Control of the setting reaction and strength development of slag-blended volcanic ash-based phosphate geopolymer with the addition of boric acid

Jean Noël Yankwa Djobo,Dietmar Stephan

doi:10.1007/s41779-021-00610-4

Jean Noël Yankwa Djobo, Dietmar Stephan

Open Access

https://doi.org/10.1007/s41779-021-00610-4

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Abstract

This work aimed to evaluate the role of the addition of blast furnace slag for the formation of reaction products and the strength development of volcanic ash-based phosphate geopolymer. Volcanic ash was replaced by 4 and 6 wt% of ground granulated blast furnace slag to accelerate the reaction kinetics. Then, the influence of boric acid for controlling the setting and kinetics reactions was also evaluated. The results demonstrated that the competition between the dissolution of boric acid and volcanic ash-slag particles is the main process controlling the setting and kinetics reaction. The addition of slag has significantly accelerated the initial and final setting times, whereas the addition of boric acid was beneficial for delaying the setting times. Consequently, it also enhanced the flowability of the paste. The compressive strength increased significantly with the addition of slag, and the optimum replaced rate was 4 wt% which resulted in 28 d strength of 27 MPa. Beyond that percentage, the strength was reduced because of the flash setting of the binder which does not allow a subsequent dissolution of the particles and their precipitation. The binders formed with the addition of slag and/or boric acid are beneficial for the improvement of the water stability of the volcanic ash-based phosphate geopolymer.

Highlights

Aluminosilicate phosphate cement (APC) or binder is a type of acid-base cement belonging to the chemically bonded phosphate cement (CBPC) group obtained at room or slightly elevated temperature by a chemical reaction between a solid precursor and an acid phosphate [1, 2]
It mainly acts on hindering the precipitation reaction by stabilizing Mg2+ ions with the formation of the complex MgB(OH)4+ which makes the Mg2+ ions less available to react with phosphate species [20], while for a higher dosage, it rather delays the dissolution reaction [22]
The work has reported the role of blast furnace slag and boric acid addition on the reaction and strength development of volcanic ash phosphate cement

Summary

Introduction

Aluminosilicate phosphate cement (APC) or binder is a type of acid-base cement belonging to the chemically bonded phosphate cement (CBPC) group obtained at room or slightly elevated temperature by a chemical reaction between a solid precursor (aluminosilicate powder) and an acid phosphate [1, 2]. The synthesis of APC that sets and hardens in a shorter time at room temperature depends mainly on the chemical composition of the aluminosilicate, the type, and the dosage of the phosphate solution [3, 12, 13]. The initial setting time of metakaolin phosphate cement ranges from 20 h to several days depending on the reactivity of metakaolin [13, 14], while volcanic ash-based phosphate geopolymer has an initial setting time of few minutes at room temperature [3].

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