Sia is pop’s most uncompromising star.

Despite her massive breakthrough hit “Chandelier,” which recently crossed 1 billion views on YouTube, Sia refuses to show her face when she performs, opting for stand-ins or a face-covering trademark blond wig at her rare concert appearances.

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With her new album, “This Is Acting” (RCA), she takes her strong-willed ways a step further, building it completely from songs she wrote for other artists who didn’t end up using them. Spoiler alert: They work well for her.

Obviously, these aren’t simple cast-offs. Sia has co-written “Diamonds” for Rihanna, “Pretty Hurts” for Beyoncé, among others. She co-wrote the first single “Alive” with Adele and Tobias Jesso Jr., for possible use on record-breaking Adele’s “25.” You can hear how Adele would have made this work, another empowering tale of survival after heartbreak with huge emotional swings. But it’s the edge in Sia’s voice, on lines like “You took it all, but I’m still breathing,” that makes the song a success.

Throughout “This Is Acting,” it’s Sia’s voice that makes the songs special. “Sweet Design,” with its distorted sampling of Sisqo’s “Thong Song” and its stop-start R&B rhythms, sounds like it was built for Beyoncé, but the way Sia attacks the verses and coos on the chorus is an unexpectedly good take.

Sia dubbed this album “This Is Acting” because she feels that’s what she does when she writes a song for someone else. These songs are less quirky and more pop-oriented than her previous albums. While the soaring “Bird Set Free” is what we have come to expect from Sia, the clever pop trifle “House on Fire,” which she co-wrote with fun.’s Jack Antonoff, is a nice change of pace. Only on “Reaper,” co-written with Kanye West, does Sia’s delivery seem out of step.

“This Is Acting” is an interesting concept, but as good as it is, Sia probably won’t be having any more trouble placing songs.