Ordinary Portland cement (OPC) has been used for centuries as a primary binding material for concrete, mortar, and stucco, but researchers at the Missouri University of Science and Technology (Missouri S&T) (Rolla, Missouri) are studying ways to potentially make concrete without using the traditional material.
Mohamed ElGawady, associate professor of civil, architectural, and environmental engineering at the school, and his students are testing mixtures of concrete made with fly ash that they believe can be stronger, more durable, and more resilient than concrete using OPC.
By using this zero-cement (ZC) concrete, the Missouri Department of Transportation (MoDOT) (Jefferson City, Missouri) can improve the durability and performance of the state’s concrete bridges and extend the service life of these bridges beyond the typical 75 years, ElGawady says. The zero-cement concrete can also improve the sustainability of the MoDOT structures, he adds.
The mixtures developed as part of the study will allow MoDOT to use zero-cement concrete to address shrinkage cracking and freeze/thaw damage currently experienced with conventional cement-containing concrete mixtures.
“The superior durability of zero-cement fly ash means also less potential maintenance and repair,” ElGawady says.
The goal is to develop zero-cement concrete mixtures that can be used by MoDOT contractors for bridge deck and girders using locally available materials. The feasibility of using zero-cement concrete for partial depth deck and prestressed girder repair works will also be explored, according to the professor.
In the testing process, ElGawady and his students plan to produce up to 3,000 samples that use fly ash from different sources, and have varying alkaline activator properties and concentrations, curing temperatures, curing times, and water content.
Conventional concrete formulations use OPC alone, or a mixture of OPC and fly ash, as the main binding material. This involves a slow hydration process for the OPC/fly ash mixture, ElGawady says. Conventional concrete also requires curing, he notes, adding that it typically takes 28 days for conventional concrete to reach its compressive strength.
Differences of Zero-Cement Concrete
Zero-cement concrete, however, combines an alumina-silica rich material such as fly ash, calcined clay, mine tailings, or blast furnace slag with an alkali activator like sodium hydroxide (NaOH), sodium silicate (Na2SiO3), potassium hydroxide (KOH), or potassium silicate (K2O3Si). This zero-cement concrete incorporates a substantially quicker chemical process, ElGawady says, where the alkaline liquid polymerizes the fly ash. This, in turn, creates a new, three-dimensional inorganic polymeric chain with enhanced strength.
In all, zero-cement concrete reaches its compressive strength in less than 24 hours—compared to the 28-to-56 days often needed in the case of conventional or high-volume fly ash concrete.
Other benefits of zero-cement concrete include high resistance to freeze/thaw, corrosion, elevated temperatures, fire, salt, and acid or alkaline environments, according to the professor, who adds that zero-cement also shows very low permeability and high tensile strength.
As a result, ElGawady believes zero-cement is more durable than conventional concrete. Moreover, he says research from the Alaska Department of Housing (Anchorage, Alaska) has shown that making zero-cement can be cheaper or equal to the price of conventional concrete.
Further Research Ahead
Looking ahead, further research from the professor’s team is planned to investigate the production of zero-cement concrete with locally sourced materials to determine the best practice to produce zero-cement concrete without jeopardizing its strength. The effects of zero-cement concrete on the bond between concrete and rebar will also be investigated, he says.
In addition, because zero-cement concrete could be used for retrofitting purposes, ElGawady plans to try to determine the cohesion and friction between existing OPC concrete and zero-cement concrete.
Source: Missouri S&T, news.mst.edu.