Try Plastic sheeting

Test specimens were cured under constant temperature 20oC and relatively humidity condition of 65 %.

Wrap it tight in a polythene cover,and place it in a desiccator containing little water and place the desiccator in a constant temperature room.

Please refer our paper "Factors influencing the strength of cement fly ash base courses"

SR Kaniraj, V Gayathri

Journal of transportation engineering, ASCE, 129 (5), 538-548 

Dear Zaman Khan...

When resorting to cement treatment, expect high heat of hydrations. The main idea behind controlled curing conditions is to minimize the loss of moisture. Accordingly, you have two options:

1. I do not recommend plastic wrapping simply because, you will not substitute the loss of moisture from desiccator to the specimen back. Instead, as soon as you compact the specimen, weigh it, and then keep it inside desiccator directly without wrapping it. If the desiccator is intact(proper seal) and you have pure distilled water below the desiccator chamber, the loss of moisture due to heat of hydration will be compensated by the water from desiccator. Make sure to weigh the sample before testing. Any loss in weight by more than .5%, you should discard the sample.

2. The other option is to keep the compacted specimen(s) inside humidity controlled chambers. Check with your concrete lab guys. They might have this chamber. But, you need to make sure that, only like samples are inside the chamber. If you have samples mixed with different activator(s) (Calcium ligno sulfonate, calcium hydroxide, calcium chloride, calcium carbonate), you must avoid using the chamber.

Let me know if you have any further queries.

First i am asking why not going for soil lime instead of soil cement since lime will react chemicaly with the chemical component of the expansive soil.

Second, if you go for soil cement, then you may directly coat the casted specimens with liquid asphalt or wax to prevent the evaporatioon of water throughout the curing process sothat that the available water content wil be consumed by thecement during hydration process.

I support the second part of the suggested answer by Saad Sarsam.

plastic sheet to keep water content constant