Lloyd E. Morris’s research while affiliated with Texas A&M University and other places

Monitor wells are frequently sealed using a cement–bentonite grout mixture because it is believed that the addition of bentonite (1) reduces shrinkage of the cement, (2) increases cement plasticity, (3) reduces curing temperatures, and (4) reduces grout weight. The design of a compound grout appears to be simple: add a fixed percentage of premium bentonite, by dry-weight, per sack of Portland cement and increase the volume of mix water from 5.2 gal/sack by 1.3 gal/2% bentonite added. This formula, however, is only valid if the bentonite is dry-blended with the cement before water is added. In most environmental applications, cement–bentonite mixing is performed at the job site. The driller has two options: (1) mix the cement with water and then add the dry bentonite or (2) mix the bentonite with water and then add the cement. Both of these lead to problems; in the first case, the bentonite does not fully blend with the grout, and in the second case the bentonite consumes all of the available water. The solution is to add more water. An 8% bentonite–cement compound grout can easily be mixed and pumped if the bentonite and cement are first dry blended, a problem not easily solved on an environmental drill site. If the cement is hydrated first, an additional 10 gal of water must be added; if the bentonite is hydrated first, at least 20 gal of water must be added to the blend. When expanded, high-yield bentonite is used,> 30 gal of additional water must be added. These high-water content environmental grouts are very low density, very low strength, and highly permeable. A strong quality assurance/quality control program for cement–bentonite grout specifications must be established.