A Vision of Stronger – and ‘Greener’ – Concrete Buildings (AMSEN)

Alan Anderson

This is an update on an earlier profile here.

In 2009, when John Mwero entered the AMSEN program as a young engineer, he described his dream of solving two problems with a single solution. He wanted to find a cost-effective way to make concrete structures cheaper and “greener” without reducing their structural integrity, and he wanted to find a productive use for the giant mounds of sugarcane waste, or bagasse, that dot the tropical landscape around his native Kenya. He remembers his mother keeping a “garden” of cane in the backyard where she would cut a piece to chew on when she went out.

Most sugarcane plants leave bagasse to rot, while others burn the bagasse to produce electricity and use some of the ash as fertilizer. But John discovered that when the bagasse is burned to ash, it takes on potential value as an industrial material. He dug into the scientific literature, finding evidence that sugarcane waste fiber ash, or SWFA, might be useful in making concrete.

One of the reasons for John’s enthusiasm is that concrete is virtually the only material locally available for large construction projects in East Africa and many other parts of the world – but it is expensive. Also, the manufacture of cement consumes huge amounts of energy and is one of the largest producers of global greenhouse gas emissions. Any measure that would decrease the proportion of cement in concrete could help address both these challenges.

On the hopeful side, he found that the waste ash itself was regarded by the sugar companies as useless, and therefore it would be available at virtually no cost. At the same time, the potential for producing ash is large. John estimated that Kenya’s sugar plants could generate up to 120 metric tons of SWFA daily by burning their bagasse, using a special spraying technique to minimize smoke emissions.

His advisors in AMSEN gave their blessing to his idea, and John plunged into three years of experimental testing to verify the utility of SWFA as a concrete additive. He knew from previous research that other kinds of ash are useful for this purpose, such as volcanic ash, fly ash from coal plants, and other concrete “extenders.” And he knew about Kenya’s great need for low-cost, high-quality housing and commercial structures. For example, the government is committed to building new housing stock in Kibera, the largest slum in Nairobi and second-largest in Africa, and it will need to do so economically. At the end of those three years, his own work has provided evidence that SWFA can bring substantial advances for Kenya in the field of concrete engineering.

The history of concrete is as complex as its chemistry. Most simply, concrete is a composite that consists of some kind of filler, such as gravel or sand, and a paste-like binder of cement mixed with water. The cement binder, manufactured when limestone and clay or similar minerals are burned at very high temperatures, combines chemically with water and “glues” the filler into a conglomerate.

Crude concretes have been known since Macedonian times, but the technology was refined by Roman and Egyptian engineers who used it as a construction material in expanding their empires through the use of enormous aqueducts, buildings, and harbors. A key to their success was the discovery of so-called pozzolan additives, which were similar in many ways to sugarcane ash. These additives, named for the volcanic ash around the city of Pozzuoli, near Naples, were a product of the high temperature of volcanoes, which produced their own cement-like powders. Such pozzolans are called concrete extenders because they add cement-like strength when reacting with water. Pozzolan mixtures have been used to build such strong and enormous structures as the cupola of the Pantheon and the dome of St. Paul’s Cathedral in London.

The next revolutionary step in improving concrete was finding a way to speed up the hardening process. In the 19th century, English engineers needed a kind of concrete sturdy enough to build bridges and harbor structures – and to harden during the 12 hours between one high tide and the next. These engineers discovered that adding up to 5 percent gypsum to the concrete would cause the initial hardening to happen within a few hours – well within their intra-tidal needs. It would then continue at a much slower pace for as long as decades. This new mixture was called Portland cement because the engineers were seeking to mimic the beauty of stone quarried from the Isle of Portland, just off the Dorset coast. This whitish-gray Portland stone, easy to work yet long-lasting, beautifies structures throughout the English-speaking world, from Buckingham Palace in London to the United Nations headquarters in New York city.

John Mwero, “standing on the shoulders” of these giants of engineering, set about to apply historic advances to local conditions. He had to ensure that sugarcane ash could be hardy and durable, both reducing the amount of expensive cement while maintaining the toughness of the final product. Here John quickly found grounds for optimism. The chemical composition of SWFA contained a high silicon dioxide content, which is indicative of a good pozzolan. That is, sufficiently siliceous materials tend to react with calcium hydroxide to prolong strength development in the concrete. As he had hoped, it can thus decrease the amount of Portland cement used in the concrete – an environmentally friendly outcome. In addition, he found that the addition of SWFA, in amounts between 4 and 10 percent by weight, resulted in improved mechanical strength, reduced pore sizes, better durability, and early strength gain. Amounts up to 20 percent could also be used with little loss in strength.

Another issue he had to address is the heat that is generated by curing concrete. Heat can increase the formation of cracks and voids that may later weaken the concrete. Heat of hydration is a special concern in construction where masses are large, such as large buildings and dams, where this heat may be trapped and cause expansion and cracking.

To test this, John created a series of mixtures, adding successively more SWFA to Portland cement, from 0 to 20 percent. As he had hoped, he found that the addition of SWFA to cement pastes in amounts between 10 and 20 percent reduced the heat of hydration. He attributed this to the “interference” of the SWFA particles with the hydration process, thus reducing the chemical reactions and the heat itself. This, concluded John, “can be useful in construction of large-mass concrete structures, where heat of hydration is usually a challenge.”

Finally, he compared the chemical shrinkage of his new concrete mixtures with those of concrete without SWFA. This shrinkage, which reduces the size of a structure early in curing, is caused by the reduction of the volume of the cement paste itself, and by the collapse of voids between particles. He found that reducing the amount of Portland cement reduced the chemical shrinkage, and that introducing new particles (of SWFA) filled some of the voids. All samples of SWFA between 10 and 20 percent reduced the amount of chemical shrinkage.

With these results, John considered the experimental phase of his work to be persuasive, and will now look for the most appropriate applications. The first step, he suggests, is to enlist the support of government agencies. “They are invested in reducing costs, and in the environmental aspects. Some agencies, such as the National Housing Corporation and the Kenya Industrial Research and Development Institute, should be receptive to this.”

He also notes that in his native region near Mombasa, most people build houses with coral blocks bound together, but that this material is too weak for structures more than two or three stories high. “Our work can be useful in a rural setting. It is stronger than coral blocks and costs less than concrete.”

He also needs to convince the sugar companies and the cement manufacturers that producing SWFA concrete at a fair price is in the industries’ and the country’s interest – and to develop a system whereby the sugarcane farmers receive part of any profits. He hopes to make his case from several professional vantage points: both as a lecturer at the university and a consultant to government and industry. He is under no illusions that the construction industry will change overnight because of his results. “One PhD will not be enough to convince the world that this is what they should be doing,” he said. But he hopes that his results will be sufficient to nudge his country toward new practices that make environmental as well as economic sense.