A Matter of Mortar

Mixing mortar is like mixing pancake batter. The most important thing is the consistency, which should be neither too wet nor too dry.

On the houses we’re building in Chiang Mai, we’re using mortar made of three parts aggregate to one part cement. Its purpose is to bind together the courses of LEGO-like interlocking blocks that we’ve been laying.

When you’re building with LEGO, even cement LEGO, you need something permanent to bind the blocks together. If the holes in the blocks were perfectly round and perfectly aligned, you could perhaps use metal rods. But the holes are square and irregular, and the blocks aren’t perfectly aligned, so you need a plastic material like mortar that can take the shape of the holes before curing in place.

The small rocks in the aggregate give the mortar strength, but they also make it difficult to pour the mortar down the inch-and-a-half-square holes where it needs to go. On the first day, we struggled to force the mortar (using a funnel and rebar) down several courses at a time.

Because the method of construction we’re using requires so much mortaring, coming up with a better method was an important priority. Once I had a little quiet time last night, I analyzed the problem and decided that, because I couldn’t think of any better way to cram the mortar down the holes, the solution must be to make the mortar wetter so that it would pour easier. The mix couldn’t be too wet, however, or else it would seep out of the walls.

Next, I needed a way to explain this new method to the Thai ladies who were doing most of the mortaring. I ruled out a lecture about the physics of surface tension and decided instead on a show and tell. First, I mixed up a bucket of mortar; then I loaded some into a funnel. I showed the ladies that the mortar was dry enough to hold in the funnel yet wet enough so that if I tapped the funnel end, globs or mortar would flow out. Finally, I demonstrated how using a piece of rebar to break the surface tension at the funnel end would allow them to release mortar easily into the holes.

All of this left me feeling rather pleased with myself. I had made effective use of my tenth-grade chemistry, and afterward the mortaring went noticeable faster than it had before.

How else could I improve things around the job site, I wondered. Then I overheard one of our crew chiefs, Jason, a staff member from Habitat Newark talking to Greg, a chief information officer from Hong Kong. Their conversation went something like this:

“The cement we’re using here is much weaker than the cement in the states.”

“Oh, really.”

“Yeah, you know the tensile strength of American cement, right?”

“Sure, three thousand pounds.”

“Well, the tensile strength of this stuff is only fifteen hundred pounds.”

“You don’t say.”

After hearing that exchange, I decided to take a break from saving the work project and found some plastic mortar buckets that needed a good washing.