Development of a Low-pH Concrete Intended for Deep Geological Repository for Radioactive Waste

Abstract

This article deals with the development of concretes intended to be used as construction materials in the Czech deep repository for radioactive waste. The basic requirements for this concrete are a reduced pH value, which must maintain a constant reduced pH with a value of around 11 in the long term, and mechanical properties comparable to conventional concrete. The raw materials for the production of the proposed low pH concrete come exclusively from the Czech Republic. Material characteristics were measured on fresh mixtures and concrete after 28, 56, and 90 days of curing. In addition to the basic raw materials (aggregate, cement, water), plasticizers, microsilica or slag, and defoaming agents were added to the mixture. The aggregate:binder ratio was approximately 5:1 and the w/c water coefficient was approximately 0.6. The mechanical properties of the final concrete were similar to the reference recipe of conventional concrete (the decrease was less than 10%) and the pH value was even below 11 after 90 days. The issue of a sprayed variant of the LPC mixture was solved within the project.

Evaluation of Physico-Chemical Characteristics of Cement Superplasticizer Based on Polymelamine Sulphonate

“…Cement is used widely in waste repositories due to its ability to buffer a high pH value of water, to maintain many radionuclides as low soluble hydroxides, to lead a substrate for sorption, and thus to retard the migration of long-lived radionuclides [37][38][39][40]. An important feature of cement systems is the formation of high-pH conditions [41].…”

“…An important feature of cement systems is the formation of high-pH conditions [41]. This occurs as the result of hydration reactions as cement is mixed with water, principally forming calciumsilicate-hydrates (CSH) of different compositions and portlandite (Ca(OH) 2 ) [33,35,[39][40][41]. The dissolution of these phases over time leads to the development of higher pH values.…”