This is the third post discussing the gear needed on Aconcagua. Here are the first and second posts.
You need some robust footwear for the 3-day trek to base camp, over dry but often rather rocky terrain. I wore my reliable Asolo leather tramping boots, which for long walks are just as comfortable as shoes. A couple of people struggled with blisters. For me, they can be completely avoided by having boots that fit properly, combined with the right socks. A couple of the people in the group wore running shoes all the way to base camp. They got away with it, but I personally wouldn't recommend this. Given the terrain, running shoes aren't great either for keeping your feet and ankles protected or for security of footing.
Double plastic boots
These are quite possibly the most important single item of gear. Keeping your feet warm and dry is essential high up on the mountain. The gear list insisted on double plastic boots as obligatory. Literally getting cold feet is one of the main reasons for failing to summit Aconcagua.
The main options presented by the gear list were the Asolo 8000 and Scarpa Inverno (known as the Vega in the UK). I ended up getting the Scarpa boots since they were the only ones the local gear importer could get in an appropriate size (andI'd like to thank Bivouac for all their help with this). They were perfectly adequate: robust, plenty warm enough with the factory liners, and easy enough to get on and off. However, the Invernos were very large and clunky. They made a duck-like gait basically inevitable, and they were so broad they created some problems for fitting gaiters and strapping crampons. Thanks to some advice from a local ski store, I inserted some silicon wedges into the heels, which notably reduced the duck-like gait. If I had my time again, I'd like to try the Asolos. A couple of team members had these and they seemed to be smaller and less clunky while being just as warm. On the other hand, some of the reviews suggest they aren't great for wide feet (like mine).
It's worth noting that I ended up getting a pair of the Invernos at least 1 1/2 sizes bigger than my usual boot size, which was not due to wearing thicker socks. Boots are one of the things that you really must try before you buy (or have the option and time to exchange if you buy online).
The gear list specifically advised people on the Aconcagua expedition not to bring Himalayan-style boots with integrated gaiters such as the Millet Everest, since it's thought that they may suffer damage on the sharp scree of Aconcagua. As it happened, several expedition members did bring these boots, and they turned out to be ideal in the heavy snow we experienced. So, they can work out well, and might be an option if you're planning to use them on further expeditions to the Himalayas or Alaska. But you can risk damage if conditions are dry.
A piece of advice: don't try to delay wearing your plastic boots by continuing from Base Camp up to Camp 1 in your trekking boots. This will simply mean that you will have to carry your plastic boots at some point and you'll put off getting used to them. At the speed you will be walking up the mountain, plastics are perfectly acceptable.
Adventure Consultants recommended getting some Intuition liners to wear instead of the factory liners in our plastic boots. These are specialist liners made by a small Canadian company using an innovative foam developed in New Zealand. They can be heat molded to better fit your boots and feet. The liner made for climbing boots is called the Denali. They are said to be warmer, lighter and quicker-drying than factory liners. I took the advice and sent away for some Intuition liners, based on my foot measurements. When they arrived, they turned out to be too small, even after heat molding. The company very kindly (and quickly) agreed to replace them with the next size up. However, even these were a bit tight around my toes, even after more heat molding. In the end, I took them to Argentina but at base camp decided to go with the factory liners, which fit perfectly and were plenty warm enough. There are people, including a couple of our guides, who swear by the Intuition liners. If you decide to get these, make sure you have the ability and time to get the right size and ensure they fit your feet and boots.
Regardless of which liners you end up using, when you are at the high camps you need to sleep with them in your sleeping bag to help them dry out.
For almost any long trekking or climbing expedition, I wear liner socks. I find that by wicking away sweat to the outer sock layer, they keep my feet drier, warmer (and, paradoxically, also cooler in hot conditions). I find that cheap, synthetic liners from the Cool Max brand work best. For outer layers I usually wear Icebreaker merino socks, of varying weights depending on the conditions. However, for Aconcagua summit day I got the thicket, warmest synthetic mountaineering socks I could find. These were reserved, unworn, for the day itself, along with a clean pair of liner socks.
In a previous post, I suggested that you could get away with fewer base layers, maybe only one set, on the high mountain. However, I do not recommend skimping on socks. Having something clean and dry on your feet makes a big difference, and I did not regret taking 5 full sets of socks with me. Above 5,000 metres, where it's freezing cold and doing anything saps precious oxygen and energy, your priorities become clear. I think I only managed to clean my teeth 2 or 3 times after we left base camp. But on a snowy afternoon at camp 1, I made a special effort to trudge down to a mostly-frozen stream. Using a small lump of soap and numbing water that gushed beneath a hole in the ice, I washed my dirty liner socks (to be dried by being hung up inside the tent and then joining the growing collection of items inside my sleeping bag).
Footwear around camp
The gear list advised us to take some old running shoes for river crossings and wearing around camp, and some down booties for wearing in the tent and around camp. I used the running shoes for the one river crossing and then left them at base camp. The booties turned out to be redundant inside the tent, and useless in the deep snow we had at all our camps. As I've suggested previously, I would replace both of these with a pair of sandals, or maybe some old tennis-style shoes that can be really squashed up.