# Triethylamine-Capped Calcium Phosphate Oligomers/Polyacrylamide Synergistically Reinforced α-Hemihydrate Gypsum Composites: A Mechanistic Study on Mechanical Strengthening via Organic/Inorganic Interpenetrating Networks

## TL;DR

A new gypsum composite with enhanced strength and longer setting time was developed using a combination of calcium phosphate, polyacrylamide, and triethylamine.

## Contribution

The study introduces a synergistic organic/inorganic interpenetrating network to improve mechanical properties of gypsum composites.

## Key findings

- The composite's initial setting time increased by 140.5% compared to pure gypsum.
- Flexural and compressive strengths were significantly higher than pure and PAM-modified gypsum systems.
- A dual-network structure formed, enhancing overall mechanical performance.

## Abstract

In this study, a novel calcium phosphate/polyacrylamide copolymer/α-type hemihydrate gypsum (CPO/PAM/α-HHG) composite material was prepared by polymerising a stable inorganic CPO precursor, end-capped with triethylamine (TEA), with an organic polyacrylamide (PAM) hydrogel to form a CPO/PAM precursor solution. Subsequently, this precursor solution was mixed with inorganic α-hemihydrate gypsum. The effects of CPO/PAM precursor addition and CPO addition on the slurry flowability, initial setting time, and mechanical properties of hardened specimens of the CPO/PAM/α-HHG composite were investigated. The structural characteristics of the composites were analysed by XRD, FE-SEM, and TGA. The results show that the initial setting time of the CPO/PAM/α-HHG composites was 26.7 min, which was 140.5% longer than that of the pure water α-HHG system and 3.9% longer than that of the PAM/α-HHG system; additionally, the oven-dried specimens had a flexural strength of 27.59 MPa and a compressive strength of 68.48 MPa, which were 77.2% and 102.0% higher than those of the pure water α-HHG system and 38.8% and 14.1% higher than those of the PAM/α-HHG system, respectively. The wet compressive strength of the CPO/PAM/α-HHG composites was improved by 11.8% compared to that of the PAM/α-HHG system. A structural analysis showed that CPO promoted the gelation process of PAM and allowed the hydration reaction process of α-HHG to be fully carried out by slowing down the gelation process of the organic network, which led to the full development of both organic and inorganic networks, ultimately forming an interspersed inorganic/organic dual-network structure, which enhanced the comprehensive mechanical properties of the composites. This study provides a new idea for the modification of α-type hemihydrate gypsum and a new method for the preparation of high-utilisation and high-performance gypsum-based composites.