[Edited to add: Some images were copyright and had the watermarks stripped off. I was not aware of that. Those images have been removed.]
Can concrete buildings be attractive, or at least neat? I was reading an article lamenting the lack of historical preservation granted to 1950s-70s Brutalist architecture, and then started thinking about concrete. Which led to Gaudi and La Sagrada Familia, and because I’m Odd, the Hundredwasser Haus in Vienna. Personally, I will not miss most Brutalist structures, although in a few cases, what replaced them is less attractive, at least to me. I understand why some Brutalist structures were constructed, but that doesn’t improve the aesthetics.
Brutalism is the term applied to the heavy, grey cement and steel and glass structures built between roughly the Bauhaus period of the 1930s and the 1970s. The 1960s were sort of the heyday for the stuff. Officially, it began in the 1950s as a “modern” aesthetic to counter the nostalgia of the 1940s and the neo-Everything styles of the late 1800s-early 1900s. It tends to be mostly steel and concrete, with basic shapes (square, oblong, a few curves, or a lot of really strange curves) and no trim. It was not painted, and loomed in a morose grey way over the cities of England, Europe, and the US. It was very much form and function, without wasting materials on decorative features. It could be built quickly if the design were simple. Some later designs push the limits of materials and structure. It was considered very modern, the style of the future. Universities adopted it, although usually with more decoration and trim.

Not simple, but cold. That’s the library at UC – San Diego. Source: https://mymodernmet.com/brutalist-architecture/

From a CNN article about saving Brutalism. This is public housing in Warsaw. https://www.cnn.com/style/article/brutalism-this-brutal-world-modern-forms/index.html
Critics leaped to attack the new design style. It was cold, hard, boring, unhuman. The use of quasi-Brutalist as the preferred building style of Communist dictators didn’t help the reputation of Brutalism, and led to the joke that it was “Stalin Baroque” or “Khrushchev Eclectic.” As much as I loathe Stalin, his taste in building style wasn’t quite that bad. It wasn’t great, but he was old-school and favored grandiose and palatial. Those are terms not applied to Brutalism, although grandiose might fit (in the negative sense, often, if you are in the Eastern Bloc). Another flaw with the style is that running pipes and conduits and wires through the buildings is very hard, unless you build a framework inside and hang paneling. Or run everything outside, which has its own flaws.
However, concrete buildings are easier to make in a hurry, weather and location permitting. They are less expensive than steel and glass, much less than stone or wood or brick in many places. Concrete scales up easily, something not true of wood and brick. If you needed something relatively fast, relatively cheap, and pretty sturdy if done right, Brutalism it was. That described a lot of the rebuilding done in the non-historic parts of Europe after 1945.
In contrast, Gaudi took cement and did weird and wonderful things with it. It looks organic, flowing and touched with color. Now, his style is NOT fast or inexpensive, and required a lot of engineering to make work, especially the great cathedral of La Sagrada Familia in Barcelona, Spain.

Source:https://www.pinterest.com/pin/632122497683053678/
The interiors can be relatively sane (like in the apartment house) or off the wall.
Antoni Gaudi worked on commission, and was pushing the limits of what was possible in the 1900s-1920s. Casa Batllo is part of that.
And then there’s the cathedral, which is almost finished. Only a hundred years or so in the making, which for a cathedral is about average. Average if you go back to the 1000s, that is.

The drippy bit is NOT what people expected, but it’s cool. https://altmarius.ning.com/profiles/blogs/catedrale-romanocatolice
It’s cool, and controversial. The source article for the above image goes into a lot of detail. https://www.happytravellingfeet.com/sagrada-familia-when-buildings-tell-stories/
I think it is the curves and the playful sense in Gaudi’s work, and that of Hundertwasser in Vienna, that appeals to me. It’s not about being modern or industrial or powerful, but about playing with forms. It has the same problems as Burtalist in terms of materials and pipes and wires, and leaks. But it feels more human.
There are virtues in both, but I don’t care for Brutalism unless it is modified and softened.