Bulletin-24—Building Blocks from Natural Lightweight Materials of New Mexico

By D. M. Clippinger, 1946, 35 pp., 8 figs., 7 plates, 1 index. Supplemented by Bulletin 28.

The increasing use in NM of concrete building blocks employing lightweight volcanic aggregates has presented many problems. Questions have arisen regarding locations of suitable lightweight aggregates, properties of the various materials, proper sizing of aggregates, properties of blocks with certain proportions of cement, and methods of manufacture. In the interest of promoting the use of the natural lightweight materials throughout the state, the NM Bureau of Mines and Mineral Resources has conducted research work on available scoria and pumice. This report embodies the results of this work.

Samples were obtained from 17 deposits of scoria and pumice within the state. Large enough samples of eight of these materials were procured so that they could be made into 8"x8"x16" hollow concrete building blocks for testing. Samples were prepared and blocks were tested in the laboratories at the NM School of Mines. Test blocks were made on one of the commercial vibrator block-machines at the plant of Edgar D. Otto and Son, Albuquerque. The factors causing variations in the properties of concrete building units are so numerous that it was not possible with the available time and facilities to delve into all phases of the subject. This bulletin supersedes a previous circular on the same subject.

The lightweight volcanic aggregates used for concrete include pumice and scoria. Pumice is a natural silicic glass that was produced by volcanism in the form of a molten froth. This froth, upon cooling rapidly, trapped tiny gas bubbles that caused it to remain extremely porous and minutely vesicular in structure. Pumice is white to light-grey or light-tan in color. Its porous texture and glassy composition make it desirable as a building aggregate. Because of it porosity, pumice is light in weight; one cubic yard in pebble form weighs between 825–900 pounds, as compared to ordinary sand and gravel which weighs about 2,600 pounds per cubic yard. The dead-air cells in the pumice give it excellent insulating properties against heat, cold, and sound. The glassy composition makes it practically fireproof; its fusing point is 2,450ºF.

Scoria is a highly vesicular cindery material occurring on the surface of volcanic flows of any composition. The samples studied ranged in composition from that of basalt to that of basic volcanic glass. In general scoria has larger cells and thicker cell walls than pumice, and therefore is slightly heavier; a cubic yard of the crushed material weighs 1,000–1,500 pounds. Scoria ranges in color from red or brown to black. Its insulating and fire-resisting properties are similar to those of pumice. Scoria is generally stronger than pumice.