Effects of Micro- and Nanosilica on the Mechanical and Microstructural Characteristics of Some Special Mortars Made with Recycled Concrete Aggregates

Abstract

In this paper, we study the influence of densified microsilica and colloidal nanosilica admixtures on the mechanical strength and the microstructural characteristics of special mortars used for immobilizing radioactive concrete waste. The experimental program focused on the replacement of cement with micro- and/or nanosilica, in different proportions, in the basic composition of a mortar made with recycled aggregates. The technical criteria imposed for such cementitious systems, used for the encapsulation of low-level radioactive waste, imply high fluidity, increased mechanical strength and lack of segregation and of bleeding. We aimed to increase the structural compactness of the mortars by adding micro- and nanosilica, all the while maintaining the technical criteria imposed, to obtain a cement matrix with high durability and increased capacity for immobilizing radionuclides. The samples from all the compositions obtained were analyzed from the point of view of mechanical strength. Also, micro- and nanosilica as well as samples of the optimal mortar compositions were analyzed physically and microstructurally. Experimental data showed that the mortar samples present maximum compressive strength for a content between 6 and 7.5% wt. of microsilica, respectively, for a content of 2.25% wt. nanosilica. The obtained results suggest a synergistic effect of micro- and nanosilica when they are used simultaneously in cementitious compositions. Thus, among the analyzed compositional variants, the mortar composition with 3% wt. microsilica and 2.25% wt. nanosilica showed the best performance, with an increase in compressive strength of 23.5% compared to the control sample (without micro- and nanosilica). Brunauer-Emmett-Teller (BET) analysis and scanning electron microscopy (SEM) images highlighted the decrease in pore diameter and the increase in structural compactness, especially for mortar samples with nanosilica content or a mixture of micro- and nanosilica. This study is useful in the field of recycling radioactive concrete resulting from the decommissioning of nuclear research or nuclear power reactors.

Keywords: colloidal nanosilica; microsilica; microstructure; physical–mechanical properties; recycled concrete aggregates; special mortars.

Figure 3

EDS spectra, with the quantitative identification of the constituent elements.

Figure 1

Images from the mixture homogenization (a), the flexural tests (b) and the compression tests (c) of the samples.

Figure 2

Micrograph of the MS sample, at 1 × 10⁵ magnification, highlighting the average particle sizes.

Figure 4

The flow time of the paste, in seconds, depending on the % wt. of the superplasticizer (a) and the % wt. of the MS (b).

Figure 5

Flexural strength of mortar samples.

Figure 6

Compressive strength of mortar samples.

Figure 7

Correlation between compressive strength and W/(C + S) ratio.

Figure 8

SEM images of the OPC mortar specimens: (a) after 7 days of curing, 5 × 10³ magnification; (b) after 28 days of curing, 5 × 10³ magnification.

Figure 9

SEM images of the MS6 mortar specimens: (a) after 7 days of curing, 1 × 10⁵ magnification; (b) after 28 days of curing, 5 × 10⁴ magnification.

Figure 10

SEM images of the CB4.5 mortar specimens: (a) after 7 days of curing, 1 × 10⁵ magnification; (b) after 28 days of curing, 2 × 10⁴ magnification.

Figure 11

SEM images of the MS3CB4.5 mortar specimens: (a) after 7 days of curing, 2 × 10⁴ magnification; (b) after 28 days of curing, 2 × 10⁴ magnification.