Performance and Mechanism of Wollastonite-Slag-Metakaolin Geopolymer under Seawater Wet-Dry Cycles

doi:10.16552/j.cnki.issn1001-1625.2025.1224

Abstract

Abstract:

To investigate the service performance of metakaolin geopolymer mortar in marine environments， this study focused on the degradation behavior and mechanisms of metakaolin geopolymer mortar modified with a wollastonite-slag composite admixture under two erosion modes： seawater wet-dry cycles and full immersion. Wollastonite-slag-metakaolin geopolymer mortars （WSM） with varying mass ratios of wollastonite to slag （1∶2， 1∶1， and 2∶1）， along with pure metakaolin mortar （MK）， were prepared. The evolution of their apparent morphology， physical properties， and mechanical performance over 0~90 erosion cycles was comparatively investigated， and the degradation mechanisms were analyzed using micro-testing methods such as SEM and XRD. The results indicate that the damage caused by wet-dry cycles is significantly more severe than that by full immersion. The former is identified as a “physical-chemical” coupled erosion process driven by salt crystallization pressure and an ion “pump” effect. The wollastonite-slag composite admixture significantly enhances erosion resistance and durability of metakaolin geopolymer mortar. Specifically， when the mass ratio of wollastonite to slag is 1∶1 （WSM15）， geopolymer mortar exhibits the optimal performance. Compared with the reference MK group， the compressive strength of WSM15 increases by 59.3%， while the mass loss rate is reduced by 30.97%. The enhanced performance originates from the physical and chemical synergistic effects of wollastonite and slag.

Key words: metakaolin geopolymer, wollastonite, slag, seawater wet-dry cycle, erosion mechanism

CLC Number:

TU57⁺8.12

WANG Xiaobin, LUO Liting, HE Zixiang, CHEN Juan. Performance and Mechanism of Wollastonite-Slag-Metakaolin Geopolymer under Seawater Wet-Dry Cycles[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(6): 2021-2033.

Figures/Tables 19

Table 1 Main chemical composition of raw materials

Material	Mass fraction/%
Material	SiO₂	Al₂O₃	Fe₂O₃	CaO	MgO	K₂O	Na₂O	SO₃
Metakaolin	54.00	43.00	0.40	0.09	0.04	0.30	0.03	—
Wollastonite	41.80	0.36	0.31	54.91	1.75	—	—	—
Slag	34.50	17.70	1.03	34.00	6.01	—	—	1.64

Table 2 Main characteristic indicators of Na2SiO3 solution

Item	Density/（g·cm^-3）	Na₂O content/%	SiO₂content/%	H₂O content/%	Solid content/%	Modulus M_s	pH value
Value	1.36	8.54	26.98	64.48	35.50	3.30	10~13

Table 3 Solute concentrations in simulated seawater

Ingredient	NaCl	MgCl₂	Na₂SO₄	CaCl₂	KCl	NaHCO₃
Concentration/（g·L^-1）	122.7	25.6	20.5	5.8	3.5	1.0

Table 4 Mix proportion of geopolymer mortar

Group	Metakaolin/B	Slag/B	Wollastonite/B	Sand/B	Added water/B	NaOH/B	Na₂SiO₃ solution/B
MK	1.000	0	0	2.500	0.209	0.088	0.453
WSM10	0.700	0.200	0.100	2.500	0.209	0.088	0.453
WSM15	0.700	0.150	0.150	2.500	0.209	0.088	0.453
WSM20	0.700	0.100	0.200	2.500	0.209	0.088	0.453

Fig.1 Apparent morphological changes of specimens under two seawater erosion conditions

Fig.2 XRD pattern of white precipitates on surface of WSM15-60-N

Fig.3 Changes in water absorption of specimens after seawater erosion

Fig.4 Fractured cross-section images of WSM15 at the 90th cycle under two seawater erosion conditions

Fig.5 Changes in mass loss rate of specimens after seawater erosion

Fig.6 Changes in compressive strength of specimens after seawater erosion

Fig.7 “Bridging” effect of wollastonite

Fig.8 Schematic diagram of ion "pump" effect and salt crystallization pressure

Fig.9 Changes of relative dynamic elastic modulus with seawater erosion cycles

Fig.10 Relationship between RE， N and Rm

Table 5 Experimental and fitted values of relative dynamic elastic modulus of geopolymer mortar

Group	R_m	N	Experimental value/%	Fitted value/%	Absolute error	Relative error/%
WSM10	0.5	0	100.00	99.91	0.09	0.09
		15	84.74	87.75	3.01	3.55
		30	74.39	77.66	3.28	4.41
		60	65.12	63.72	1.40	2.16
		90	55.31	58.08	2.76	4.99
WSM15	1.0	0	100.00	99.93	0.07	0.07
		15	91.65	88.20	3.45	3.76
		30	81.84	78.56	3.28	4.01
		60	74.95	65.49	9.46	12.62
		90	70.56	60.72	9.84	13.95
WSM20	2.0	0	100.00	99.96	0.04	0.04
		15	91.21	89.11	2.10	2.31
		30	74.68	80.34	5.66	7.58
		60	65.63	69.02	3.39	5.17
		90	61.76	66.01	4.26	6.89

Fig.11 XRD patterns of WSM15-0， MK-60， and WSM15-60

Fig.12 SEM image and EDS spectrum of MK-60

Fig.13 SEM image and EDS spectrum of WSM15-0

Fig.14 SEM image and EDS spectrum of WSM15-60

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