As the population of sub-Saharan Africa grows, so does the demand for housing and other infrastructure that requires concrete, leading to an increased need for cement for its production. Nigeria - the most populous country in the African region - has embarked on a massive infrastructural development to meet among others the demand for housing caused by rapid urbanization and a high annual population growth. The production of cement, an essential component of concrete, presents two major challenges. First, a large amount of energy is required for its production and grinding. This is a problem especially in developing countries, making cement more expensive. Second, cement production accounts for about 6 % to 10 % of global carbon dioxide emissions. It’s already well established that carbon dioxide (CO2) emission from cement production could be reduced through a circular carbon economy of reducing, reusing, recycling, and or removing CO2. This study considers the option of reducing CO2 emission by partial replacement of cement with a supplementary cementitious material (SCM). The performance of two SCMs locally available in Nigeria, calcined clay (CC) and rice husk ash (RHA), as partial replacements for cement in mortar and concrete was determined. The first part deals with the determination of the pozzolanic potential of Nigerian calcined clays (NCC) relative to German calcined clay and RHA as partial substitutes for cement in general. For this purpose, four naturally occurring clay samples (two kaolinite-rich and two smectite-rich) were collected from different areas in the vicinity of the Ashaka cement plant in Gombe State, Nigeria, and calcined in a laboratory. Mineralogical characterization of the clays was carried out by XRD. The hydration kinetics of the calcined clay-cement systems were monitored by isothermal calorimetry. Workability was determined using the ﬂow table method. The reactivity of the CCs was determined by the solubility of Si and Al ions and the strength activity index (SAI). The results obtained were then compared with the German CC for later use in the production of concrete, since such a quantity needed for the concrete batches is difficult to transport from Nigeria. Rice husks from the vicinity of Zaria in Kaduna State, Nigeria, were calcined in an electric furnace to obtain the RHA and ground with a laboratory-scale mill. The RHA powder was characterized and subsequently evaluated. For the application of the CC and RHA characterized above in concrete, the experimental program of this study considers the use of self-compacting concrete (SCC) to produce durable concrete that has good resistance to chloride ions penetration. This is because both CC and RHA have higher water demand and specific surface area (SSA) compared to cement, which makes their use as a partial replacement for cement in SCC more critical. The objective is to consider the challenging physical properties of CC and RHA and design a powder type of SCC, with a higher replacement level of ordinary Portland cement (OPC) to achieve an eco-efficient and durable SCC. Therefore, first, the OPC is partially replaced by 15 vol-% limestone powder (LP), resulting in a Portland limestone cement (PLC), and up to 40 vol-% of the PLC is replaced by binary and ternary blends of CC and RHA. A rational mix design procedure for SCC was used for proportioning the quantities of SCC. The paste phase of the SCC was first designed considering the physical properties of CC and RHA and used as a medium to embed the fine aggregate (FA), which forms a self-compacting mortar (SC-M). Finally, the SC-M was considered as a medium for embedding the coarser aggregate (CA) that forms the SCC. Two volumetric water-to-powder ratios (Vw/Vp) were used considering the physical properties of CC and RHA, first Vw/Vp = 0.875 for the reference mixtures (PLC-SCC) and SCC mixtures containing up to 40 vol-% CC as partial replacement to PLC in SCC. For the use of RHA in SCC, an individual Vw/Vp was used for each level of PLC replacement by RHA. For comparison of the reference SCC, and SCC produced with binary and ternary blends of CC and RHA, a Vw/Vp = 1.275 was used. The effect of partially replacing PLC with CC and RHA on the fresh properties of the SCC was investigated using a variety of tests. All the CCs and RHA studied met the requirements of ASTM C618 for the use of natural pozzolans as a partial replacement for hydraulic cement. The properties of the German CC are similar to those of the metasmectite Nigerian clays (classified as 2:1 CC). The metasmectitic clays exhibited greater SSA, higher water demand, and less reactive Si and Al ions than the metakaolinitic clays (classified as 1:1 CC). The two CC groups require the addition of a superplasticizer (SP) to achieve a similar workability class to the OPC mortar system. They can be used to replace OPC at replacement levels of up to 45 %, in combination with LP, there by achieving at least a "Level 1" reduction in greenhouse gas emissions. Similarly, RHA is characterized as having high water demand and SSA compared to CC and can be used to replace up to 40 vol-% of cement in SCC. Both CC and RHA have a higher water demand than PLC, with the water demand value of CC being one-third higher than that of PLC which required only an adjustment of SP to achieve a degree of deformability comparable to the reference SCC. Using a Vw/Vp = 0.875 made even a partial replacement up to 40 vol-% CC possible although with an increased viscosity. The use of RHA as a partial replacement for PLC in SCC is more critical because its water demand is more than three times higher and, unlike the CC-SCC system, the SP adjustment is not sufficient to deform the RHA-SCC system. Partial replacement of PLC with RHA up to 40 vol-% is also possible if an increased Vw/Vp ≥ 1.575 is used. Using a Vw/Vp = 1.275, all the SCC specimens achieved the short-term segregation resistance of SCC, and even SCC produced with 20 vol-% RHA as PLC partial replacement retained its flowability up to 30 min after mixing. For the flowability retention evaluation, the use of 20 vol-% RHA as a partial replacement to PLC is considered critical because the excess mixing water was absorbed after 30 min of mixing, making the SCC mix stiffer and rapidly losing its flowability. Therefore, SCC produced with 20 vol-% CC can be used to produce both precast and ready-mix SCC. However, the use of 20 vol-% RHA and the ternary blend 10 vol-% CC + 10 vol-% RHA developed high flow resistance and showed rapid loss of flowability after 30 min of mixing. Therefore, their flowability retention needs to be improved for applications beyond 30 minutes.
At a lower Vw/Vp = 0.875, partial replacement of PLC with 20 and 40 vol-% CC slightly increased the compressive strength of SCC and its chloride migration resistance, changing the SCC quality in terms of chloride resistance from "Normal" to "Good" and resulting in a CO2 savings of 32 %. The increase of the Vw/Vp to 1.275 increased the average pore diameter of the reference SCC and SCC produced with 20 vol-% CC, which changed the initial grade of SCC with 20 vol-% CC from "Good" to "Normal" classification. While the SCC produced with RHA and the blend of CC and RHA with a Vw/Vp = 1.275 slightly increased the 28-day compressive strength of SCC. The chloride migration resistance of 20 vol-% RHA is 3 times that of SCC produced with only PLC, while that of the ternary blend 10 vol-% CC + 10 vol-% RHA is twice that of SCC produced with PLC only. RHA is capable of improving the chloride migration resistance of SCC and should be used to improve the microstructural densification of SCC produced with only PLC. «
As the population of sub-Saharan Africa grows, so does the demand for housing and other infrastructure that requires concrete, leading to an increased need for cement for its production. Nigeria - the most populous country in the African region - has embarked on a massive infrastructural development to meet among others the demand for housing caused by rapid urbanization and a high annual population growth. The production of cement, an essential component of concrete, presents two major challeng... »