKSF-9	19%	1.4	2.0	16%

. Alkali equivalent refers to the percentage of the mass of sodium oxide in the alkali activator to the total mass of the cementitious material, namely AE%; water glass modulus refers to the molar ratio of silicon dioxide to sodium hydroxide in the alkali activator solution; bone glue ratio refers to the ratio of bone material to cementitious material; and the addition of silicon powder adopts the internal mixing method, that is, the proportion of 8%, 12%, and 16% is used instead of metakaolin. According to the previous experimental study, the alkali equivalent was designed as 13%, 16%, and 19%; the water glass modulus was 1.2, 1.4, and 1.6; the bone glue ratio was 1.0, 1.5, and 2.0; and the silicon powder content was 8%, 12%, and 16%. The preparation of test samples for a uniaxial compressive strength test had three specimens in each group. The geopolymer mortar is cured at a relative humidity of 90%, room temperature (20 ± 5) °C for 3 days and 7 days [1], and the test plan is shown in Table 4. The TAW-2000 rock uniaxial testing machine was used to measure the compressive strength of metakaolin-silica fume geopolymer mortar for 3 and 7 days. The strain loading rate was controlled at 1 KN/S during the loading process. The preparation process of the test is shown in Figure 3.

2.

Influence of defoamer on compressive strength, density, fluidity, and water absorption.

By adding different types and dosages of defoamers, the effect of defoamers on the mechanical properties, fluidity, density, and water-absorption of geopolymer mortar was observed. The mix proportion is shown in

Table 6. Mixture ratio of different defoamers and dosages.

Experiment Number	Binding Material	Alkali Activator	Water	Polyether Defoaming Agent	Organic Silicon Defoaming Agent
MKSF-0%	191.02	116.74	64.2	0	0
MKSFP-0.05%	191.02	116.74	64.2	0.095	0
MKSFP-0.10%	191.02	116.74	64.2	0.191	0
MKSFP-0.15%	191.02	116.74	64.2	0.287	0
MKSFP-0.20%	191.02	116.74	64.2	0.382	0
MKSFP-0.25%	191.02	116.74	64.2	0.478	0
MKSFO-0.05%	191.02	116.74	64.2	0	0.095
MKSFO-0.10%	191.02	116.74	64.2	0	0.191
MKSFO-0.15%	191.02	116.74	64.2	0	0.287
MKSFO-0.20%	191.02	116.74	64.2	0	0.382
MKSFO-0.25%	191.02	116.74	64.2	0	0.478

. The geopolymer mortar with polyether defoamer is labeled as MKSFP, and the geopolymer mortar with silicone defoamer is labeled as MKSFO. The dosage of the two defoamers is 0.05–0.25% of the cementitious material, and 0% is the best ratio optimized by orthogonal test without adding any defoamer.

The samples were tested for water absorption. The samples were taken out after curing for 28 days at a temperature of 20 ± 2 °C, dried at a temperature of 78 ± 3 °C for 48 h, weighed, and then put the samples into a water tank with a height of 35 mm and kept the temperature at 20 ± 3 °C. In a constant temperature room with a relative humidity of 80%, the samples were soaked for 48 h to remove, wiped off the surface moisture with a dry rag, and weighed:

$$\mathrm{Percentage} \mathrm{of} \mathrm{water} \mathrm{absorption} (\%)=\left(\frac{W_s-W_d}{W_d}\right)\times 100$$

where:

W_d = oven dry weight of specimen (kg);

W_s = saturated weight of specimen (kg);

Samples were tested for density. First, the mass M₁ of the volumetric cylinder was weighed, the volumetric cylinder was filled with the mortar mixture at one time. It was evenly pounded from the edge to the center 25 times with a tamper and vibrated for 10 s on the vibration, then the excess colloid at the mouth of the volumetric cylinder was scraped off. The total mass of the alternating material and graduated cylinder M2 was weighed out:

$$\mathrm{D}=\frac{M_2-M_1}{\mathrm{V}}\times 100$$

where:

