On another matter, Calcalist (an Israeli financial paper) had an
article (Hebrew-only AFACT, althoguh a small portion was translated
here) about the cement which went into Gaza. The high points:
1) The article gives as an example a tunnel which was discovered at the end of 2013, about 2.5 km in length (which appears to be fairly typical). According to IDF Engineering Corps estimates, it used 25,000 concrete plates with a total weight of about 800 tons.
2) Hamas skimmed off the cement coming into Gaza. In particular, there were different qualities of cement coming in through the smuggling tunnels (the high quality-stuff was either smuggled in from Turkey or "legally" brought via Israel, the rest was often of very low quality; also, the low-quality cement was often packaged as high-quality), and Hamas appropriated the high-quality stuff. The situation was bad enough that the head of Gaza's building contractors' association formally requested devices for testing cement from USAID, though apparently the request was not granted. While the article doesn't say, this probably contributed to the death toll, because a building built with the low-quality cement is more likely to collapse even if it wasn't directly hit.
3) Two years ago, Israel approved a Qatari initiative to build $400 million worth of projects in Gaza. For that, a significant amount of cement was brought in from Egypt. That ement was effectively handed over to Hamas, who apparently used much of it for their own projects. The article points out that the Qatari projects slowed down significantly due to lack of materials after Egypt shut down the smuggling tunnels, which shouldn't have happened if the material had been going to where it should.
EDIT - might as well put the translated section linked above here:
Where did Hamas get the cement to produce its concrete-lined attack tunnels, especially in light of the Israeli blockade and an overall shortage of cement? While the civilian sector in Gaza suffered from a lack of building materials, Hamas had no problem obtaining thousands of tons of cement. Unfortunately, part of the cement that Israel allowed to enter Gaza for humanitarian needs ended up in Hamas' hands.
In addition, cement was brought in through smuggling tunnels from Egypt for years, especially during the period when the Muslim Brotherhood ruled Egypt (June 2012 to June 2013). During that time, an average of between 400 and 3,000 tons of cement, iron, and building stone entered Gaza daily via the tunnels. High-quality cement came from Turkey to the port at El-Arish in Sinai. The Egyptian authorities at the time were aware of the flow but did nothing to stop it.
Beginning in 2011, complaints by Gaza contractors about the quality of the cement coming through the tunnels started appearing in the Palestinian press, with concerns voiced that newly built homes would one day collapse. It appears that in the private sector, cheaper Egyptian cement was often switched for what was labeled as high quality cement. One cement merchant in Gaza explained that suppliers would deliver a truckload of cement sacks. The outer ones were Turkish cement, while those beneath were Egyptian. Anyone who complained or tried to return the cement quickly found himself threatened by the military arm of Hamas, and dropped his complaint.
Two years ago, Israel approved Qatari financing of reconstruction projects in Gaza at a cost of $400 million. A large amount of Egyptian military grade cement was allowed into Gaza through the Rafah crossing. The moment the cement entered Gaza, Hamas took control of it. The Qataris didn't really care what happened to the cement and the other building materials they paid for.
When the Egyptian army closed the smuggling tunnels, a serious delay in Qatar's building projects in Gaza was announced. The local reaction was surprise, since Egypt had sent to Gaza all the materials that were needed for the Qatari projects. (Calcalist-Hebrew)