Mexican building supplies giant Cemex SAB de CV said Monday that a joint venture in which it has a 49.9 percent stake has completed the sale of 12 active quarries in the United States and other assets for $420 million.

The assets are being sold to SPO Partners & Co., an investment firm in San Francisco.

The operations include two granite quarries in Georgia and 10 limestone quarries — nine in Tennessee and one in Virginia.

Cemex said its joint venture partner Ready Mix USA LLC has been operating the quarires and both companies considered them non-strategic.

A portion of the funds from the sale will be used to reduce Ready Mix's debt and to pay $100 million each to the two joint venture partners.

Cemex will use some of the funds to reduce debt. The Mexican company has been taking steps to improve its cash levels and debt structure, including refinancing $15 billion of debt and raising about $2.3 billion in new debt. Cemex also has sold $2.2 billion in stock and convertible securities.

Last October, Cemex sold its Australian business to Holcim Group for $1.7 billion.