D = the density of the mortar mix (kg/m³)

M₁ = mass of volumetric cylinder (kg)

M₂ = capacity cylinder and mortar quality (kg);

V = Volume of volumetric cylinder (L);

Fluidity of colloidal materials. The small slump cone was filled with colloidal material, (top width 36 mm, bottom 60 mm, height 60 mm), the excess slurry material above the cone removed, and then the slump hammer was lifted vertically. After 30 s, the vertical direction was selected to measure the average diameter of the colloidal material. The values were measured three times for each group of materials, and the average value of the three times was selected as the result.

3.

Microscopic test of geopolymer mortar

The crushed test sample after the uniaxial compressive strength test was naturally air-dried, dehydrated, and cut into a 5 mm × 5 mm × 5 mm cube as a sample for scanning electron microscope observation. This device can analyze the composition of the sample while analyzing the image, and can give the point, line, and surface distribution of the composition, and give the digital image. The debris on the surface of the sample was carefully cleaned. The sample was firmly fixed with adhesive tape. The JSM-6700F cold field emission scanning electron microscope was used to observe the microstructure of the sample under different magnifications, as shown in Figure 4. The arrangement and structure distribution of mortar particles were observed under the microscope. Energy chromatography was used to observe the distribution state of Na, Al, and Si element pairs and further explore the reaction mechanism of geopolymer mortar [29,30].

3. Results

3.1. Compressive Strength Results and Analysis

Table 7. Test results and range analysis.

Experiment Number	Alkali Content	Modulus	Bone Glue Ratio	Silica Fume	Comprehensive Strength (MPa)
					3 d	7 d
MKSF-1	13%	1.2	1.5	16%	52.9	78.3
MKSF-2	13%	1.4	1.0	8%	51.2	74.4
MKSF-3	13%	1.6	2.0	12%	46.4	64.4
MKSF-4	16%	1.4	2.0	8%	54.1	74.7
MKSF-5	16%	1.2	1.0	12%	39.8	66.4
MKSF-6	16%	1.6	1.5	16%	29.4	37.1
MKSF-7	19%	1.6	1.0	12%	26.1	58.3
MKSF-8	19%	1.2	1.5	8%	39.2	62.2
MKSF-9	19%	1.4	2.0	16%	49.6	71.1
K1	150.5	131.9	117.1	144.5	3 d Compressive strength Analysis
K2	123.3	154.9	121.5	112.3
K3	114.9	101.9	150.1	131.9
H1	50.17	43.97	39.03	48.17
H2	41.1	51.63	40.5	37.43
H3	38.3	33.97	50.03	43.97
Range Ri	11.87	17.66	11	10.74
Primary and secondary order	B > A > C > D
Superior level	A1	B2	C3	D1
Optimal combination	A1B2C3D1
K1	217.1	206.9	199.1	211.3	7 d Compressive strength Analysis
K2	178.2	220.2	177.6	189.1
K3	191.6	159.8	209.9	186.5
H1	72.37	68.97	66.37	70.43
H2	59.4	73.4	59.2	63.03
H3	63.87	53.27	70.07	62.17
Range Ri	12.97	20.13	10.87	8.26
Primary and secondary order	B > A > C > D
Superior level	A1	B2	C3	D1
Optimal combination	A1B2C3D1

is the uniaxial compressive strength of geopolymer mortar for 3 and 7 days in the orthogonal test. As shown in Figure 5. The test data adopt the method of range analysis to analyse the factor A, that is, the alkali equivalent. This factor is in the first column and will include three data with the same factor at the same level in a group, that is, in the column of alkali equivalent, MKSF-1, MKSF-2, and MKSF-3 are the first group, MKSF-4, MKSF-5, and MKSF-6 are, the second group of the same factor with different levels, MKSF-7, MKSF-8, and MKSF-9 is the third group, and so on. Nine groups of experiments are divided into three groups under the same level and the same factor. Other factors will only appear once, and the other conditions are the same. This will replace the original 3³ = 27 groups of experiments with 9 groups of experiments evenly and dispersedly, and at the same time, the number of experiments can be reduced.

