Why is grout poured with such a high slump? Is this necessary? Wouldn't slumps that were closer to those for concrete provide higher strength walls?
Grout is generally very fluid to allow the grout to flow around bars. The bars are sometimes placed close to the masonry units or the sides of the cores. The spaces between the bars and the masonry units are much smaller than you would typically have in concrete construction between bars and forms. In most cases, the masonry is also very absorptive. This will draw moisture out of the grout, reducing the water-cement ratio and causing it to stiffen. If the grout starts with a low water-cement ratio, it may not retain sufficient water to hydrate properly.