# Assessing the Applicability of Lanthanide-Based Upconverting Nanoparticles for Optically Monitoring Cement Hydration and Tagging Building Materials

## TL;DR

This paper explores using lanthanide-based nanoparticles to monitor cement hydration and tag building materials, finding optimal conditions for their use without impairing cement properties.

## Contribution

The study identifies suitable lanthanide nanoparticles and application conditions for monitoring cement hydration and tagging materials without significantly affecting hydration processes.

## Key findings

- 25 nm-sized oleate-coated UCNPs at 0.1 wt% provided optimal luminescence probing without delaying cement hydration.
- Higher UCNP concentrations (1.0 wt%) delayed hydration processes, especially in the first 24 hours.
- Luminescence stability over one year supports UCNP use as durable optical tags for construction materials.

## Abstract

Chemically stable, lanthanide-based photon upconversion
micro-
and nanoparticles (UCNPs) with their characteristic multicolor emission
bands in the ultraviolet (UV), visible (vis), near-infrared (NIR),
and short-wave infrared (SWIR) are promising optical reporters and
barcoding tags. To assess the applicability of UCNPs for the monitoring
of early stage cement hydration processes and as authentication tags
for cementitious materials, we screened the evolution of the luminescence
of self-made core-only NaYF4:Yb,Er UCNPs and commercial
μm-sized Y2O2S:Yb,Er particles during
the first stages of cement hydration, which largely determines the
future properties of the hardened material. Parameters explored from
the UCNP side included particle size, morphology, surface chemistry
or coating, luminescence properties, and concentration in different
cement mixtures. From the cement side, the influence of the mineral
composition of the cement matrix was representatively examined for
ordinary Portland cement (OPC) and its constituents tricalcium aluminate
(C3A), tricalcium silicate (C3S), and gypsum at different water to
cement ratios. Based on reflection and luminescence measurements,
enabling online monitoring, which were complemented by XRD and isothermal
heat-flow calorimetric measurements to determine whether the incorporation
of these particles could impair cement hydration processes, well suited
lanthanide particle reporters could be identified as well as application
conditions. In addition, thereby the reporter influence on cement
hydration kinetics could be minimized while still preserving a high
level of information content. The best performance for the luminescence
probing of changes during early stage cement hydration processes was
observed for 25 nm-sized oleate (OA)-coated UCNPs added in a concentration
of 0.1 wt %. Higher UCNP amounts of 1.0 wt % delayed cement hydration
processes size- and surface coating-specifically in the first 24 h.
Subsequent luminescence stability screening studies performed over
a period of about one year support the applicability of UCNPs as optical
authentication tags for construction materials.