The principle of range analysis is to first calculate the sum of the three sets of data of the same level and the same factor, that is, Ki, then calculate the average value of Ki Hi = Ki/3, and finally calculate the range Ri = Max(Hi) − Min(Hi). The table shows that the greater the range Ri, the greater the influence of this factor on the strength of geopolymer mortar. The range analysis of the compressive strength test results of geopolymer mortar at different ages of 3 and 7 days is carried out. The order of factors affecting the strength of geopolymer mortar from large to small is water glass modulus > alkali equivalent > bone glue ratio > silicon powder content. The reasons are as follows:

Compare the average values of the compressive strengths of the three groups at the same level under the same factor, and Figure 6 shows the trend of the level of the influencing factors.

(a)

The level of alkali equivalent is used as a measure of the quality of Na₂O in the alkali activator, which determines the level of PH value. In a relatively alkaline environment, the hydration reaction rate of geopolymers can be accelerated, and the silico-alumina raw materials can be better dissolved, so the increase of the base equivalent and controlled within a certain range can be to increase the colloidal strength. It can be seen from Figure 6a,e that the mechanical properties of the geopolymer first decreased and then increased with the increase of the alkali equivalent. Too much alkalinity results in the excessive formation of early precipitation of the aluminosilicate product [31,32], thereby affecting the colloidal strength. It can be seen that when the alkali equivalent is 13%, the compressive strength of geopolymer mortar reaches the highest;

(b)

Water glass is an alkaline activator, and the modulus affects the compressive strength of geopolymer mortar, as seen in Figure 6b,f. When the modulus of water glass is 1.2–1.4 m, and when the modulus of water glass is 1.4, the compressive strength reaches the maximum, and when the modulus is greater than 1.4 m, the compressive strength of the sample gradually decreases. When the modulus is 1.2–1.4 m, the content of oligomeric [SiO₂] in water glass gradually increases, and the content of high polymer [SiO₂] decreases, and the alumino-siliceous raw material in metakaolin undergoes depolymerization, which makes the polymerized strength increase. When the modulus is greater than 1.4, too much high polymer [SiO₂] reduces the pH value and cannot react with the silicon-alumina raw materials in metakaolin in a low-alkaline environment, and the hydration rate decreases, which is not conducive to the efficient progress of the geopolymer reaction and is therefore detrimental to the strength after polymerization. It can be seen that in the geopolymer mortar, when the modulus of water glass is 1.4 m, it is the best modulus;

(c)

The effect of the bone glue ratio on the compressive strength of geopolymer mortar. As shown in Figure 6c,g, it can be seen from the figure that the bone-to-adhesive ratio is 1.0–2.0. With the increase of the bone glue ratio, the compressive strength of the sample decreases first and then increases, but more aggregates are not conducive to the working performance of the mortar, so the bone glue ratio of the mortar needs to be controlled within a certain range;

(d)

It can be seen from Figure 6d,h that the content of silica fume affects the compressive strength of high-strength geopolymer mortar. Within the range of 8–16% of silica fume content, the compressive strength of mortar increases with the increase of silicon as powder decreases, and the compressive strength reaches the highest when the content of silicon powder is 8%. This is because the silica fume gives full play to the role of fine particles, and the addition of the fine-grained silica fume improves the gradation of the geopolymer cementitious material, fills the pores and enhances the density of the slurry [18,33],; the specific surface area of the silica fume is large, adsorbing a large amount of free water to improve the internal compactness of the mortar, and the content of silica fume will increase the silicon-aluminum ratio of the base material, and the increase of the silicon-aluminum ratio will increase the degree of reaction in the geopolymer polymerization reaction and improve the strength. When a large amount of silica fume is not conducive to the polymerization reaction of mortar, an appropriate amount of silica fume has a favorable effect on the compressive strength. It can be seen that in the geopolymer mortar, when the content of silica fume is 8%, the improvement of the compressive strength is the best.

