Abstract

Objective To prepare ternary geopolymers with excellent mechanical properties and eco-friendly characteristics， this study used metakaolin， fly ash， and coal gangue as raw materials and a phosphate solution as the activator. The simplex centroid design method （SCDM） was used to optimize the dosage ratios of metakaolin， fly ash， and coal gangue.

Methods The activity indices of three raw materials， i.e.， metakaolin， fly ash， and coal gangue， were evaluated. Preliminary optimization of their dosage ratios was conducted using SCDM， and the control， single-doped， and co-doped group samples were prepared. The influence of raw material dosage ratios on the mechanical properties， flowability， and pore structure of the samples was analyzed. The mechanism of hydration product formation in ternary geopolymers was revealed through microscopic morphology analysis. The optimal dosage ratios of the three raw materials for preparing ternary geopolymers were determined.

Results and Discussion At a curing age of 28 days， the activity indices of metakaolin， fly ash， and coal gangue were measured as 104.2%， 85.2%， and 51.1%， respectively. The incorporation of coal gangue resulted in varying degrees of decreases in compressive strength and fluidity of ternary geopolymers， while its effect on flexural strength was negligible. In contrast， the addition of fly ash effectively improved the fluidity and mitigated compressive strength reduction caused by coal gangue. Most co-doped group samples exhibited superior compressive strength and fluidity compared to the control and single-doped group samples. The pore structure analysis revealed that the control group samples primarily contained air pores with a limited number of small capillary pores and gel pores， showing a relatively dense structure. The single-doped group samples exhibited a higher proportion of small capillary pores and air pores. Although the co-doped group samples were still dominated by small capillary pores and air pores， the porosity of small capillary pores was increased to varying degrees， and the number of air pores was reduced compared to single-doped group， indicating an improved pore structure. The phosphate activator was observed to react with precursor materials such as metakaolin and fly ash through geopolymerization， forming amorphous gels， including phosphoaluminate gels ［—Al—O—P—］_n and aluminosilicate hydrate gels ［—Si—O—Al—O—P—］_n. These gels greatly enhanced the mechanical properties of geopolymers. To achieve a balance between mechanical properties， fluidity， and economic efficiency， at a curing age of 28 days， the compressive and flexural strengths of ternary geopolymers were set to be no less than 60% of the control group. Under the condition of compressive strength is equal or more than 45.0 MPa， flexural strength is equal or more than 5.8 MPa， and fluidity is more than 180 mm， the optimal mass fractions of each raw material in the optimized dosage were determined as follows： metakaolin 74%~83%， fly ash 3%~15%， and coal gangue 15%~27%.

Conclusion Using SCDM， the optimal dosage of raw materials in ternary geopolymer mortar is achieved， striking a balance between mechanical properties， workability， and cost-effectiveness while contributing to environmental sustainability.

Keywords： geopolymer； simplex centroid design method； metakaolin； fly ash； coal gangue

Get Citation: WANG Jinsong，DAN Li， LIU Liqun，et al. Formulation optimization for ternary geopolymers based on simplex centroid design method［J］. China Powder Science and Technology，2025， 31（5）：1-13.

Received: 2024-10-13. Revised: 2025-04-14, Online: 2025-05-06.

Funding Project: 国家自然科学基金项目，编号 ：42177074。

First Author: 王劲松（1972—），男，教授，博士，博士生导师，湖南省“121”人才工程人选，研究方向为环境功能材料。E-mail：xhwjs@163. com。

DOI：10.13732/j.issn.1008-5548.2025.05.013

CLC No: TU526；TB4          Type Code: A

Serial No: 1008-5548（2025）05-0001-13