Internal Curing (IC) for cement is a practical way of supplying additional water throughout the concrete mixture. In other words, achieving better hydration and less waste of valuable cement. This is done by consuming absorbed water in Light Expanded Clay Aggregate (LECA) which is replacing some of the conventional aggregate in the mixture.

Internal curing of concrete

Internal Curing: “supplying water throughout a freshly placed cementitious mixture using reservoirs, via pre-wetted lightweight aggregates, that readily release water as needed for hydration or to replace moisture lost through evaporation or self-desiccation”

Internal curing at the contact zone

Internal curing provides what concrete needs that conventional curing cannot provide for the concrete. Additional water helps preventing early age shrinkage and increases hydration of cementitious materials throughout the concrete.
The other important aspect of curing is temperature.The concrete can’t be too cold or too warm. As fresh concrete gets cooler, the hydration reaction slows down. The temperature of the concrete is what’s important here. In cold temperature condition below 10 °C, hydration suddenly slows down and below temperature 4 °C, it stops.
Warm concrete has the opposite problem: The hydration reaction goes too fast, and since the reaction is exothermic (produces heat), it can quickly cause temperature differentials within the concrete that can lead to cracking. Cement reacts too quickly, it wouldn’t have time for the crystals to grow properly so it doesn’t develop as much strength as it should be.
Once concrete sets, hydration creates partially-filled pores in the cement paste which causes stress that results in shrinkage. Internal Curing (IC) provides readily available additional water throughout the concrete, so hydration can continue while more of the pores in the cement paste remain saturated. This reduces shrinkage and early age curling/warping, increases strength, and lowers the permeability of the concrete, making it more resistant to chloride penetration.
Internal curing is often referred as “curing concrete from the inside out.” Due to the inherently low permeability of the surrounding environment, internal curing is particularly beneficial in concrete with a low water-cement ratio (w/c) where external curing has little effect on hydration in the internal portion of the concrete.
Typically, concrete has been cured from the outside in; Internal Curing (IC) is the kind of curing from the inside out. Internal water is supplied via internal reservoirs found in prewetted lightweight fine aggregates (leca).

Difference between internal cured concrete and normal cured concrete
Advantages:

With an emphasis on durability, and with the trend to high performance concrete, internal curing is needed to reduce both autogenous shrinkage and cracking. The internal curing process magically happens within an ordinary concrete mixture. Since concrete cracking is most often the result of overt shrinkage, the life of concrete can be severely reduced as early age cracks grow in size.

Cracks weaken the concrete by exposing its insides to many deleterious things from its in situ environment. Over time, concrete cracks become exacerbated as small cracks grow into larger cracks.

Time dependent improvement in the quality of concrete containing pre-wet lightweight aggregate is greater than normal weight aggregate.

According to specifications, the amount of lightweight aggregate needed is based on the absorption and desorption of the aggregate being used. Most practical concrete applications, seven pounds of Internal Curing water per 100 pounds of cementitious material, provide an appropriate value for the amount of Internal Curing moisture needed. However, the amount of Internal Curing water may be increased to accommodate evaporation or to satisfy the higher water demand in mixtures with supplemental cementitious materials.