Summarizing the above effects of alkali equivalent, water glass modulus, bone glue ratio, and silica fume content on the strength of geopolymer mortar, the optimal mixing ratio of compressive strength of geopolymer mortar is alkali equivalent 13%, water glass modulus 1.4, bone glue ratio 2.0, and silicon powder content of 8%. The optimal mix ratio selected by the orthogonal test was experimentally studied, and its seven-day uniaxial compressive strength was 81.22 MPa.

Figure 6. Influence mortar 3 days, 7 days level change trend.

Figure 6. Influence mortar 3 days, 7 days level change trend.

Compressive strength: Figure 7 shows the 3-day and 7-day compressive strength values of polyether defoamer and silicone defoamer with different dosages. The 3-day and 7-day compressive strength of MKSF-0% is higher than that of the sample after adding defoamer the compressive strength value, and the defoamer changes the cohesive force of the inorganic macromolecular polymer produced by the geopolymer reaction, thereby reducing the overall strength of the mortar, which is reflected in the subsequent fluidity test, microscopic test, and further discussions. In addition, it can be seen from

Table 8. The compressive strength of the benchmark group and different amounts of defoamer.

No.	Polyether Defoaming Agent/g	Organic Silicon Defoaming Agent/g	Comprehensive Strength/MPa
			3 d	7 d
MKSF-0%	0	0	52.46	81.22
MKSFP-0.05%	0.095	0	49.17	72.31
MKSFP-0.10%	0.191	0	47.81	73.6
MKSFP-0.15%	0.287	0	51.20	72.29
MKSFP-0.20%	0.382	0	51.91	74.38
MKSFP-0.25%	0.478	0	50.81	72.71
MKSFO-0.05%	0	0.095	49.08	73.12
MKSFO-0.10%	0	0.191	48.31	75.21
MKSFO-0.15%	0	0.287	51.07	76.54
MKSFO-0.20%	0	0.382	48.97	53.06
MKSFO-0.25%	0	0.478	47.12	67.28

that when the content of polyether defoamer is 0.05–0.25%, as the content of polyether defoamer gradually increases, the compressive strength value of geopolymer mortar increases first and then decreases, and reaches the maximum strength of the polyether defoamer content at 0.20%, because the defoamer will eliminate the bubbles inside the mortar to a certain extent, reduce the generation of pores, reduce the internal porosity of the mortar, and improve the mortar resistance to a certain extent. When the content of the silicone defoamer is 0.05–0.25%, as the content of silicone defoamer gradually increases, the compressive strength value of geopolymer mortar first increases and then decreases, and reaches 0.15%. The reason for the maximum strength of the silicone defoamer dosage is the same as the reaction principle of the polyether defoamer, which reduces the porosity inside the mortar and improves the compactness to increase the strength value of the mortar.

At the same time, we found that the compressive strength of geopolymer mortar after the addition of two defoamers was lower than that without the addition of defoamer, because the compressive strength of geopolymer depends on the microstructure and the intrinsic properties of gelling ions. The mechanical properties of the mortar and the degree of reaction of the raw materials, the defoamer changes the cohesiveness of the inorganic polymer mortar produced by the depolymerization-polymerization reaction of the geopolymer, and this change to the mortar itself is far more than that of the defoamer. The resulting decrease in porosity and the increase in density bring changes. Therefore, we will find that the addition of defoamer will not improve the 3-day and 7-day compressive strength values of geopolymer mortar.

Mobility and density:

Table 9. Mobility and density.

