Will an OPC concrete become less or more resistant to sulphates with addition of extra OPC.

I believe it will become more susceptible to sulphate attack if more cement is present as there will be more hydrated C₃A which can react with the sulphate. If you want to confer sulphate resistance to OPC then add ground granulated blast furnace slag.

I would like to know more about alkali silica reaction in the concrete. Kindly clarify the following. I am aware that alkalis in the cement will react with reactive silica of the aggregates and cause cracks. I would like to know how fast these cracks are likely to appear once the concrete is poured. Is it possible that within a day or two after pouring the concrete, cracks are likely to be surfaced due to ASR problem? Fly ash addition in cement is supposed to mitigate ASR problem in the concrete. Is there any reason due to which fly ash is likely to increase the ASR problem? What tests are to be conducted on fly ash, to find out the suitability for mixing in the cement? What tests are to be conducted on aggregates to understand the reactivity with alkalis present in the cement?

No, it is not possible for the alkali-silica reaction to cause cracking of concrete within a day or two of pouring the concrete. The expansion reaction caused by the alkali -silica reaction causes cracks to appear a number of years after the concrete. Fly ash addition should not increase the susceptibility to alkali-silica reaction. The pozzolanic reaction of the fly ash fills the pores of the concrete and reduces the tendency for cracking. However, the fly ash should be of low alkali content. You must test the type of silica present in the aggregates, reactive forms are opal, chalcedony, some types of quartz, cristobalite, tridymite and various siliceous glasses.