Adsorption Characteristics and Action Mechanism of Na-Montmorillonite on Polycarboxylate Superplasticizer

doi:10.16552/j.cnki.issn1001-1625.2025.0720

Abstract

Abstract:

The competitive adsorption between the mud （mainly clay minerals such as bentonite） of concrete aggregate and polycarboxylate superplasticizer （PCE） adversely affects the performance of cement and concrete. The purpose of this paper was to explore the effect of Na-montmorillonite on the adsorption behavior of three kinds of PCE （JSJ-1， JSJ-2， JSJ-3） and cement performance. The molecular structure and parameters of PCE were characterized by Fourier transform infrared spectroscopy （FT-IR）， nuclear magnetic resonance hydrogen spectroscopy （¹H-NMR） and gel permeation chromatography （GPC）. The fluidity and setting time were measured by cement paste test. The structural changes and adsorption mechanism of Na-montmorillonite after adsorption were analyzed by X-ray diffraction （XRD） and simultaneous thermal analysis （TG-DSC）. The results show that the presence of large molecular groups in PCE makes the cement paste completely lose fluidity when the Na-montmorillonite content is 5% （mass fraction）. The adsorption kinetics study shows that the adsorption of three types of PCE by Na-montmorillonite conforms to the pseudo-second-order kinetic model （R²>0.99）， mainly chemical adsorption， and the maximum adsorption capacity is 338.98 mg/g （JSJ-1）， 294.12 mg/g （JSJ-2）， 280.90 mg/g （JSJ-3）. The Langmuir isotherm fitting results show that the adsorption process is uniformly covered by a monolayer （R²>0.99）. The structure analysis of Na-montmorillonite after adsorption confirms that the PCE side chain is embedded in the interlayer of Na-montmorillonite to form a stable intercalation structure， which delays the decomposition temperature range of PCE to 200 ~ 450 °C.

Key words: Na-montmorillonite, cement, polycarboxylate superplasticizer, adsorption characteristic, thermodynamics, action mechanism

CLC Number:

TQ172.4⁺6

LI Meng, KONG Desong, HUANG Teng, HE Zhihao, WANG Enwen, WEI Yunlong. Adsorption Characteristics and Action Mechanism of Na-Montmorillonite on Polycarboxylate Superplasticizer[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(2): 390-403.

Figures/Tables 19

Fig.1 Infrared spectra of three types of polycarboxylate superplasticizers

Fig.2 GPC spectra of three types of polycarboxylate superplasticizers

Fig.3 1H nuclear magnetic resonance spectra of three types of polycarboxylate superplasticizers

Fig.4 Fluidity of cement with addition of three types of polycarboxylate superplasticizers

Fig.5 Setting time of cement with addition of three types of polycarboxylate superplasticizers

Fig.6 Adsorption kinetics fitting curves of Na-montmorillonite on three types of polycarboxylate superplasticizers

Table 1 Adsorption kinetic parameters

Kinetic parameter		PCE type
Kinetic parameter		JSJ-1	JSJ-2	JSJ-3
Quasi-first-order kinetic model	q_e/（mg·g^-1）	18.927 96	0.419 44	14.226 55
	k₁	0.005 88	0.008 38	0.004 27
	R²	0.623 96	0.602 60	0.500 19
Quasi-second-order kinetic model	q_e/（mg·g^-1）	495.049 51	497.512 44	490.196 08
	k₂	0.004 00	0.006 78	0.017 70
	R²	0.999 99	1.000 00	1.000 00

Fig.7 Effects of temperature and initial concentration on adsorption performance of polycarboxylate superplasticizer

Fig.8 Adsorption isothermal model of polycarboxylate superplasticizer by Na-montmorillonite

Table 2 Adsorption isothermal model parameters

PCE type	Temperature/℃	Langmuir			Freundlich
PCE type	Temperature/℃	q_m/（mg·g^-1）	K_L	R²	1/n	K_F	R²
	25	256.410 3	0.002 3	0.997 7	0.782 7	1.235 3	0.995 6
Na-MMT+JSJ-1	35	338.983 1	0.001 4	0.999 9	0.840 4	0.848 0	0.997 6
	45	327.868 9	0.001 3	0.997 7	0.837 2	0.781 1	0.996 5
	25	215.517 2	0.005 4	0.996 7	0.718 5	2.627 6	0.990 9
Na-MMT+JSJ-2	35	232.558 1	0.003 0	0.995 3	0.795 4	1.330 6	0.987 8
	45	294.117 6	0.001 9	0.998 6	0.843 7	0.950 7	0.993 2
	25	280.898 9	0.001 5	0.996 7	0.860 7	1.096 6	0.993 2
Na-MMT+JSJ-3	35	210.084 0	0.002 2	0.996 5	0.787 2	0.966 2	0.996 1
	45	168.918 9	0.002 5	0.998 5	0.738 3	0.935 8	0.993 3

Fig.9 Adsorption thermodynamics of Na-montmorillonite on polycarboxylate superplasticizer

Table 3 Thermodynamic parameters of adsorption

Sample	T/K	∆H/（kJ·mol^-1）	∆S/（kJ·mol^-1·K^-1）	∆G/（kJ·mol^-1）
Na-MMT+JSJ-1	293.15			-0.757 3
	303.15	-5.638 2	-0.016 3	-0.695 4
	313.15			-0.427 6
Na-MMT+JSJ-2	293.15			-2.019 7
	303.15	-4.673 0	-0.048 4	-2.008 4
	313.15			-1.840 0
Na-MMT+JSJ-3	293.15			-1.269 7
	303.15	-15.709 7	-0.008 8	-0.834 0
	313.15			-0.299 9

Fig.10 Infrared spectra of Na-montmorillonite after adsorption of polycarboxylate superplasticizer at 35 ℃

Fig.11 GPC images of Na-montmorillonite before and after adsorption of polycarboxylate superplasticizer

Table 4 Molecular weight and distribution changes of Na-montmorillonite before and after adsorption of polycarboxylate superplasticizer

Sample	Adsorption state	M_p	M_n	M_w	PDI
Na-MMT+JSJ-1	Before	7 256	6 516	9 804	1.504 6
Na-MMT+JSJ-1	After	13 734	10 748	22 605	2.103 2
Na-MMT+JSJ-2	Before	25 090	21 276	33 413	1.570 5
Na-MMT+JSJ-2	After	9 105	4 066	14 219	3.497 1
Na-MMT+JSJ-3	Before	29 016	23 793	39 881	1.676 2
Na-MMT+JSJ-3	After	11 079	4 019	16 362	4.071 2

Fig.12 XRD patterns of Na-montmorillonite before and after adsorption of polycarboxylate superplasticizer

Table 5 Layer spacing of Na-montmorillonite before and after adsorption of polycarboxylate superplasticizer

Sample	2θ/（°）	d_（001）/Å
Na-MMT	7.325	12.055
Na-MMT+JSJ-1	7.068	12.497
Na-MMT+JSJ-2	6.534	13.517
Na-MMT+JSJ-3	6.653	13.275

Fig.13 XPS analysis of Na-montmorillonite before and after adsorption of polycarboxylate superplasticizer

Fig.14 TG-DSC curves of Na-montmorillonite before and after adsorption of polycarboxylate superplasticizer

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