No	Polyether Defoaming Agent/g	Organic Silicon Defoaming Agent/g	Fluidity/mm	Density/(kg·mg)
MKSF-0%	0	0	134.8	2993
MKSFP-0.05%	0.095	0	137.2	3100
MKSFP-0.10%	0.191	0	144.1	3130
MKSFP-0.15%	0.287	0	149.5	3240
MKSFP-0.20%	0.382	0	155.3	3320
MKSFP-0.25%	0.478	0	153.0	3220
MKSFO-0.05%	0	0.095	135.5	3060
MKSFO-0.10%	0	0.191	140.5	3100
MKSFO-0.15%	0	0.287	148.1	3140
MKSFO-0.20%	0	0.382	146.5	3210
MKSFO-0.25%	0	0.478	142.8	3180

shows the changes in the flow and density properties of mortar with different dosages of polyether defoamer and silicone defoamer. It is not difficult to find that the fluidity and density of the geopolymer colloid have been improved by both defoamers. As shown in Figure 8 and Figure 9, the fluidity of the polyether defoamer is the largest when the content of the polyether defoamer is 0.20%. The fluidity is the largest when the dosage of the agent is 0.15%. Taking MKSF-0% as an example, the fluidity of the 0.20% polyether defoamer group is increased by 15.2%, and the fluidity of the 0.15% silicone defoamer group increases by 9.8%. At the same time, the density of the two defoamers increases the most when the dosage is 0.20%. Taking MKSF-0% as an example in the geopolymer mortar test, the density of the 0.20% polyether defoamer group increased by 10.9%, and the silicone defoamer increased by 0.15%. The density of the agent group is increased by 7.2%, and the defoamer can eliminate the air bubbles in the mortar, reduce the surface tension between the mortars, reduce the cohesion, and improve the compactness. It can be seen that both defoamers greatly improve the fluidity of geopolymer mortar, but the polyether defoamer improves the fluidity and density of mortar the most, so the use of polyether defoamer is more conducive to mortar mobility.

Water absorption: Figure 10 shows the change of water absorption rate of geopolymer mortar under different dosages of defoamer. The water absorption rate of geopolymer mortar decreases with the increase of the dosage of two defoamers. Compared with the MKSF-0% group, the water absorption rate of the polyether defoamer with a dosage of 0.25% is reduced by 16.6%, and the water absorption rate of the polyether defoamer with a dosage of 0.25% The water absorption rate of the silicone defoamer is reduced by 16.1%. This is because the defoamer can greatly reduce the bubbles existing in the mortar, making the mortar colloid material more dense after the reaction, thereby increasing the density and reducing the water absorption.

3.2. Influence of Defoamer on the Properties of Geopolymer Mortar

The microstructure of MKSF-0%, MKSFP-0.20%, and MKSFO-0.15% mortars which had been cured for 7 days were observed under 3000 times and 500 times magnification and analyzed by energy chromatograph under 500 times magnification. Figure 11 shows the microstructure of MKSF-0%, MKSFP-0.20%, and MKSFO-0.15% mortars. It can be seen from the figure that compared with the mortar of the MKSFP-0.20% and MKSFO-0.15% groups, the MKSF-0% group has a dense structure and the gaps between the particles are filled. The surface of the MKSFP-0.20% and the MKSFO-0.15% group has more cracks, and the surface is rough. In the comparison test of the three groups of mortars, the cracks near the cementitious material in the MKSF-0% group are less and dense, while the cracks in the MKSFP-0.20% and MKSFO-0.15% groups are wider and clearer. The cementitious material generated by the polymerization reaction of aluminosilicate materials reduces the cementation between mortars and increases the voids between mortars. Therefore, compared with the two defoamer groups, the surface density of the benchmark group is better. Therefore, compared with the two defoamer groups of MKSFP-0.20% and MKSFO-0.15%, the surface compactness of the MKSF-0% reference group is better, and the flaky units are wrapped and cemented together by the gel of the geopolymer. Independent fine particles, aggregates, and flocs have formed giant particles with strong network links, and the arrangement between gel flocs and giant particles is more compact and has a more uniform microstructure, and the phenomenon of overhead between particles is greatly reduced.

