Method for Estimating Quantity of Non-Hydrated Cement in a Cement Recycling System

Abstract

Abstract The objective of this research was to develop a method for the rapid estimation of the quantity of residual cement in sludge water and to control its hydration reaction using sodium gluconate in order to enable the residual cement in sludge water to be effectively utilized. The quantity of residual cement in sludge water can be estimated by measuring the heat of hydration liberated in 24 h using a conduction calorimeter. It is possible to recommence sodium glucon-ate-controlled cement hydration using magnesium nitrate. A method was developed to simulate the rate of the hydration reaction, which makes it possible to estimate the quantity of residual cement in sludge water by measuring the rate of heat liberation of hydration over 10 h. 1. Introduction In today’s society, where there is a need to implement low carbon recycling systems, the reduction and reuse of waste sludge water from the production of ready-mixed concrete at factories is an important issue. Data from 2006 indicate that about 1 million tons of sludge are generated annually in Japan. Sludge is a mixture of cement and very fine aggregate that occurs as waste at construction sites or forms when residual recycled concrete is sieved to remove aggregates. Cleaning water that contains a sludge component, which is referred to as sludge water, is formed when water is used to clean equipment such as ready-mixed-concrete mixers and agitators. To reduce the quantity of sludge formation at ready-mixed-concrete factories, Japanese Industrial Standard A 5308 “Ready-mixed Concrete” prescribes a method of using mortar that adheres to the inside of truck agitator drums. This allows for a solid sludge component of up to 3% (the mass of the solid sludge component as a percentage of the unit cement quantity in the concrete mix) to exist in sludge water when it is used as concrete mixing water. However, if sludge water is added, the concrete fluidity is reduced and the quantity of water required to produce the concrete increases. This method is not used widely because of its high cost and compli-cated management of sludge water. The number of ready-mixed-concrete factories that use sludge water as concrete mixing water is low and the amount of indus-trial waste that they generate after the dewatering process is large. Sludge water contains fine particles that originate from cement that has already been hydrated, and from non-hydrated cement and aggregates. By using a set retarder such as sodium gluconate (GLNa) or similar, the hydration reaction of the unhydrated cement in the sludge water can be controlled (Atarashi