Born in Alessandria, he was initially a student of the sculptor and architect Angelo Cavanna in Novi Ligure.
In 1893, he was named professor of the Accademia Ligustica di Belle Arti in Genoa.
In Genoa, he was known for his portraits, but also completed large fresco or decorative cycles, including for the Società promotrice di Belle Arti, Palazzo Raggio in via Balbi, Castello Raggio a Cornigliano, Villa Weil, and the Villa Bisio in Albaro.
Among other works Il Canto del mattino and Ritorno dai campi The contemporary critic Ercole Arturo Marescotti described his paintings at the Promotrice di Genoa: The paintings of Viazzi portray the truth of the environment: what first impresses the observer: is the truth that most stubbornly look for in the abstruse, in the baroque, and almost despise them for failing to remain modest and simple: the truth of which you can overcome without studying the great and true mastery that is nature: ... Passionate lover of the truth, the atmosphere, but to arrive (there you have) sacrifice (the reality) for the effect, I would say almost to destroy the color found there, Viazzi well shows that what worries him most is not only vulgar effect ... and much less complacency before the intonation: his only care, his only study, is the tone, the totality of the painting.
He cares very little to talk to the eye, but seeks, puts all his concerns, because his painting speaks to the heart.