Spectra of geopolymer mortar samples MKSF-0%, MKSFP-0.20%, and MKSFO-0.15% in the selected area are shown in Figure 11, which shows the elemental composition and chemical composition of each sample in the selected area. EDS provides qualitative evidence for the analysis of cementitious materials in geopolymer mortar.

Table 10. Si/Al and Na/Al molar ratios of MKSF-0%, MKSFP-0.20%, and MKSFO-0.15%.

EDS Sample	Si/Al (Molar Ratio)	Na/Al (Molar Ratio)
MKSF-0%	1.04	0.85
MKSFP-0.20%	1.0	0.84
MKSFO-0.15%	1.04	0.85

shows Si/Al and Na/Al molar ratios in geopolymer mortars. It can be seen from the analysis table that the atomic percentage of Na element in the three groups of samples is maintained at a low level compared to Si element and Al element, and the Na element in the geopolymer comes from the alkali activator water glass, so the Na element is introduced. In addition, it is worth noting that the atomic percentage of Si in geopolymer is at a relatively high level, which is related to the large amount of SiO₂ in metakaolin and water glass. Metakaolin and water glass in geopolymer cementitious materials are rich in Si. The Si/Al and Na/Al molar ratios can be used as qualitative indicators. From the EDS analysis table, it can be seen that the Si/Al and Na/Al in the three groups of experiments swing around 1.0 and 0.85, respectively, and n(Si):n(Na):n(Al) is almost the same. The addition of ions did not affect the process of inorganic polymer polymerization in the geopolymer reaction process.

In order to evaluate the effect of defoamers on geopolymer mortar, SEM and EDES were used to observe the microstructure of MKSF-0%, MKSFP-0.20%, and MKSFO-0.15%. In the MKSFP-0.20% and MKSFO-0.15% groups, the distribution of Na, Si, and Al is relatively uniform without enrichment. In the MKSF-0% group, we found that the content of Na element is less but the distribution is uniform, and there is no enrichment phenomenon. The distribution of Si and Al elements has triangular grooves. The reason for this phenomenon is that metakaolin and water glass were caused by insufficient reaction, and, except for the groove-shaped part, the rest of the distribution is relatively uniform, and there is no enrichment phenomenon. Figure 12 shows that, compared with MKSFP-0.20% and MKSFO-0.15%, MKSF-0% has a more uniform and dense microstructure, which is mainly due to the directional encapsulation and connection of the lamellar units after mortar polymerization, so that the geopolymer cementitious material has obvious regularity and spatial distribution. The directionality, due to the addition of defoaming agent, affects the bonding connection between the lamellar units generated after the polymerization of geopolymer mortar, which will make the mortar lamellar units present a certain dispersion in the spatial distribution and the arrangement of the lamellar units after polymerization looser, thus weakening the filling effect of the gel material. It can be seen that the addition of defoamer affects the cementation of aggregates after the polymerization of silicate materials to a certain extent, forming an overhead structure. Therefore, it can be seen from the microstructure of MKSFP-0.20% and MKSFO-0.15% that a wide range of micro-cracks and pores are distributed on the surface of mortar. The reason for this phenomenon is positively correlated with the decrease of compressive strength of geopolymer by defoaming agent.

Since the surface quality of the mortar can be observed on a macro scale, the improvement effect of the MKSF-0% group on the surface quality of the mortar can be qualitatively judged by the naked eye. Additionally, the number is large, while the MKSFP-0.20% and MKSFO-0.15% group mortars have different degrees of improvement, and the improvement effect is very significant. Among them, the MKSFP-0.20% group has almost no honeycomb pitting phenomenon. The quality effect is the best, which is mainly due to the fact that when the mortar is vibrated, the bubbles overflowing outwards come into contact with the defoamer on the surface of the mortar, so that the residual air bubbles on the surface of the mortar are reduced. In addition, the defoamer has a certain water-reducing effect, and the contact with the surface of the mortar reduces the water–cement ratio, making it more compact after hardening, so the apparent quality of the specimen is improved.

