but的使用方法

He always does the work, but in his own sweet time.

He tried to save it, but in vain.

这两句话中，but词性为连词吗？连词的话前后内容可以不并列吗？

But 在此的词性依然是连词。

张道真《现代英语使用方法词典》第 174 页：

（but）间或引起状语等：

Hetried to save it, but in vain.

Heworked hard but without any success.

Sheglanced quickly about, but seeing only the empty room.

另：InBritain the government advises people to wear masks, but to little effect.