Technical Paper

Figure 2:

Figure 1:

generally, mainly as moist absorbed from the environment plus some water present in the mortar used for jointing. However, attention shall be paid to the fact where and how much monolithic refractory materials have been used in combination with the bricked constructions.

In deviation from commonly used air-setting refractories which harden as they dry, a heat-setting refractory although containing bonding agents, requires heat to develop strength (table 2).

3: Curing

Curing is the process following placement of the refractory lining, during which period strength is developed by hydration of the cement component or a chemical reaction in the refractory material. The first is particular important for cast and gunned refractory materials. The latter is mainly present in plastic or refractory mixes for special applications.

During mixing of a hydraulically setting refractory material with potable water, principally three different quantities of water can be distinguished:

1. Water, in a proper ratio with the cement, required for the hydration reactions;

2. Water required for the workability;

3. Extra water which will be absorbed in the porous aggregates more or less immediately;

And let’s not forget:

Only water that’s good for yourself, is good enough for refractories !!

3.1: Curing of hydraulically setting castables and gunned refractory mixes

During the hydration process this water (ref. 1) will be bound as hydrates to the cement clinker. An exothermic hydration reaction develops after some time, e.g. 2 - 8 hours after placement and may reach up to 100 °C (!), depending on type of cement, lining geometry and ambient conditions. (figures 1 and 2).

May 2018 Issue

± 20 %w H O 2

± 5 %w H O 2

± 0 - 80 %w H O 2

Figure 3:

The heat evaporates and/or drives the water away which it unavoidable needs for full hydration, hence full strength development may be hampered.

The deficit is in first instance filled up with surrounding 'free water'. But this free water acts also as a kind of granulate in the mixture besides the other aggregates during the setting process of the installed material. And at the same time it must act as cool down medium for control of the hydration reaction in the lining, hence losses have to be substituted, which generally shows by drying spots on the lining surface. (figure 3).

This substitution can be achieved by using intensive water spraying during the heat development and thereafter, or by using a coating or curing compound, forming a film which reduces the evaporation of water at the surface, but which does not contribute to the (necessary) cooling. (figure 2).

Too fast loss of water may cause formation of unwanted hydrates from the cement clinker, insufficient bond, excess shrinkage of the lining and may lead to improper strength development of the lining.

Refractories using alumina cement develop about 80% of the specified strength already within 24 hours, and even after 6 - 8 hours the concrete is strong enough to carry moderate loads. The fast initial setting makes it so very useful for gunned applications. However, in doing so it places very high emphasis on these initial first and important hours.

In normal practice this period coincides with the installation of the



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