Figure 13 shows the surface morphology and failure modes of MKSF-0%, MKSFP-0.20%, and MKSFO-0.15%. It can be clearly seen that the addition of defoamer effectively improves the generation of honeycomb and pockmarked surfaces on the surface of the mortar, so that the surface of the mortar has high flatness and gloss, which shows that the defoamer can effectively change the surface of the mortar and morphological characteristics, so as to achieve the purpose of improving the apparent quality of the mortar. The MKSF-0% group has obvious characteristics after failure. The failure specimen presents two positive and inverted trapezoids with basically the same size and symmetrically arranged along the abscissa. There was no uneven stress. In the test samples of MKSFP-0.20% and MKSFO-0.15% groups, it can be seen that the arrangement of the specimens after failure is in a normal trapezoid and inverted conical arrangement. This is because the geopolymer mortar itself is brittle and the splitting tensile strength is smaller. The specimen is not constrained during the lateral force received during the compression process, so that the cracks generated by the side extrusion will be ejected, resulting in the destruction of the integrity of the geopolymer mortar.

4. Conclusions

Geopolymer mortar has obvious advantages in mechanical properties. Geopolymer mortars modified with defoamers have better properties. Based on the uniaxial compressive strength test, this study discussed the effects of alkali equivalent, water glass modulus, bone glue ratio, and silica fume content on the strength of geopolymer mortar. By adding different defoamers and mortars, the performance changes of the geopolymer mortar improved by the defoamer were evaluated in detail, and the research results provided parameters for the application and promotion of geopolymer mortar in engineering.

• The compressive strength test of nine geopolymer mortars was designed and completed based on the principle of the orthogonal test, the optimal ratio of geopolymer mortar can be obtained by analyzing the test results: the optimal modulus is 1.4 m, the optimal alkali the equivalent is 13%, the optimal bone-to-adhesive ratio is 2.0, and the optimal amount of silica fume is 8%. The primary and secondary factors affecting the compressive strength are water glass modulus > alkali equivalent > bone-to-adhesive ratio > silica fume content. The n(SiO₂)/n(Al₂O₃) is 3.0, and the strength is 81.22 MPa;
• Through the analysis of the geopolymer mortar with defoamer added, it can be concluded that the compressive strength is the largest when the content of polyether defoamer is 0.20%, and the compressive strength when the content of silicone defoamer is 0.15% maximum; however, adding defoamer will not increase the compressive strength value of geopolymer mortar. Adding a defoamer will improve the fluidity and density of mortar and reduce the water absorption rate of mortar. When the content of polyether defoamer and silicone defoamer is 0.20% and 0.15%, respectively, the fluidity of mortar increases the most, relative to the benchmark group increased by 15.2% and 9.8%, respectively. When the content of polyether defoamer and silicone defoamer was 0.2%, the density of mortar increased the most, which was 10.9% and 7.2% higher than that of the benchmark group, respectively. When the content of polyether defoamer and silicone defoamer is 0.25%, the water absorption of mortar decreases the most, which is 16.6% and 16.1% lower than that of the benchmark group, respectively;
• Microstructure and product analysis show that the inorganic macromolecular polymer formed by the action of the geopolymer has a dense structure, the particle gaps are fully filled, and the surface is smooth and has a more dense and uniform structure. The microscopic morphology of the polyether group and the silicone group is significantly different from the reference group. The surface of the two defoamer groups is obviously more cracked, and the surface is rough. There are few and dense cracks nearby, while the cracks near the polyether group and the silicone group are wider and clearer, which indicates that the defoaming agent produces a cementitious material in the polymerization reaction of the aluminosilicate material to reduce the cementation between mortars and increase the voids between the interior of the mortar. After adding the defoamer, the honeycomb and hemp surface of the mortar surface has high flatness and gloss, which shows that the defoamer can effectively change the characteristics of the mortar surface and achieve the purpose of improving the apparent quality of the mortar.