Effect of Alkali Equivalent on Properties of Phosphogypsum- Alkali-Activated Slag Foam Concrete

doi:10.16552/j.cnki.issn1001-1625.2025.0962

Abstract

Abstract:

The utilization of phosphogypsum and slag in the preparation of foam concrete is beneficial for the efficient use of solid wastes. In this study， CaO+Na₂CO₃ was fixed as activator， the phosphogypsum content was 50% （mass fraction）. The effects of different alkali equivalents （2.5%， 3.0%， 3.5%， and 4.0%， mass fraction） on properties of phosphogypsum-alkali-activated slag foam concrete （PAF） were investigated. The hydration products and pore characteristics were analyzed by XRD， TG-DTG， SEM and BSE. The results show that as the alkali equivalent increases from 2.5% to 4.0%， the fluidity， wet density， compressive strength and flexural strength of PAF decrease， while the water absorption and drying shrinkage increase. The softening coefficient increases first and then decreases， reaching a peak at 3.0%， indicating the best water resistance of sample. When the alkali equivalent is 3.0%， the formation of C-（A）-S-H gel and AFt crystals is maximized and uniformly distributed， the pore size is minimal， and the roundness approaches 1， forming a dense and continuous microstructure. Under this condition， PAF exhibits higher mechanical properties， lower water absorption， and reduced shrinkage deformation， resulting in the optimal overall performance. This study provides a theoretical basis and technical support for the resource utilization of phosphogypsum and the mix proportion optimization of solid-waste-based foamed concrete.

Key words: phosphogypsum-alkali-activated slag foam concrete, alkali equivalent, drying shrinkage, average roundness value, softening coefficient, water resistance

CLC Number:

TU528

CHEN Zihan, GUO Yudong, LYU Qinfei, LIANG Yongning, JI Tao. Effect of Alkali Equivalent on Properties of Phosphogypsum- Alkali-Activated Slag Foam Concrete[J]. BULLETIN OF THE CHINESE CERAMIC SOCIETY, 2026, 45(4): 1315-1323.

Figures/Tables 17

Table 1 Main chemical composition of undisturbed PG

Composition	Na₂O	MgO	Al₂O₃	SiO₂	P₂O₅	SO₃	K₂O	CaO	Cl	Fe₂O₃	F
Mass fraction/%	0.26	0.11	0.38	3.63	1.26	56.81	0.10	35.95	0.03	0.23	1.11

Table 2 Property index of slag

Density/（g·cm^-3）	Specific surface area/（m²·kg^-1）	7 d activityindex/%	28 d activityindex/%	Mobilityratio/%	Moisturecontent/%	Loss onignition/%
3.10	429.00	84.20	98.50	98.00	0.25	1.24

Fig.1 Particle size distribution of slag

Fig.2 XRD pattern of slag

Table 3 Mix proportion of PAF with different alkali equivalents

Sample	Mix proportion/kg
Sample	PG	CaO	Na₂CO₃	Water reducer	Coagulant	Water	Foaming agent
A2.5	500	11.3	21.4	5	30	500	281.4
A3.0	500	13.4	25.7	5	30	500	281.4
A3.5	500	15.8	29.9	5	30	500	281.4
A4.0	500	18.1	34.2	5	30	500	281.4

Fig.3 XRD patterns of PAF with different alkali equivalents

Fig.4 TG-DTG curves of PAF with different alkali equivalents

Table 4 Mass loss of PAF in different temperature ranges calculated based on TG curves

Sample	Mass loss/%
Sample	△m₁	△m₂	△m₃	Total
A2.5	1.77	3.75	1.23	9.90
A3.0	9.90	7.99	0.97	27.22
A3.5	11.25	5.99	1.20	29.03
A4.0	4.84	6.78	1.77	20.16

Fig.5 SEM images of hydration products of PAF with different alkali equivalents

Fig.6 Porosity profile of PAF with different alkali equivalents

Table 5 Porosity parameters of PAF with different alkali equivalents

Sample	Porosity/%	Porosity（d≤50 μm）/%	Porosity（50 μm<d<100 μm）/%	Porosity（d≥100 μm）/%	Averagepore size/μm	Average roundnessvalue
A2.5	34.24	33.79	0.34	0.10	11.42	1.99
A3.0	51.00	50.29	0.56	0.15	7.66	1.88
A3.5	57.51	55.55	0.98	0.98	15.92	2.67
A4.0	65.85	63.35	1.98	0.46	17.35	3.07

Fig.7 Fluidity and wet density of PAF with different alkali equivalents

Fig.8 Compressive strength and flexural strength of PAF with different alkali equivalents

Fig.9 Softening coefficient of PAF with different alkali equivalents

Fig.10 Influence factors of water resistance of PAF with different alkali equivalents

Fig.11 Drying shrinkage of PAF with different alkali equivalents

Fig.12 Influence factors of drying shrinkage of PAF with different alkali equivalents

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